docs: mark --compare-dest/--copy-dest/--link-dest implemented

Document receiver-side basis semantics, relative-to-destination-root
confinement, content-verified matching, hard-link vs copy vs compare-only
behavior, cross-filesystem copy fallback, repetition/priority, --delete
exclusion, the implied --incremental and -s incompatibility, and each exact
divergence from rsync (no dest deletion on compare-dest stale entries, no
attribute re-application on basis hits, no re-linking of already-up-to-date
dest files, sources matched from basis dirs kept under --remove-source-files).

RSYNC_COMPAT.md

@@ -171,9 +171,9 @@ This document maps rsync's full feature set to FastSync's current implementation
 |------|-------------------|-----------------|-------|
 | `--checksum` | Skip based on checksum | ✅ Implemented | With `--incremental`, compares xxHash64 content checksums; `-c` remains compression |
 | `--checksum-choice=STR` | Choose checksum algorithm | ❌ Not Implemented | xxHash used internally |
-| `--compare-dest=DIR` | Compare dest files relative to DIR | ❌ Not Implemented | Removed because it had no effect |
+| `--compare-dest=DIR` | Compare dest files relative to DIR | ✅ Implemented | DIR is a receiver-side basis relative to the destination root (confined below it; absolute/`..` rejected). On the receiver's per-file check (implies `--incremental`) an exact match = same size + mtime (unless `--size-only`/`-I`) **and** equal xxHash64 of the sender's file; a match suppresses the data transfer. compare-dest never copies: it only skips a file the destination does **not** already hold (sparse destination, rsync parity), and is consulted before the normal delta/full paths. Repeatable; searched in command-line order, first match wins. Divergences: when the destination already holds a *different* version rsync deletes it but FastSync instead transfers the data (keeps the mirror complete; never deletes without `--delete`); attribute-only differences on a match are not re-applied (data is skipped so the sender never sends metadata); content is verified by xxHash64, stricter than rsync's default quick check. Requires `--incremental` (implied); needs the per-file check so it is incompatible with `-s` chunk serialization. Wire: new config fields (protocol bumped to 2.8.0) |
-| `--copy-dest=DIR` | Include copies of unchanged files | ❌ Not Implemented | Removed because it had no effect |
+| `--copy-dest=DIR` | Include copies of unchanged files | ✅ Implemented | Same basis rules as `--compare-dest`, but an exact match materializes a **local copy** of the DIR file into the destination (via the normal atomic temp+rename store path, so `--existing`/`--ignore-existing`/`--update`/`--backup`/`--delay-updates` all still apply) instead of transferring data. Repeatable; command-line order = priority. Content is xxHash64-verified before the copy. Divergences: attributes come from the basis file (sender metadata is not sent on a match); a match that differs only in attributes is copied with the basis attributes rather than rsync's "copy + fix attributes". Requires `--incremental` (implied); incompatible with `-s`. Wire: protocol 2.8.0 |
-| `--link-dest=DIR` | Hardlink to files when unchanged | ❌ Not Implemented | Removed because it had no effect |
+| `--link-dest=DIR` | Hardlink to files when unchanged | ✅ Implemented | Same basis rules as `--copy-dest`, but an exact match installs an atomic **hard link** to the DIR file (temp hard link + rename) so no data or disk space is used; where the link is impossible (basis on another filesystem, filesystem refuses links) it falls back cleanly to a byte-identical local copy, never a corrupt/partial file. `--delay-updates` stages the link and publishes by rename, so the final entry stays a real hard link. Repeatable (searched in command-line order, first match wins). Content is xxHash64-verified before linking. Divergences: an already up-to-date destination file is not re-linked to a basis file (only files that would otherwise be written are linked); attributes on a link are the basis inode's own (metadata is never written through the shared inode, which would mutate the basis file); basis dirs are excluded from `--delete`; a `--remove-source-files` source matched from a basis dir is retained (treated as skipped). Requires `--incremental` (implied); incompatible with `-s`. Wire: protocol 2.8.0 |
 | `--fuzzy`, `--no-fuzzy` | Find similar file for basis | ❌ Not Implemented | |
 
 ## 12. Compression

src/client/client_cli.c

@@ -114,6 +114,27 @@ static int set_nonneg_int_option(int* dest, const char* value, const char* optio
   return 0;
 }
 
+/* Validate and append one --compare-dest/--copy-dest/--link-dest directory.
+ * The path is interpreted on the receiver relative to the destination root,
+ * so it must be a non-empty relative path with no "." / ".." components (an
+ * absolute or escaping path is rejected up front instead of failing on the
+ * server). Returns 0 on success, -1 on error. */
+static int set_basis_dest_option(Config* config, BasisDestType type, const char* value,
+                                 const char* option_name) {
+  if (!value || !value[0]) {
+    log_message(LOG_LEVEL_ERROR, "missing argument for %s", option_name);
+    return -1;
+  }
+  if (config_basis_append(config, type, value) != 0) {
+    log_message(LOG_LEVEL_ERROR,
+                "%s requires a non-empty relative directory name with no '.', '..', or absolute "
+                "path (resolved below the destination root)",
+                option_name);
+    return -1;
+  }
+  return 0;
+}
+
 static int set_stderr_mode(const char* value) {
   if (strcmp(value, "errors") == 0 || strcmp(value, "e") == 0)
     log_set_stderr_mode(LOG_STDERR_ERRORS);
@@ -974,6 +995,36 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
       }
       log_message(LOG_LEVEL_ERROR, "%s is not supported yet (xxHash64 is used)", argv[i]);
       return -1;
+    } else if (strncmp(argv[i], "--compare-dest=", 15) == 0) {
+      if (set_basis_dest_option(config, BASIS_DEST_COMPARE, argv[i] + 15, "--compare-dest") != 0)
+        return -1;
+    } else if (opt_is(argv[i], "--compare-dest", NULL)) {
+      if (i + 1 >= argc) {
+        log_message(LOG_LEVEL_ERROR, "missing argument for %s", argv[i]);
+        return -1;
+      }
+      if (set_basis_dest_option(config, BASIS_DEST_COMPARE, argv[++i], "--compare-dest") != 0)
+        return -1;
+    } else if (strncmp(argv[i], "--copy-dest=", 12) == 0) {
+      if (set_basis_dest_option(config, BASIS_DEST_COPY, argv[i] + 12, "--copy-dest") != 0)
+        return -1;
+    } else if (opt_is(argv[i], "--copy-dest", NULL)) {
+      if (i + 1 >= argc) {
+        log_message(LOG_LEVEL_ERROR, "missing argument for %s", argv[i]);
+        return -1;
+      }
+      if (set_basis_dest_option(config, BASIS_DEST_COPY, argv[++i], "--copy-dest") != 0)
+        return -1;
+    } else if (strncmp(argv[i], "--link-dest=", 12) == 0) {
+      if (set_basis_dest_option(config, BASIS_DEST_LINK, argv[i] + 12, "--link-dest") != 0)
+        return -1;
+    } else if (opt_is(argv[i], "--link-dest", NULL)) {
+      if (i + 1 >= argc) {
+        log_message(LOG_LEVEL_ERROR, "missing argument for %s", argv[i]);
+        return -1;
+      }
+      if (set_basis_dest_option(config, BASIS_DEST_LINK, argv[++i], "--link-dest") != 0)
+        return -1;
     } else if (argv[i][0] == '-') {
       char* escaped = output_escape(argv[i], false);
       fprintf(stderr, "Unknown option: %s\n", escaped ? escaped : "<allocation failed>");
@@ -1010,6 +1061,13 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
     config->files_from_set = set;
   }
 
+  /* The "unchanged" decision for --compare-dest/--copy-dest/--link-dest must
+   * be made on the receiver against the basis directories, which requires the
+   * per-file STATUS_CHECK handshake: basis-dir options therefore imply
+   * --incremental (and, via the block below, metadata) on the sender. */
+  if (config_has_basis(config))
+    config->use_incremental = true;
+
   /* Incremental and delta transfers need metadata unless the user disabled it. */
   if ((config->use_incremental || config->use_delta) && !config->use_metadata &&
       !config->metadata_explicitly_disabled) {

src/client/client_send.c

@@ -613,7 +613,10 @@ static int incremental_check(Client* client, File* file, const Config* config,
     return -1;
   if (!send_n_data(client->file_descriptor, &mtime_nsec, sizeof(mtime_nsec)))
     return -1;
-  if (config->checksum) {
+  /* With alternate basis directories the receiver must be able to verify the
+   * content of every candidate basis file, so the sender supplies its xxHash64
+   * for every file even when --checksum was not requested. */
+  if (config->checksum || config_has_basis(config)) {
     uint64_t checksum;
     if (!file_checksum(file, &checksum) ||
         !send_n_data(client->file_descriptor, &checksum, sizeof(checksum)))

src/client/client_validation.c

@@ -11,6 +11,12 @@ bool validate_config(const Config* config) {
     print_usage();
     return false;
   }
+  if (config_has_basis(config) && config->use_chunk_serialization) {
+    log_message(LOG_LEVEL_ERROR,
+                "--compare-dest/--copy-dest/--link-dest require per-file incremental checks and "
+                "cannot be combined with -s (chunk serialization)");
+    return false;
+  }
   if (config->use_sendfile && (config->use_chunk_serialization || config->use_compression)) {
     log_message(LOG_LEVEL_ERROR, "-f/--sendfile cannot be combined with -c (compression) or -s "
                                  "(chunk serialization)");

src/client/usage.c

@@ -58,6 +58,13 @@ void print_usage(void) {
   printf("  -@, --modify-window <sec>  Modification time tolerance\n");
   printf("  -u, --update        Skip files newer than the source on receiver\n");
   printf("  --existing          Skip files not already present at destination\n");
+  printf("  --compare-dest <dir>  Treat DIR (relative to destination root) as an extra\n");
+  printf("                    comparison basis: unchanged files are not transferred\n");
+  printf("                    (requires --incremental, which is implied)\n");
+  printf("  --copy-dest <dir>   Like --compare-dest, but copies the unchanged file from DIR\n");
+  printf("                    into the destination instead of transferring its data\n");
+  printf("  --link-dest <dir>   Like --copy-dest, but hard-links the unchanged file from DIR\n");
+  printf("                    into the destination (repeatable; earlier DIRs win)\n");
   printf("  --checksum-choice, --cc <alg>  Checksum algorithm (not supported yet; xxHash64 is "
          "used)\n");
   printf("  --delta             Delta transfer for changed files (requires --incremental)\n");

src/server/receiver.c

@@ -214,7 +214,7 @@ static bool receiver_save_file(File* file, void* context_pointer) {
     result = file_save_to_disk_full(context->config->receive_root_directory, file, context->config);
   }
   if (result != FILE_SAVE_ERROR && context->config->remove_source_files && !file->is_dir &&
-      !receiver_outcomes_append(&context->outcomes, (unsigned char)result)) {
+      !file->skip && !receiver_outcomes_append(&context->outcomes, (unsigned char)result)) {
     file_destroy(file);
     return false;
   }

src/shared/config.c

@@ -109,9 +109,8 @@ static void config_set_defaults(Config* config) {
   config->rsync_path = NULL;
   config->old_args = false;
   config->temp_dir = NULL;
-  config->compare_dest = NULL;
+  config->basis_dirs = NULL;
-  config->copy_dest = NULL;
+  config->basis_count = 0;
-  config->link_dest = NULL;
   config->partial_dir = NULL;
   config->suffix = NULL;
   config->delete_before = false;
@@ -188,6 +187,63 @@ Config* config_create(void) {
   return config;
 }
 
+bool config_has_basis(const Config* config) {
+  return config && config->basis_count > 0;
+}
+
+/* A basis-dir path travels from the client to the receiver and is resolved
+ * below the destination root, so it must be a non-empty relative path with no
+ * "." or ".." component and no traversal: an absolute or escaping path would
+ * make the receiver read or link files outside its authorized root. */
+bool config_basis_path_valid(const char* path) {
+  if (!path || path[0] == '\0' || path[0] == '/' || has_path_traversal(path))
+    return false;
+  if (strcmp(path, ".") == 0)
+    return false;
+  char* dup = str_dup(path);
+  if (!dup)
+    return false;
+  bool ok = true;
+  char* saveptr = NULL;
+  for (char* part = strtok_r(dup, "/", &saveptr); part; part = strtok_r(NULL, "/", &saveptr)) {
+    if (strcmp(part, ".") == 0) {
+      ok = false;
+      break;
+    }
+    if (strcmp(part, "..") == 0) {
+      ok = false;
+      break;
+    }
+  }
+  free(dup);
+  return ok;
+}
+
+int config_basis_append(Config* config, BasisDestType type, const char* path) {
+  if (!config || !config_basis_path_valid(path) ||
+      (type != BASIS_DEST_COMPARE && type != BASIS_DEST_COPY && type != BASIS_DEST_LINK) ||
+      config->basis_count >= MAX_BASIS_DIRS)
+    return -1;
+  BasisDest* grown = realloc(config->basis_dirs, (config->basis_count + 1) * sizeof(BasisDest));
+  if (!grown)
+    return -1;
+  config->basis_dirs = grown;
+  /* Normalize a user-supplied trailing slash away so the stored path matches
+     the delete-walker prefix form exactly. */
+  size_t len = strlen(path);
+  while (len > 1 && path[len - 1] == '/')
+    len--;
+  char* dup = malloc(len + 1);
+  if (!dup)
+    return -1;
+  memcpy(dup, path, len);
+  dup[len] = '\0';
+  config->basis_dirs[config->basis_count].type = type;
+  config->basis_dirs[config->basis_count].path = dup;
+  config->basis_count++;
+  return 0;
+}
+
 bool config_is_remote_dest(const char* s) {
   if (s == NULL)
     return false;
@@ -244,9 +300,13 @@ void config_delete(Config* config) {
   free(config->rsh_command);
   free(config->rsync_path);
   free(config->temp_dir);
-  free(config->compare_dest);
+  for (int i = 0; i < config->basis_count; i++) {
-  free(config->copy_dest);
+    free(config->basis_dirs[i].path);
-  free(config->link_dest);
+    config->basis_dirs[i].path = NULL;
+  }
+  free(config->basis_dirs);
+  config->basis_dirs = NULL;
+  config->basis_count = 0;
   free(config->partial_dir);
   free(config->suffix);
   free(config->address);
@@ -333,6 +393,17 @@ static bool send_resume_options(int fd, const Config* c) {
          send_str(fd, c->chmod_spec ? c->chmod_spec : "") && send_skip_compress_options(fd, c);
 }
 
+static bool send_basis_options(int fd, const Config* c) {
+  if (!send_int(fd, c->basis_count))
+    return false;
+  for (int i = 0; i < c->basis_count; i++) {
+    if (!send_int(fd, (int)c->basis_dirs[i].type) ||
+        !send_str(fd, c->basis_dirs[i].path ? c->basis_dirs[i].path : ""))
+      return false;
+  }
+  return true;
+}
+
 static bool receive_core_fields(int fd, Config* c) {
   int value;
   if (!receive_wire_bool(fd, &c->eight_bit_output))
@@ -479,12 +550,35 @@ static bool receive_resume_options(int fd, Config* c) {
   return true;
 }
 
+static bool receive_basis_options(int fd, Config* c) {
+  int count;
+  if (!receive_int(fd, &count))
+    return false;
+  if (count < 0 || count > MAX_BASIS_DIRS)
+    return false;
+  for (int i = 0; i < count; i++) {
+    int type;
+    if (!receive_int(fd, &type) || type <= BASIS_DEST_NONE || type > BASIS_DEST_LINK)
+      return false;
+    char* path = receive_str(fd);
+    if (!path)
+      return false;
+    bool ok = config_basis_path_valid(path);
+    if (ok)
+      ok = config_basis_append(c, (BasisDestType)type, path) == 0;
+    free(path);
+    if (!ok)
+      return false;
+  }
+  return true;
+}
+
 bool config_send(int file_descriptor, const Config* config) {
   protocol_session_set_max_alloc(NULL, config->max_alloc);
   if (!send_core_fields(file_descriptor, config) || !send_delta_fields(file_descriptor, config) ||
       !send_file_options(file_descriptor, config) ||
       !send_selection_options(file_descriptor, config) ||
-      !send_resume_options(file_descriptor, config))
+      !send_resume_options(file_descriptor, config) || !send_basis_options(file_descriptor, config))
     return false;
   Status status;
   if (!receive_status(file_descriptor, &status))
@@ -516,7 +610,8 @@ Config* config_receive(int file_descriptor) {
       !receive_delta_fields(file_descriptor, config) ||
       !receive_file_options(file_descriptor, config) ||
       !receive_selection_options(file_descriptor, config) ||
-      !receive_resume_options(file_descriptor, config))
+      !receive_resume_options(file_descriptor, config) ||
+      !receive_basis_options(file_descriptor, config))
     goto error;
   if (config->compress_choice[0] != '\0' && strcmp(config->compress_choice, "zstd") != 0 &&
       strcmp(config->compress_choice, "none") != 0) {

src/shared/config.h

@@ -12,6 +12,22 @@ typedef enum { TRANSPORT_TCP, TRANSPORT_SSH } TransportType;
    Config can carry it; the concrete type lives in delay_updates.h. */
 typedef struct DelayUpdatesContext DelayUpdatesContext;
 
+/* Alternate basis-directory modes (--compare-dest / --copy-dest /
+ * --link-dest).  Each flag adds one entry to the ordered Config->basis_dirs
+ * list; the receiver consults entries in command-line order and stops at the
+ * first exact match, mirroring rsync's basis-dir priority rules. */
+typedef enum {
+  BASIS_DEST_NONE = 0,
+  BASIS_DEST_COMPARE, /* compare only: never copies, never materializes */
+  BASIS_DEST_COPY,    /* local copy of the matched basis file */
+  BASIS_DEST_LINK     /* hard link to the matched basis file */
+} BasisDestType;
+
+typedef struct BasisDest {
+  BasisDestType type;
+  char* path; /* relative to the destination root (receiver-confined) */
+} BasisDest;
+
 typedef struct Config {
   char* version;
   char* send_directory;
@@ -134,9 +150,12 @@ typedef struct Config {
   char* rsync_path;
   bool old_args;
   char* temp_dir;
-  char* compare_dest;
+  /* Alternate basis directories, ordered by command-line appearance.  Each
-  char* copy_dest;
+   * entry's type selects compare/copy/link behavior on an exact match.  These
-  char* link_dest;
+   * cross the wire so the receiver can consult them; they are interpreted
+   * relative to the destination root and confined there. */
+  BasisDest* basis_dirs;
+  int basis_count;
 
   // PR #174: Partial transfer resumption
   char* partial_dir;
@@ -174,8 +193,10 @@ typedef struct Config {
   DelayUpdatesContext* delay_context;
 } Config;
 
-#define PROTOCOL_VERSION "2.7.0"
+#define PROTOCOL_VERSION "2.8.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
+/* Upper bound on total basis-dir entries (rsync caps --link-dest at 20). */
+#define MAX_BASIS_DIRS 64
 
 Config* config_create(void);
 void config_delete(Config* config);
@@ -184,4 +205,11 @@ Config* config_receive(int file_descriptor);
 bool config_is_remote_dest(const char* s);
 void config_parse_ssh_dest(Config* config);
 
+/* True when at least one --compare-dest/--copy-dest/--link-dest was set. */
+bool config_has_basis(const Config* config);
+/* Append one basis-dir entry. Returns 0 on success, -1 on allocation failure. */
+int config_basis_append(Config* config, BasisDestType type, const char* path);
+/* Validate a client-provided basis-dir path (relative, confined, non-empty). */
+bool config_basis_path_valid(const char* path);
+
 #endif

src/shared/file.c

@@ -83,6 +83,7 @@ File* file_create(const char* path) {
   file->metadata = NULL;
   file->skip = false;
   file->is_dir = false;
+  file->basis_link = NULL;
   return file;
 }
 
@@ -98,6 +99,8 @@ void file_destroy(void* item) {
   file->path = NULL;
   free(file->send_path);
   file->send_path = NULL;
+  free(file->basis_link);
+  file->basis_link = NULL;
   free(file);
 }
 
@@ -630,6 +633,106 @@ bool file_to_disk_secure_no_replace(const char* path, const void* data,
                                   preserve_executability, false, true, false, temp_dir);
 }
 
+/* Atomic --link-dest install.  The destination is replaced (via a temporary
+ * name and a final rename) with a hard link to `basis_path`.  When a hard
+ * link cannot be created (the basis lives on a different filesystem, the
+ * filesystem refuses hard links, ...) the install falls back to writing a
+ * local copy from `data`/`data_size`, which the caller has already verified is
+ * byte-identical to the basis file.  `metadata` is only applied on that copy
+ * fallback; a successful hard link keeps the basis inode's own attributes
+ * (applying metadata through the shared inode would mutate the basis file).
+ * Returns false only when both the link and the copy fallback fail. */
+bool file_to_disk_secure_link(const char* path, const char* basis_path, const void* data,
+                              unsigned long long data_size, const FileMetadata* metadata,
+                              bool preserve_executability, bool use_fsync, const char* temp_dir) {
+  if (!path || !basis_path)
+    return false;
+  char* leaf = NULL;
+  int dirfd = file_open_secure_parent(path, &leaf, true);
+  if (dirfd < 0)
+    return false;
+
+  int scratch_dirfd = -1;
+  if (temp_dir) {
+    scratch_dirfd = file_open_private_dir(temp_dir);
+    if (scratch_dirfd < 0) {
+      int saved_errno = errno;
+      log_message(LOG_LEVEL_ERROR, "could not open --temp-dir scratch directory '%s': %s", temp_dir,
+                  strerror(saved_errno));
+      close(dirfd);
+      free(leaf);
+      return false;
+    }
+  }
+
+  char* basis_leaf = NULL;
+  int basis_dirfd = file_open_secure_parent(basis_path, &basis_leaf, false);
+  bool linked = false;
+  if (basis_dirfd >= 0 && basis_leaf != NULL) {
+    int tmp_size = snprintf(NULL, 0, ".%s.tmp.%ld.%llu", leaf, (long)getpid(), ~0ULL);
+    char* tmp = NULL;
+    if (tmp_size >= 0)
+      tmp = malloc((size_t)tmp_size + 1);
+    if (!tmp) {
+      log_message(LOG_LEVEL_ERROR, "memory allocation failed while hard-linking basis file");
+    } else {
+      for (unsigned int i = 0; i < 100 && !linked; ++i) {
+        if (scratch_dirfd >= 0)
+          snprintf(tmp, (size_t)tmp_size + 1, ".%s.tmp.%ld.%llu", leaf, (long)getpid(),
+                   next_temp_sequence());
+        else
+          snprintf(tmp, (size_t)tmp_size + 1, ".%s.tmp.%ld.%u", leaf, (long)getpid(), i);
+        if (linkat(basis_dirfd, basis_leaf, scratch_dirfd >= 0 ? scratch_dirfd : dirfd, tmp, 0) ==
+            0) {
+          linked = true;
+          break;
+        }
+        if (errno != EEXIST)
+          break; /* EXDEV / EPERM / ...: give up and fall back to a copy */
+      }
+      if (linked) {
+        int target_dirfd = scratch_dirfd >= 0 ? scratch_dirfd : dirfd;
+        if (use_fsync) {
+          int tfd = openat(target_dirfd, tmp, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
+          if (tfd < 0 || fsync(tfd) != 0) {
+            linked = false;
+            if (tfd >= 0)
+              close(tfd);
+          } else {
+            close(tfd);
+          }
+        }
+        if (linked && renameat(target_dirfd, tmp, dirfd, leaf) != 0)
+          linked = false;
+        if (!linked)
+          unlinkat(target_dirfd, tmp, 0);
+      }
+      free(tmp);
+    }
+  }
+  if (basis_dirfd >= 0)
+    close(basis_dirfd);
+  free(basis_leaf);
+  basis_leaf = NULL;
+
+  if (!linked) {
+    if (scratch_dirfd >= 0)
+      close(scratch_dirfd);
+    close(dirfd);
+    free(leaf);
+    /* The basis file could not be linked in (missing, cross-device, refused
+       by the filesystem).  Write a byte-identical local copy instead. */
+    return file_to_disk_secure_with_fsync(path, data, data_size, false, false, metadata,
+                                          preserve_executability, use_fsync, temp_dir);
+  }
+
+  if (scratch_dirfd >= 0)
+    close(scratch_dirfd);
+  close(dirfd);
+  free(leaf);
+  return true;
+}
+
 bool file_write_to_disk(const char* path, const void* data, unsigned long long data_size,
                         bool inplace, bool sparse) {
   if (!path || (!data && data_size != 0) || has_path_traversal(path))

src/shared/file.h

@@ -63,5 +63,12 @@ bool file_to_disk_secure_no_replace(const char* path, const void* data,
                                     unsigned long long data_size, bool sparse,
                                     const FileMetadata* metadata, bool preserve_executability,
                                     const char* temp_dir);
+/* Atomic --link-dest install: replace `path` with a hard link to `basis_path`
+   (via a temp name + rename); fall back to a byte-identical local copy from
+   `data` when the link is impossible (EXDEV/EPERM/unsupported filesystem).
+   `metadata` is applied only on the copy fallback. */
+bool file_to_disk_secure_link(const char* path, const char* basis_path, const void* data,
+                              unsigned long long data_size, const FileMetadata* metadata,
+                              bool preserve_executability, bool use_fsync, const char* temp_dir);
 
 #endif

src/shared/file_receive.c

@@ -75,10 +75,17 @@ static FileSaveResult file_stage_delayed_update(const char* root_directory,
   /* The staged location is brand new (stale leftovers from a prior crash were
      wiped by prepare), so the plain atomic temp+rename engine installs the
      complete file there.  --temp-dir scratch is deliberately not layered on
-     top of the delay-updates staging tree. */
+     top of the delay-updates staging tree.  A --link-dest basis file is hard
-  bool ok =
+     linked into the staging tree (so publication's rename keeps the link). */
-      file_to_disk_secure_with_fsync(staged_path, file->data->data, file->data->size, false, sparse,
+  bool ok;
+  if (file->basis_link) {
+    ok = file_to_disk_secure_link(staged_path, file->basis_link, file->data->data, file->data->size,
                                   metadata, preserve_executability, config->use_fsync, NULL);
+  } else {
+    ok = file_to_disk_secure_with_fsync(staged_path, file->data->data, file->data->size, false,
+                                        sparse, metadata, preserve_executability, config->use_fsync,
+                                        NULL);
+  }
   if (!ok) {
     free(staged_path);
     return FILE_SAVE_ERROR;
@@ -272,16 +279,27 @@ FileSaveResult file_save_to_disk_full(const char* root_directory, const File* fi
     while (temp_len > 1 && confined_temp[temp_len - 1] == '/')
       confined_temp[--temp_len] = '\0';
   }
-  bool ok =
+  /* A --link-dest basis hit installs an atomic hard link (with a byte-copy
-      config && config->ignore_existing
+     fallback); --inplace and the update/no-replace write variants do not
+     apply to a fresh hard link, whose inode attributes already match.  The
+     existing/ignore-existing/update/backup preamble above has already made the
+     policy decision. */
+  bool ok;
+  if (config && file->basis_link) {
+    ok = file_to_disk_secure_link(disk_path, file->basis_link, file->data->data, file->data->size,
+                                  metadata, preserve_executability, config->use_fsync,
+                                  confined_temp);
+  } else {
+    ok = config && config->ignore_existing
              ? file_to_disk_secure_no_replace(disk_path, file->data->data, file->data->size, sparse,
                                               metadata, preserve_executability, confined_temp)
          : config && config->update
              ? file_to_disk_secure_update(disk_path, file->data->data, file->data->size, inplace,
                                           sparse, metadata, preserve_executability, confined_temp)
-          : file_to_disk_secure_with_fsync(disk_path, file->data->data, file->data->size, inplace,
+             : file_to_disk_secure_with_fsync(disk_path, file->data->data, file->data->size,
-                                           sparse, metadata, preserve_executability,
+                                              inplace, sparse, metadata, preserve_executability,
                                               config && config->use_fsync, confined_temp);
+  }
   free(confined_temp);
   confined_temp = NULL;
   if (!ok)
@@ -505,6 +523,143 @@ static File* receive_delta_file(int fd, const Config* config, const char* check_
   return NULL;
 }
 
+/* ---- Alternate basis directories (--compare-dest / --copy-dest / --link-dest) ----
+ * The receiver consults the ordered basis-dir list only when the destination
+ * entry is NOT already up to date.  An "exact match" requires an equal size,
+ * an equal mtime (unless --size-only), and an equal content xxHash64, so a
+ * hard link / local copy is only ever made from byte-identical content. */
+
+typedef struct BasisMatch {
+  bool hit;
+  BasisDestType type;
+  char* basis_path; /* owned absolute path of the matched basis file */
+  struct stat st;   /* fstat() of the matched basis file */
+  Data* content;    /* owned basis bytes (or empty Data), NULL when not loaded */
+} BasisMatch;
+
+static void basis_match_free(BasisMatch* match) {
+  if (!match)
+    return;
+  free(match->basis_path);
+  match->basis_path = NULL;
+  data_destroy(match->content);
+  match->content = NULL;
+  match->hit = false;
+  match->type = BASIS_DEST_NONE;
+}
+
+/* Open `path` (via the secure, root-confined primitives) and require it to be
+   a regular file of exactly `expected_size` bytes.  Returns an open read-only
+   descriptor and its fstat on success. */
+static bool basis_open_regular(const char* path, unsigned long long expected_size, int* out_fd,
+                               struct stat* out_st) {
+  char* leaf = NULL;
+  int parent_fd = file_open_secure_parent(path, &leaf, false);
+  if (parent_fd < 0)
+    return false;
+  int fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
+  free(leaf);
+  close(parent_fd);
+  if (fd < 0)
+    return false;
+  struct stat st;
+  if (fstat(fd, &st) != 0 || !S_ISREG(st.st_mode) ||
+      (unsigned long long)st.st_size != expected_size) {
+    close(fd);
+    return false;
+  }
+  *out_fd = fd;
+  *out_st = st;
+  return true;
+}
+
+/* Read the whole remaining content of an open descriptor.  A zero-length file
+   yields an empty Data (data pointer NULL). */
+static Data* basis_read_content(int fd, unsigned long long size) {
+  if (size == 0)
+    return data_create_reserve(0);
+  if (size > MAX_RECEIVE_WHOLE_FILE_SIZE || size > SIZE_MAX)
+    return NULL;
+  void* buf = protocol_alloc((size_t)size);
+  if (!buf)
+    return NULL;
+  size_t got = 0;
+  while (got < (size_t)size) {
+    ssize_t n = read(fd, (char*)buf + got, (size_t)size - got);
+    if (n <= 0) {
+      free(buf);
+      return NULL;
+    }
+    got += (size_t)n;
+  }
+  return data_create(buf, (size_t)size);
+}
+
+/* --ignore-times forces every file to be updated, so no basis hit is ever
+   declared (matching rsync, where -I prevents link-dest from linking). */
+static bool basis_quick_matches(const Config* config, const struct stat* st, time_t check_mtime,
+                                long check_mtime_nsec) {
+  if (config->size_only)
+    return true;
+  long mtime_nsec = 0;
+#ifdef __linux__
+  mtime_nsec = st->st_mtim.tv_nsec;
+#endif
+  return metadata_mtime_matches(st->st_mtime, mtime_nsec, check_mtime, check_mtime_nsec,
+                                config->modify_window);
+}
+
+/* Search the basis-dir list in command-line order and return the first exact
+   match.  When load_content is true the matched bytes are kept in out->content
+   so the caller can materialize the file without re-reading it. */
+static bool basis_match_find(const Config* config, const char* check_path,
+                             unsigned long long check_size, time_t check_mtime,
+                             long check_mtime_nsec, uint64_t check_checksum, bool load_content,
+                             BasisMatch* out) {
+  memset(out, 0, sizeof(*out));
+  if (!config || !config_has_basis(config) || config->ignore_times)
+    return false;
+  for (int i = 0; i < config->basis_count; i++) {
+    const BasisDest* entry = &config->basis_dirs[i];
+    char* basis_dir = path_cat(config->receive_root_directory, entry->path);
+    if (!basis_dir)
+      continue;
+    char* candidate = path_cat(basis_dir, check_path);
+    free(basis_dir);
+    if (!candidate)
+      continue;
+
+    int fd;
+    struct stat st;
+    if (basis_open_regular(candidate, check_size, &fd, &st)) {
+      if (basis_quick_matches(config, &st, check_mtime, check_mtime_nsec)) {
+        Data* content = basis_read_content(fd, check_size);
+        if (content) {
+          uint64_t basis_hash = check_size == 0 ? delta_xxhash64("", 0)
+                                : content->data ? delta_xxhash64(content->data, content->size)
+                                                : 0;
+          if (basis_hash == check_checksum) {
+            out->hit = true;
+            out->type = entry->type;
+            out->basis_path = candidate;
+            candidate = NULL; /* ownership transferred to out */
+            out->st = st;
+            out->content = load_content ? content : NULL;
+            if (!load_content)
+              data_destroy(content);
+            close(fd);
+            return true;
+          }
+        }
+        data_destroy(content);
+      }
+      close(fd);
+    }
+    free(candidate);
+  }
+  return false;
+}
+
 File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   if (!config || !skipped) {
     send_status(fd, STATUS_ERROR);
@@ -531,7 +686,8 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
     send_status(fd, STATUS_ERROR);
     return NULL;
   }
-  if (config->checksum && !receive_n_data(fd, &check_checksum, sizeof(check_checksum))) {
+  if ((config->checksum || config_has_basis(config)) &&
+      !receive_n_data(fd, &check_checksum, sizeof(check_checksum))) {
     free(check_path);
     return NULL;
   }
@@ -642,6 +798,76 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
     return NULL;
   }
 
+  /* ---- Alternate basis directories ---- */
+  if (config_has_basis(config)) {
+    BasisMatch basis;
+    basis_match_find(config, check_path, check_size, (time_t)check_mtime, (long)check_mtime_nsec,
+                     check_checksum, true, &basis);
+    if (basis.hit) {
+      if (basis.type == BASIS_DEST_COMPARE) {
+        /* compare-dest never copies: an exact match only suppresses the data
+           for a file the destination does not already hold (sparse backup).
+           When the destination holds a DIFFERENT version FastSync falls back to
+           a normal transfer rather than deleting the stale entry the way rsync
+           does (see RSYNC_COMPAT.md). */
+        basis_match_free(&basis);
+        if (!has_old_file) {
+          if (!send_status(fd, STATUS_OK)) {
+            close(old_fd);
+            free(full_path);
+            free(check_path);
+            return NULL;
+          }
+          free(old_data);
+          close(old_fd);
+          free(full_path);
+          free(check_path);
+          *skipped = true;
+          return NULL;
+        }
+      } else {
+        /* copy-dest / link-dest: materialize the unchanged file locally so the
+           sender can skip the data.  The store engine re-applies the normal
+           existing/ignore-existing/update/backup/delay-updates policy. */
+        File* materialized = file_create(check_path);
+        if (materialized && basis.content) {
+          materialized->data = basis.content;
+          basis.content = NULL;
+          materialized->metadata = file_metadata_create(&basis.st);
+          materialized->skip = true; /* receiver must not ack this as a data file */
+          if (basis.type == BASIS_DEST_LINK) {
+            materialized->basis_link = basis.basis_path;
+            basis.basis_path = NULL;
+          }
+          if (!materialized->metadata) {
+            file_destroy(materialized);
+            materialized = NULL;
+          }
+        } else {
+          file_destroy(materialized);
+          materialized = NULL;
+        }
+        if (materialized) {
+          if (!send_status(fd, STATUS_OK)) {
+            file_destroy(materialized);
+            close(old_fd);
+            free(full_path);
+            free(check_path);
+            return NULL;
+          }
+          free(old_data);
+          close(old_fd);
+          free(full_path);
+          free(check_path);
+          *skipped = false;
+          return materialized;
+        }
+        /* Materialization setup failed: fall through to the normal transfer. */
+      }
+    }
+    basis_match_free(&basis);
+  }
+
   if (try_delta && old_data != NULL) {
     bool delta_failed = false;
     File* delta_file =
@@ -843,10 +1069,29 @@ int receive_manifest(int fd, const Config* config, int* next_status) {
   /* With --delay-updates the staged (not yet published) files live directly
      under the receive root in the staging directory; the delete walker must
      not treat them as extras or it would remove every staged file before it
-     can be published. */
+     can be published.  Alternate basis directories (--compare-dest /
-  const char* skip_staging = config->delay_updates ? DELAY_UPDATES_STAGING_DIR : NULL;
+     --copy-dest / --link-dest) are also excluded: they are extra comparison
-  bool deletion_ok = delete_extras_limited(config->receive_root_directory, manifest,
+     snapshots the user pointed at, not destination content, and deleting them
-                                           MAX_SERVER_DELETE_COUNT, skip_staging);
+     would destroy the very files a --link-dest run just linked into place. */
+  int skip_count = (config->delay_updates ? 1 : 0) + config->basis_count;
+  const char** skip_prefixes = NULL;
+  bool deletion_ok = false;
+  if (skip_count > 0) {
+    skip_prefixes = calloc((size_t)skip_count, sizeof(char*));
+    if (!skip_prefixes) {
+      array_list_delete(manifest);
+      send_status(fd, STATUS_ERROR);
+      return -1;
+    }
+    int idx = 0;
+    if (config->delay_updates)
+      skip_prefixes[idx++] = DELAY_UPDATES_STAGING_DIR;
+    for (int i = 0; i < config->basis_count; i++)
+      skip_prefixes[idx++] = config->basis_dirs[i].path;
+  }
+  deletion_ok = delete_extras_limited(config->receive_root_directory, manifest,
+                                      MAX_SERVER_DELETE_COUNT, skip_prefixes, skip_count);
+  free(skip_prefixes);
   array_list_delete(manifest);
   if (!deletion_ok)
     send_status(fd, STATUS_ERROR);

src/shared/file_types.h

@@ -29,6 +29,12 @@ typedef struct {
   /* True when this entry is an explicit directory entry (--dirs mode): the
    * receiver creates the directory instead of writing a regular file. */
   bool is_dir;
+  /* Receiver-only, --link-dest: when set, install the destination entry as a
+   * hard link to this absolute (root-confined) path instead of writing
+   * `data`.  The matching code has already verified the link target's content
+   * equals the incoming file, and `data` is kept as the cross-filesystem
+   * fallback (a local copy) if the hard link cannot be created. */
+  char* basis_link;
 } File;
 
 /* The path that should be sent on the wire and used for the receiver-side

src/shared/multiprocessing.c

@@ -297,7 +297,7 @@ int write_thread(void* pipeline_context) {
     /* Record the per-file outcome so a --remove-source-files sender learns
        which sources were actually written versus skipped on the receiver.
        Explicit directory entries have no source and are never acknowledged. */
-    if (context->config->remove_source_files && !file->is_dir &&
+    if (context->config->remove_source_files && !file->is_dir && !file->skip &&
         !receiver_outcomes_append(&context->outcomes, (unsigned char)result)) {
       file_destroy(file);
       pipeline_context_receiver_note_bytes_released(context, file_bytes);

src/shared/utils.c

@@ -204,9 +204,22 @@ static bool is_dir_in_manifest(const char* rel_path, ArrayList* manifest) {
   return false;
 }
 
+/* True when the relative path is, or lies below, one of the protected
+   prefixes.  A prefix "a" therefore protects "a" and "a/b/c" but not "ab". */
+static bool path_under_skip_prefix(const char* rel_path, const char* const* prefixes,
+                                   int prefix_count) {
+  for (int i = 0; i < prefix_count; i++) {
+    size_t prefix_len = strlen(prefixes[i]);
+    if (strncmp(rel_path, prefixes[i], prefix_len) == 0 &&
+        (rel_path[prefix_len] == '\0' || rel_path[prefix_len] == '/'))
+      return true;
+  }
+  return false;
+}
+
 static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifest,
                              size_t max_delete, size_t* deleted_count,
-                             const char* skip_root_child) {
+                             const char* const* skip_prefixes, int skip_prefix_count) {
   int scanfd = dup(dirfd);
   if (scanfd < 0)
     return false;
@@ -220,18 +233,19 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
   while ((entry = readdir(dir)) != NULL) {
     if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
       continue;
-    /* A --delay-updates run keeps its staging directory as a direct child of
-       the receive root.  Its contents are not manifest entries yet (they are
-       published after deletion), so descending into it would delete every
-       staged file as an "extra".  Skip only the top-level staging name; nested
-       directories with the same name are ordinary destination content. */
-    if (rel_path[0] == '\0' && skip_root_child && strcmp(entry->d_name, skip_root_child) == 0)
-      continue;
     char* child_rel = path_cat((char*)rel_path, entry->d_name);
     if (!child_rel) {
       operation_ok = false;
       continue;
     }
+    /* A --delay-updates run keeps its staging directory below the receive
+       root, and basis-dir snapshots live there too.  Their contents are not
+       manifest entries, so descending into them would delete every staged /
+       basis file as an "extra". */
+    if (path_under_skip_prefix(child_rel, skip_prefixes, skip_prefix_count)) {
+      free(child_rel);
+      continue;
+    }
     struct stat st;
     if (fstatat(dirfd, entry->d_name, &st, AT_SYMLINK_NOFOLLOW) != 0) {
       if (errno != ENOENT)
@@ -249,7 +263,7 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
       bool child_removed = false;
       if (childfd >= 0) {
         child_removed = delete_extras_fd(childfd, child_rel, manifest, max_delete, deleted_count,
-                                         skip_root_child);
+                                         skip_prefixes, skip_prefix_count);
         if (!child_removed)
           operation_ok = false;
         close(childfd);
@@ -301,7 +315,7 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
 }
 
 bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete,
-                           const char* skip_root_child) {
+                           const char* const* skip_prefixes, int skip_prefix_count) {
   if (!manifest)
     return false;
   int rootfd;
@@ -318,14 +332,15 @@ bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t ma
   if (rootfd < 0)
     return false;
   size_t deleted_count = 0;
-  bool ok = delete_extras_fd(rootfd, "", manifest, max_delete, &deleted_count, skip_root_child);
+  bool ok = delete_extras_fd(rootfd, "", manifest, max_delete, &deleted_count, skip_prefixes,
+                             skip_prefix_count);
   if (close(rootfd) != 0)
     ok = false;
   return ok;
 }
 
 bool delete_extras(const char* dest_root, ArrayList* manifest) {
-  return delete_extras_limited(dest_root, manifest, SIZE_MAX, NULL);
+  return delete_extras_limited(dest_root, manifest, SIZE_MAX, NULL, 0);
 }
 
 bool has_path_traversal(const char* path) {

src/shared/utils.h

@@ -10,12 +10,14 @@ char* output_escape(const char* string, bool eight_bit_output);
 char* path_cat(const char* path1, const char* path2);
 bool glob_match(const char* pattern, const char* str);
 bool delete_extras(const char* dest_root, ArrayList* manifest);
-/* Remove files/dirs under dest_root that are not listed in manifest.  When
+/* Remove files/dirs under dest_root that are not listed in manifest.  The
-   skip_root_child is non-NULL, a direct child of dest_root with that exact
+   delete walker never descends into (and so never removes) an entry whose
-   name is left untouched (used to protect the --delay-updates staging
+   relative path equals one of the skip_prefixes or lies below one: used to
-   directory, which holds files that are still to be published). */
+   protect the --delay-updates staging directory (files still to be published)
+   and the --compare-dest/--copy-dest/--link-dest basis trees (snapshots the
+   transfer links from, never destination content). */
 bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete,
-                           const char* skip_root_child);
+                           const char* const* skip_prefixes, int skip_prefix_count);
 bool utils_set_authorized_root(int fd, const char* canonical_path);
 /* The fd-only compatibility form is fail-closed for path-based operations;
  * callers should use utils_set_authorized_root with the canonical identity. */

tests/integration/test_features.py

@@ -1971,3 +1971,213 @@ class TestFilters:
     """--filter/-C/-F rule layer: excludes prune, ordering is first-match-wins,
     the default with no matching rule is include, and legacy --exclude remains
     an independent layer."""
+
+
+def _pin_mtime(path, ts):
+    os.utime(path, (ts, ts))
+
+
+class TestBasisDestDirs:
+    """--compare-dest / --copy-dest / --link-dest alternate basis directories.
+
+    FastSync's basis directories are relative to the destination root and are
+    confined below it.  The "unchanged" decision is receiver-side and requires
+    the per-file --incremental handshake (implied by these flags), so the basis
+    snapshot must reproduce the exact destination-relative mirror path of the
+    incoming files.
+    """
+
+    STAGING = ".fastsync-stage"
+    TS = 1577836800  # 2020-01-01 00:00:00 UTC, used to pin matching mtimes
+
+    # files: rel-path -> (source content, basis content or None, matched?)
+    UNCHANGED = "unchanged.txt"
+    CHANGED = "changed.txt"
+    ADDED = "added.txt"
+
+    def _make_source(self, name, source_files):
+        src = os.path.join(TEST_DATA_DIR, name)
+        clean_dir(src)
+        for rel, content in source_files.items():
+            full = os.path.join(src, rel)
+            os.makedirs(os.path.dirname(full), exist_ok=True)
+            with open(full, "wb") as fh:
+                fh.write(content)
+            _pin_mtime(full, self.TS)
+        return src
+
+    def _basis_root(self, dest, source):
+        received = get_dest_received_dir(dest, source)
+        rel = os.path.relpath(received, dest)
+        return os.path.join(dest, rel)
+
+    def _seed_basis(self, dest, source, basis_dir, basis_files):
+        base = os.path.join(dest, basis_dir, os.path.relpath(
+            get_dest_received_dir(dest, source), dest))
+        for rel, content in basis_files.items():
+            full = os.path.join(base, rel)
+            os.makedirs(os.path.dirname(full), exist_ok=True)
+            with open(full, "wb") as fh:
+                fh.write(content)
+            _pin_mtime(full, self.TS)
+        return base
+
+    def _source_tree(self, prefix):
+        return {
+            self.UNCHANGED: b"stable content v1\n",
+            self.CHANGED: b"changed content now\n",
+            self.ADDED: b"brand new content\n",
+        }
+
+    def _basis_tree(self, prefix):
+        # unchanged.txt matches the source; changed.txt differs in CONTENT but
+        # shares size/mtime pinning; added.txt is missing from the basis.
+        return {
+            self.UNCHANGED: b"stable content v1\n",
+            self.CHANGED: b"ANCIENT DIFFERENT CONTENT!\n",
+        }
+
+    def test_compare_dest_skips_matching_and_transfers_missing(self, shared_server):
+        source = self._make_source("basis_compare_src", self._source_tree("c"))
+        dest = os.path.join(TEST_DATA_DIR, "basis_compare_dst")
+        clean_dir(dest)
+        self._seed_basis(dest, source, "cbasis", self._basis_tree("c"))
+        result, _ = run_client(source, dest,
+                               flags=["--compare-dest=cbasis"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"compare-dest failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        # compare-dest never copies: an exact basis match is skipped, leaving a
+        # sparse destination (rsync parity).
+        assert not os.path.exists(os.path.join(received, self.UNCHANGED)), \
+            "compare-dest materialized the unchanged file"
+        # Files the destination lacks AND the basis cannot satisfy are still
+        # transferred normally.
+        assert _read_file(os.path.join(received, self.CHANGED)) == \
+            self._source_tree("c")[self.CHANGED], "changed file not transferred"
+        assert _read_file(os.path.join(received, self.ADDED)) == \
+            self._source_tree("c")[self.ADDED], "added file not transferred"
+
+    def test_compare_dest_content_mismatch_forces_transfer(self, shared_server):
+        # The basis holds a file with a DIFFERENT body: even though it shares
+        # the mtime pin, the xxHash check fails and the data must be sent.
+        source = self._make_source("basis_compare_mismatch_src", {self.UNCHANGED: b"real data\n"})
+        dest = os.path.join(TEST_DATA_DIR, "basis_compare_mismatch_dst")
+        clean_dir(dest)
+        basis = self._seed_basis(dest, source, "cbasis", {self.UNCHANGED: b"stale data!!\n"})
+        result, _ = run_client(source, dest, flags=["--compare-dest=cbasis"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"compare-dest mismatch failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        assert _read_file(os.path.join(received, self.UNCHANGED)) == b"real data\n", \
+            "content mismatch did not fall back to a normal transfer"
+        assert os.stat(os.path.join(received, self.UNCHANGED)).st_ino != \
+            os.stat(os.path.join(basis, self.UNCHANGED)).st_ino
+
+    def test_copy_dest_copies_unchanged_and_transfers_changed(self, shared_server):
+        source = self._make_source("basis_copy_src", self._source_tree("cp"))
+        dest = os.path.join(TEST_DATA_DIR, "basis_copy_dst")
+        clean_dir(dest)
+        basis = self._seed_basis(dest, source, "cpbasis", self._basis_tree("cp"))
+        result, _ = run_client(source, dest, flags=["--copy-dest=cpbasis"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"copy-dest failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        unchanged = os.path.join(received, self.UNCHANGED)
+        assert _read_file(unchanged) == b"stable content v1\n", "unchanged file not materialized"
+        # A real local copy, NOT a hard link to the basis file.
+        assert os.stat(unchanged).st_ino != os.stat(os.path.join(basis, self.UNCHANGED)).st_ino
+        # Changed content falls back to a normal transfer of the sender data.
+        assert _read_file(os.path.join(received, self.CHANGED)) == \
+            self._source_tree("cp")[self.CHANGED]
+        assert _read_file(os.path.join(received, self.ADDED)) == \
+            self._source_tree("cp")[self.ADDED]
+
+    def test_link_dest_hardlinks_and_falls_back(self, shared_server):
+        source = self._make_source("basis_link_src", self._source_tree("ln"))
+        dest = os.path.join(TEST_DATA_DIR, "basis_link_dst")
+        clean_dir(dest)
+        basis = self._seed_basis(dest, source, "lnbasis", self._basis_tree("ln"))
+        result, _ = run_client(source, dest, flags=["--link-dest=lnbasis"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"link-dest failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        unchanged = os.path.join(received, self.UNCHANGED)
+        basis_file = os.path.join(basis, self.UNCHANGED)
+        # Real hard link: same inode as the DIR file, nlink >= 2, no data copy.
+        assert os.path.exists(unchanged)
+        assert os.stat(unchanged).st_ino == os.stat(basis_file).st_ino, \
+            "link-dest did not produce a hard link"
+        assert os.stat(unchanged).st_nlink >= 2
+        # Content mismatch must fall back to a plain transfer (not a link).
+        changed = os.path.join(received, self.CHANGED)
+        assert _read_file(changed) == self._source_tree("ln")[self.CHANGED]
+        assert os.stat(changed).st_ino != os.stat(os.path.join(basis, self.CHANGED)).st_ino
+
+    @pytest.mark.parametrize("flag", ["--compare-dest", "--copy-dest", "--link-dest"])
+    def test_basis_dir_missing_is_a_clean_noop(self, shared_server, flag):
+        # A basis directory that does not exist must simply transfer everything.
+        source = self._make_source("basis_missing_src", {self.UNCHANGED: b"content\n"})
+        dest = os.path.join(TEST_DATA_DIR, "basis_missing_dst")
+        clean_dir(dest)
+        result, _ = run_client(source, dest, flags=[f"{flag}=nope"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"{flag} with missing dir failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        assert _read_file(os.path.join(received, self.UNCHANGED)) == b"content\n"
+
+    def test_link_dest_multithreaded(self, shared_server):
+        source = self._make_source("basis_link_mt_src", self._source_tree("mt"))
+        dest = os.path.join(TEST_DATA_DIR, "basis_link_mt_dst")
+        clean_dir(dest)
+        basis = self._seed_basis(dest, source, "mtbasis", self._basis_tree("mt"))
+        result, _ = run_client(source, dest, flags=["--link-dest=mtbasis", "-m"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"-m link-dest failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        assert os.stat(os.path.join(received, self.UNCHANGED)).st_ino == \
+            os.stat(os.path.join(basis, self.UNCHANGED)).st_ino
+        assert _read_file(os.path.join(received, self.ADDED)) == \
+            self._source_tree("mt")[self.ADDED]
+
+    def test_link_dest_with_delay_updates_stages_and_publishes_link(self, shared_server):
+        source = self._make_source("basis_link_delay_src", {self.UNCHANGED: b"v1\n"})
+        dest = os.path.join(TEST_DATA_DIR, "basis_link_delay_dst")
+        clean_dir(dest)
+        basis = self._seed_basis(dest, source, "delaybasis", {self.UNCHANGED: b"v1\n"})
+        result, _ = run_client(source, dest,
+                               flags=["--link-dest=delaybasis", "--delay-updates"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"delay-updates link-dest failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        unchanged = os.path.join(received, self.UNCHANGED)
+        assert os.stat(unchanged).st_ino == \
+            os.stat(os.path.join(basis, self.UNCHANGED)).st_ino
+        assert not os.path.isdir(os.path.join(dest, self.STAGING)), \
+            "delay-updates staging tree was not cleaned up"
+
+    def test_delete_does_not_touch_basis_dir(self):
+        """--delete removes genuine extras but must never treat a basis-dir
+        snapshot (which a --link-dest run just linked from) as destination
+        content."""
+        source = self._make_source("basis_delete_src", {self.UNCHANGED: b"v1\n"})
+        dest = os.path.join(TEST_DATA_DIR, "basis_delete_dst")
+        clean_dir(dest)
+        basis = self._seed_basis(dest, source, "delbasis", {self.UNCHANGED: b"v1\n"})
+        received = get_dest_received_dir(dest, source)
+        os.makedirs(received, exist_ok=True)
+        extra = os.path.join(received, "extra.txt")
+        with open(extra, "wb") as fh:
+            fh.write(b"extra")
+        with ServerManager() as server:
+            server.start(extra_args=["--allow-delete"])
+            result, _ = run_client(source, dest, flags=["--link-dest=delbasis", "--delete"],
+                                   port=server.port)
+            assert result.returncode == 0, \
+                f"delete+link-dest failed: {result.stderr[:300]}"
+            assert not os.path.exists(extra), "genuine extra file was not deleted"
+            assert _read_file(os.path.join(received, self.UNCHANGED)) == b"v1\n"
+            assert os.path.exists(os.path.join(basis, self.UNCHANGED)), \
+                "basis directory was deleted by --delete"
+            assert os.stat(os.path.join(received, self.UNCHANGED)).st_ino == \
+                os.stat(os.path.join(basis, self.UNCHANGED)).st_ino

tests/test_client_cli.c

@@ -595,6 +595,86 @@ static void test_parse_args_relative_no_implied_mkpath() {
   config_delete(cfg);
 }
 
+/* Parse --compare-dest/--copy-dest/--link-dest, including the =value and
+   separate-argument forms, and verify the ordered (repeatable) basis list. */
+static void test_parse_args_basis_dirs() {
+  Config* cfg = config_create();
+  int positional_args[2];
+  int positional_count = 0;
+  char* argv[] = {"fastsync", "--link-dest=prior", "/src", "/dst"};
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_TRUE(config_has_basis(cfg));
+  EXPECT_EQ_INT(cfg->basis_count, 1);
+  EXPECT_EQ_INT(cfg->basis_dirs[0].type, BASIS_DEST_LINK);
+  EXPECT_EQ_STR(cfg->basis_dirs[0].path, "prior");
+  /* Basis dirs are honored by the receiver-side per-file check, so they imply
+     --incremental (and, unless disabled, metadata) on the sender. */
+  EXPECT_TRUE(cfg->use_incremental);
+  EXPECT_TRUE(cfg->use_metadata);
+  config_delete(cfg);
+
+  cfg = config_create();
+  positional_count = 0;
+  char* argv2[] = {"fastsync", "--compare-dest", "cmp", "/src", "/dst"};
+  EXPECT_EQ_INT(parse_args(cfg, 5, argv2, positional_args, &positional_count), 0);
+  EXPECT_EQ_INT(cfg->basis_count, 1);
+  EXPECT_EQ_INT(cfg->basis_dirs[0].type, BASIS_DEST_COMPARE);
+  EXPECT_EQ_STR(cfg->basis_dirs[0].path, "cmp");
+  config_delete(cfg);
+
+  /* Repetition is supported: entries keep command-line order and type. */
+  cfg = config_create();
+  positional_count = 0;
+  char* argv3[] = {"fastsync",      "--link-dest=a", "--compare-dest=b",
+                   "--link-dest=c", "--copy-dest=d", "/src",
+                   "/dst"};
+  EXPECT_EQ_INT(parse_args(cfg, 7, argv3, positional_args, &positional_count), 0);
+  EXPECT_EQ_INT(cfg->basis_count, 4);
+  EXPECT_EQ_INT(cfg->basis_dirs[0].type, BASIS_DEST_LINK);
+  EXPECT_EQ_STR(cfg->basis_dirs[0].path, "a");
+  EXPECT_EQ_INT(cfg->basis_dirs[1].type, BASIS_DEST_COMPARE);
+  EXPECT_EQ_STR(cfg->basis_dirs[1].path, "b");
+  EXPECT_EQ_INT(cfg->basis_dirs[2].type, BASIS_DEST_LINK);
+  EXPECT_EQ_STR(cfg->basis_dirs[2].path, "c");
+  EXPECT_EQ_INT(cfg->basis_dirs[3].type, BASIS_DEST_COPY);
+  EXPECT_EQ_STR(cfg->basis_dirs[3].path, "d");
+  config_delete(cfg);
+
+  /* Nested relative basis dirs are allowed (they resolve below the root). */
+  cfg = config_create();
+  positional_count = 0;
+  char* argv4[] = {"fastsync", "--copy-dest=snap/2026-01", "/src", "/dst"};
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv4, positional_args, &positional_count), 0);
+  EXPECT_EQ_INT(cfg->basis_count, 1);
+  EXPECT_EQ_STR(cfg->basis_dirs[0].path, "snap/2026-01");
+  config_delete(cfg);
+}
+
+/* Absolute, escaping, or degenerate basis-dir values must be rejected up
+   front: they would resolve outside the destination root on the receiver. */
+static void test_parse_args_basis_invalid_paths() {
+  static const char* const invalid[] = {"/abs", "..", "a/../b", "."};
+  for (size_t i = 0; i < sizeof(invalid) / sizeof(invalid[0]); i++) {
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--link-dest", (char*)invalid[i], "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), -1);
+    config_delete(cfg);
+  }
+}
+
+/* Basis dirs require the per-file incremental handshake, which -s disables. */
+static void test_validate_config_basis_rejects_chunk_serialization() {
+  Config* cfg = valid_client_config();
+  EXPECT_EQ_INT(config_basis_append(cfg, BASIS_DEST_LINK, "prior"), 0);
+  cfg->use_chunk_serialization = true;
+  EXPECT_FALSE(validate_config(cfg));
+  cfg->use_chunk_serialization = false;
+  EXPECT_TRUE(validate_config(cfg));
+  config_delete(cfg);
+}
+
 /* --del is recognized as the rsync alias, but its timing mode is not implemented. */
 static void test_parse_args_delete_during_alias_unimplemented() {
   static const char* const options[] = {"--del", "--delete-during"};
@@ -632,9 +712,6 @@ static void test_parse_args_rejects_unimplemented_options() {
                                         "-e",
                                         "--rsh",
                                         "--rsync-path",
-                                        "--compare-dest",
-                                        "--copy-dest",
-                                        "--link-dest",
                                         "--delete-before",
                                         "--address",
                                         "--bind-address",
@@ -1589,4 +1666,7 @@ void test_client_cli() {
   test_parse_args_files_from();
   test_parse_args_filter_rules();
   test_parse_args_from0_cvs_filter_file_flags();
+  test_parse_args_basis_dirs();
+  test_parse_args_basis_invalid_paths();
+  test_validate_config_basis_rejects_chunk_serialization();
 }

tests/test_config.c

@@ -438,6 +438,96 @@ static void test_config_delay_updates_reserved_backup_rejected() {
   config_delete(c);
 }
 
+/* Basis-dir lists survive the config wire: each entry's type and path must
+   round-trip unchanged. */
+static void test_config_basis_roundtrip() {
+  if (is_running_under_valgrind())
+    return;
+  Config* send_cfg = config_create();
+  EXPECT_NOT_NULL(send_cfg);
+  send_cfg->send_directory = str_dup("/send/src");
+  send_cfg->receive_root_directory = str_dup("/send/dst");
+  EXPECT_EQ_INT(config_basis_append(send_cfg, BASIS_DEST_LINK, "prior"), 0);
+  EXPECT_EQ_INT(config_basis_append(send_cfg, BASIS_DEST_COMPARE, "snap/2026-01"), 0);
+  EXPECT_EQ_INT(config_basis_append(send_cfg, BASIS_DEST_COPY, "copy"), 0);
+
+  int p[2];
+  EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+  io_set_fds(p[0], p[1]);
+  io_set_bwlimit(0);
+
+  pid_t pid = fork();
+  if (pid == 0) {
+    close(p[1]);
+    io_set_fds(p[0], p[0]);
+    Config* recv = config_receive(p[0]);
+    bool ok = recv != NULL && recv->basis_count == 3 && recv->basis_dirs != NULL;
+    if (ok) {
+      ok = recv->basis_dirs[0].type == BASIS_DEST_LINK &&
+           strcmp(recv->basis_dirs[0].path, "prior") == 0;
+      ok = ok && recv->basis_dirs[1].type == BASIS_DEST_COMPARE &&
+           strcmp(recv->basis_dirs[1].path, "snap/2026-01") == 0;
+      ok = ok && recv->basis_dirs[2].type == BASIS_DEST_COPY &&
+           strcmp(recv->basis_dirs[2].path, "copy") == 0;
+    }
+    config_delete(recv);
+    close(p[0]);
+    close(p[1]);
+    _exit(ok ? 0 : 1);
+  } else {
+    close(p[0]);
+    io_set_fds(p[1], p[1]);
+    bool sent = config_send(p[1], send_cfg);
+    int status;
+    waitpid(pid, &status, 0);
+    close(p[1]);
+    config_delete(send_cfg);
+    EXPECT_TRUE(sent);
+    EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0);
+  }
+}
+
+/* The receiver must reject a basis-dir path that would escape the destination
+   root.  The values are injected directly (bypassing the client-side append
+   validator) so the receiver-side wire validation is what is exercised. */
+static void test_config_basis_wire_rejects_escaping() {
+  if (is_running_under_valgrind())
+    return;
+  Config* c = config_create();
+  EXPECT_NOT_NULL(c);
+  c->send_directory = str_dup("/src");
+  c->receive_root_directory = str_dup("/dst");
+  c->basis_count = 1;
+  c->basis_dirs = calloc(1, sizeof(BasisDest));
+  c->basis_dirs[0].type = BASIS_DEST_LINK;
+  c->basis_dirs[0].path = str_dup("../../etc");
+  EXPECT_FALSE(roundtrip_config_ok(c));
+  config_delete(c);
+
+  c = config_create();
+  EXPECT_NOT_NULL(c);
+  c->send_directory = str_dup("/src");
+  c->receive_root_directory = str_dup("/dst");
+  c->basis_count = 1;
+  c->basis_dirs = calloc(1, sizeof(BasisDest));
+  c->basis_dirs[0].type = BASIS_DEST_LINK;
+  c->basis_dirs[0].path = str_dup("/abs");
+  EXPECT_FALSE(roundtrip_config_ok(c));
+  config_delete(c);
+
+  /* A well-formed list still round-trips even with a manually built struct. */
+  c = config_create();
+  EXPECT_NOT_NULL(c);
+  c->send_directory = str_dup("/src");
+  c->receive_root_directory = str_dup("/dst");
+  c->basis_count = 1;
+  c->basis_dirs = calloc(1, sizeof(BasisDest));
+  c->basis_dirs[0].type = BASIS_DEST_COPY;
+  c->basis_dirs[0].path = str_dup("safe");
+  EXPECT_TRUE(roundtrip_config_ok(c));
+  config_delete(c);
+}
+
 static void test_config_is_remote_dest() {
   /* Valid SSH-style destinations */
   EXPECT_TRUE(config_is_remote_dest("user@host:/path"));
@@ -474,6 +564,8 @@ void test_config() {
     test_config_string_null_vs_empty_roundtrip();
     test_config_temp_dir_roundtrip();
     test_config_delay_updates_reserved_backup_rejected();
+    test_config_basis_roundtrip();
+    test_config_basis_wire_rejects_escaping();
   }
   test_config_is_remote_dest();
 }