feat: basis-dir config/CLI for --compare-dest/--copy-dest/--link-dest

Replace the vestigial single compare_dest/copy_dest/link_dest Config fields with
an ordered BasisDest list (type + path per entry) that is serialized to the
receiver and interpreted relative to the destination root.  Paths must be
relative with no '.'/'..' components (confined like --backup-dir); trailing
slashes are normalized.  Wire layout changes, so PROTOCOL_VERSION -> 2.8.0.

CLI: each flag is parsed in both --flag=DIR and --flag DIR forms, is
repeatable, and keeps command-line order as basis priority.  Supplying any
basis dir implies --incremental (and therefore metadata) on the sender because
the unchanged decision is receiver-side; combining basis dirs with -s chunk
serialization is rejected in validate_config.  Usage text updated.

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_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/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->copy_dest = NULL;
-  config->link_dest = NULL;
+  config->basis_dirs = NULL;
+  config->basis_count = 0;
   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);
-  free(config->copy_dest);
-  free(config->link_dest);
+  for (int i = 0; i < config->basis_count; i++) {
+    free(config->basis_dirs[i].path);
+    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;
-  char* copy_dest;
-  char* link_dest;
+  /* Alternate basis directories, ordered by command-line appearance.  Each
+   * entry's type selects compare/copy/link behavior on an exact match.  These
+   * 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