docs: precise basis-dir caveats (shared-inode --inplace, attribute provenance, 256MiB limit)

- --link-dest destination entries share the basis inode: a later --inplace run
  against such a path mutates the basis snapshot through the shared inode
  (recommend --copy-dest when the destination must stay independently writable).
- basis-hit files take mode/uid/gid and mtime from the basis file, not the
  sender's metadata (with --size-only the mtime can differ from the source).
- --remove-source-files sources satisfied by a basis dir are retained.
- basis runs refuse files above the 256 MiB whole-file limit up front (FastSync
  caps every whole-file payload path at 256 MiB; rsync supports arbitrary sizes);
  the config frame always carries a basis-count field (protocol 2.8.0).

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 | ✅ 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) |
+| `--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, `//` collapsed and trailing `/` dropped). On the receiver's per-file check (implies `--incremental`) an exact match = same size + mtime (unless `--size-only`; `-I` disables matching) **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. Sizing: FastSync's whole-file payload limit is 256 MiB on **every** transfer path (not basis-specific); rsync applies basis dirs to arbitrary sizes, so FastSync refuses a basis run whose source contains a larger file up front with a clear error before any transfer. Wire: a basis-count field is always present on the config frame (protocol bumped to 2.8.0, so clients and servers must both be 2.8.0) |
-| `--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 |
+| `--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: a basis-hit destination keeps the basis file's own mode/uid/gid and mtime (the sender sends no metadata on a skip), so with `--size-only` its mtime can differ from the source and attribute-only differences are 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 | ✅ 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 |
+| `--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 and caveats: an already up-to-date destination file is not re-linked to a basis file (only files that would otherwise be written are linked); a link keeps the basis inode's own mode/uid/gid and mtime — metadata is never written through the shared inode (that would mutate the basis file), so a later `--inplace` run that rewrites such a destination path **will mutate the basis snapshot** through the shared inode (use `--copy-dest` when the destination must stay independently writable); with `--size-only` the linked mtime can differ from the source; a `--remove-source-files` source satisfied by a basis dir is treated as skipped and therefore **retained** (never removed); basis dirs are excluded from `--delete`. 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_send.c

@@ -218,6 +218,49 @@ static bool files_from_list_valid(const Config* config) {
   return no_implied_dirs_files_from_valid(config);
 }
 
+/* Basis directories are honored by the receiver's per-file incremental check,
+   which (like every whole-file payload path in FastSync) is bounded by
+   MAX_RECEIVE_WHOLE_FILE_SIZE.  rsync would apply basis dirs to files of any
+   size; FastSync cannot, so when basis dirs are requested this preflight scan
+   refuses the run up front with a clear diagnostic instead of letting the
+   receiver abort the whole transfer mid-stream with no client explanation.
+   Returns true when the tree can be transferred. */
+static bool basis_oversize_preflight(const Config* config) {
+  PreparedScanner prepared;
+  if (!prepare_scanner(config, 0, &prepared))
+    return false;
+  DirectoryScanner* scanner =
+      directory_scanner_create_with_options(config->send_directory, &prepared.options);
+  prepared_scanner_destroy(&prepared);
+  if (!scanner)
+    return false;
+  bool ok = true;
+  Chunk* chunk;
+  while ((chunk = directory_scanner_next(scanner)) != NULL) {
+    for (int i = 0; i < chunk->element_count; i++) {
+      File* f = chunk->items[i];
+      if (f == NULL || f->is_dir || f->data == NULL || f->data->size <= MAX_RECEIVE_WHOLE_FILE_SIZE)
+        continue;
+      char* escaped = output_escape(file_wire_path(f), config->eight_bit_output);
+      log_message(LOG_LEVEL_ERROR,
+                  "%s is %llu bytes, larger than the %llu-byte whole-file transfer limit; "
+                  "--compare-dest/--copy-dest/--link-dest cannot sync files above this limit",
+                  escaped ? escaped : "<allocation failed>", (unsigned long long)f->data->size,
+                  (unsigned long long)MAX_RECEIVE_WHOLE_FILE_SIZE);
+      free(escaped);
+      ok = false;
+      break;
+    }
+    chunk_destroy(chunk);
+    if (!ok)
+      break;
+  }
+  if (directory_scanner_failed(scanner))
+    ok = false;
+  directory_scanner_destroy(scanner);
+  return ok;
+}
+
 /* Select the configured transport for both transfer execution paths. */
 static Client* connect_transfer_client(const Config* config) {
   if (config->transport == TRANSPORT_SSH) {
@@ -1159,6 +1202,8 @@ int send_files(Config* config) {
     return send_dry_run_manifest(config);
   if (!files_from_list_valid(config))
     return 1;
+  if (config_has_basis(config) && !basis_oversize_preflight(config))
+    return 1;
 
   Client* client = connect_transfer_client(config);
   if (!client) {
@@ -1298,6 +1343,8 @@ int send_files_multithreaded(Config** config_ptr) {
     return send_dry_run_manifest(config);
   if (!files_from_list_valid(config))
     return 1;
+  if (config_has_basis(config) && !basis_oversize_preflight(config))
+    return 1;
 
   long pages = sysconf(_SC_AVPHYS_PAGES);
   long page_size = sysconf(_SC_PAGE_SIZE);

src/shared/config.c

@@ -194,52 +194,76 @@ bool config_has_basis(const Config* config) {
 /* 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. */
+ * make the receiver read or link files outside its authorized root.
-bool config_basis_path_valid(const char* path) {
+ *
+ * Returns a malloc'd CANONICAL copy of an accepted path, or NULL when the path
+ * is rejected.  Canonicalization collapses interior empty components ("a//b" ->
+ * "a/b"), drops "." components and trailing "/"s, so validation, the delete
+ * walker prefix match and the receiver's basis lookup all agree on one form.
+ * The normalizer is the single source of truth for both config_basis_path_valid
+ * and config_basis_append. */
+static char* basis_path_normalize(const char* path) {
   if (!path || path[0] == '\0' || path[0] == '/' || has_path_traversal(path))
-    return false;
+    return NULL;
   if (strcmp(path, ".") == 0)
-    return false;
+    return NULL;
   char* dup = str_dup(path);
   if (!dup)
-    return false;
+    return NULL;
-  bool ok = true;
+  size_t out_len = 0;
-  char* saveptr = NULL;
+  char* out = malloc(strlen(path) + 1);
-  for (char* part = strtok_r(dup, "/", &saveptr); part; part = strtok_r(NULL, "/", &saveptr)) {
+  if (!out) {
-    if (strcmp(part, ".") == 0) {
+    free(dup);
-      ok = false;
+    return NULL;
-      break;
   }
+  char* saveptr = NULL;
+  bool ok = true;
+  for (char* part = strtok_r(dup, "/", &saveptr); part; part = strtok_r(NULL, "/", &saveptr)) {
     if (strcmp(part, "..") == 0) {
       ok = false;
       break;
     }
+    if (strcmp(part, ".") == 0)
+      continue;
+    if (out_len > 0)
+      out[out_len++] = '/';
+    size_t len = strlen(part);
+    memcpy(out + out_len, part, len);
+    out_len += len;
   }
   free(dup);
-  return ok;
+  if (!ok || out_len == 0) {
+    free(out);
+    return NULL;
+  }
+  out[out_len] = '\0';
+  return out;
+}
+
+bool config_basis_path_valid(const char* path) {
+  char* normalized = basis_path_normalize(path);
+  if (!normalized)
+    return false;
+  free(normalized);
+  return true;
 }
 
 int config_basis_append(Config* config, BasisDestType type, const char* path) {
-  if (!config || !config_basis_path_valid(path) ||
+  if (!config ||
       (type != BASIS_DEST_COMPARE && type != BASIS_DEST_COPY && type != BASIS_DEST_LINK) ||
       config->basis_count >= MAX_BASIS_DIRS)
     return -1;
+  char* normalized = basis_path_normalize(path);
+  if (!normalized)
+    return -1;
   BasisDest* grown = realloc(config->basis_dirs, (config->basis_count + 1) * sizeof(BasisDest));
-  if (!grown)
+  if (!grown) {
+    free(normalized);
     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_dirs[config->basis_count].path = normalized;
   config->basis_count++;
   return 0;
 }
@@ -563,9 +587,9 @@ static bool receive_basis_options(int fd, Config* c) {
     char* path = receive_str(fd);
     if (!path)
       return false;
-    bool ok = config_basis_path_valid(path);
+    /* config_basis_append validates and canonicalizes the path; a rejected
-    if (ok)
+       path (absolute / traversal / empty) drops the whole connection. */
-      ok = config_basis_append(c, (BasisDestType)type, path) == 0;
+    bool ok = config_basis_append(c, (BasisDestType)type, path) == 0;
     free(path);
     if (!ok)
       return false;

src/shared/file_receive.c

@@ -849,12 +849,14 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
         }
         if (materialized) {
           if (!send_status(fd, STATUS_OK)) {
+            basis_match_free(&basis);
             file_destroy(materialized);
             close(old_fd);
             free(full_path);
             free(check_path);
             return NULL;
           }
+          basis_match_free(&basis);
           free(old_data);
           close(old_fd);
           free(full_path);
@@ -1069,29 +1071,38 @@ 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.  Alternate basis directories (--compare-dest /
+     can be published.  That staging name is protected only as a DIRECT child
-     --copy-dest / --link-dest) are also excluded: they are extra comparison
+     of the receive root so a nested destination directory that happens to be
-     snapshots the user pointed at, not destination content, and deleting them
+     named .fastsync-stage is still ordinary content.  Alternate basis
-     would destroy the very files a --link-dest run just linked into place. */
+     directories (--compare-dest / --copy-dest / --link-dest) are excluded at
+     any depth: they are extra comparison snapshots the user pointed at, not
+     destination content, and deleting them 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;
+  DeleteSkipEntry* skips = NULL;
   bool deletion_ok = false;
   if (skip_count > 0) {
-    skip_prefixes = calloc((size_t)skip_count, sizeof(char*));
+    skips = calloc((size_t)skip_count, sizeof(DeleteSkipEntry));
-    if (!skip_prefixes) {
+    if (!skips) {
       array_list_delete(manifest);
       send_status(fd, STATUS_ERROR);
       return -1;
     }
     int idx = 0;
-    if (config->delay_updates)
+    if (config->delay_updates) {
-      skip_prefixes[idx++] = DELAY_UPDATES_STAGING_DIR;
+      skips[idx].prefix = DELAY_UPDATES_STAGING_DIR;
-    for (int i = 0; i < config->basis_count; i++)
+      skips[idx].top_level_only = true;
-      skip_prefixes[idx++] = config->basis_dirs[i].path;
+      idx++;
+    }
+    for (int i = 0; i < config->basis_count; i++) {
+      skips[idx].prefix = config->basis_dirs[i].path;
+      skips[idx].top_level_only = false;
+      idx++;
+    }
   }
   deletion_ok = delete_extras_limited(config->receive_root_directory, manifest,
-                                      MAX_SERVER_DELETE_COUNT, skip_prefixes, skip_count);
+                                      MAX_SERVER_DELETE_COUNT, skips, skip_count);
-  free(skip_prefixes);
+  free(skips);
   array_list_delete(manifest);
   if (!deletion_ok)
     send_status(fd, STATUS_ERROR);

src/shared/utils.c

@@ -204,22 +204,26 @@ 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
+/* True when child_rel is, or lies below, a protected entry.  A prefix "a"
-   prefixes.  A prefix "a" therefore protects "a" and "a/b/c" but not "ab". */
+   therefore protects "a" and "a/b/c" but not "ab".  Entries with top_level_only
-static bool path_under_skip_prefix(const char* rel_path, const char* const* prefixes,
+   set only protect DIRECT children of the receive root (at_root); nested
-                                   int prefix_count) {
+   directories that share such a name stay ordinary destination content. */
-  for (int i = 0; i < prefix_count; i++) {
+static bool path_under_skip_prefix(const char* child_rel, bool at_root,
-    size_t prefix_len = strlen(prefixes[i]);
+                                   const DeleteSkipEntry* skips, int skip_count) {
-    if (strncmp(rel_path, prefixes[i], prefix_len) == 0 &&
+  for (int i = 0; i < skip_count; i++) {
-        (rel_path[prefix_len] == '\0' || rel_path[prefix_len] == '/'))
+    if (skips[i].top_level_only && !at_root)
+      continue;
+    size_t prefix_len = strlen(skips[i].prefix);
+    if (strncmp(child_rel, skips[i].prefix, prefix_len) == 0 &&
+        (child_rel[prefix_len] == '\0' || child_rel[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,
+                             size_t max_delete, size_t* deleted_count, const DeleteSkipEntry* skips,
-                             const char* const* skip_prefixes, int skip_prefix_count) {
+                             int skip_count) {
   int scanfd = dup(dirfd);
   if (scanfd < 0)
     return false;
@@ -238,11 +242,14 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
       operation_ok = false;
       continue;
     }
-    /* A --delay-updates run keeps its staging directory below the receive
+    /* A --delay-updates run keeps its staging directory as a direct child of
-       root, and basis-dir snapshots live there too.  Their contents are not
+       the receive root, and basis-dir snapshots live below it too.  Their
-       manifest entries, so descending into them would delete every staged /
+       contents are not manifest entries, so descending into them would delete
-       basis file as an "extra". */
+       every staged / basis file as an "extra".  Only the staging name (a
-    if (path_under_skip_prefix(child_rel, skip_prefixes, skip_prefix_count)) {
+       top-level-only prefix) and the basis prefixes are protected: a nested
+       destination directory that happens to be called .fastsync-stage is
+       ordinary content. */
+    if (path_under_skip_prefix(child_rel, rel_path[0] == '\0', skips, skip_count)) {
       free(child_rel);
       continue;
     }
@@ -263,7 +270,7 @@ 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_prefixes, skip_prefix_count);
+                                         skips, skip_count);
         if (!child_removed)
           operation_ok = false;
         close(childfd);
@@ -315,7 +322,7 @@ 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* const* skip_prefixes, int skip_prefix_count) {
+                           const DeleteSkipEntry* skips, int skip_count) {
   if (!manifest)
     return false;
   int rootfd;
@@ -332,8 +339,7 @@ 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_prefixes,
+  bool ok = delete_extras_fd(rootfd, "", manifest, max_delete, &deleted_count, skips, skip_count);
-                             skip_prefix_count);
   if (close(rootfd) != 0)
     ok = false;
   return ok;

src/shared/utils.h

@@ -10,14 +10,21 @@ 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.  The
+/* One protected entry for the delete walker.  When top_level_only is true the
-   delete walker never descends into (and so never removes) an entry whose
+   prefix is skipped only as a DIRECT child of dest_root (the --delay-updates
-   relative path equals one of the skip_prefixes or lies below one: used to
+   staging directory, which must not hide genuine extras inside a nested
-   protect the --delay-updates staging directory (files still to be published)
+   destination directory that happens to share the staging name); otherwise the
-   and the --compare-dest/--copy-dest/--link-dest basis trees (snapshots the
+   prefix is skipped at any depth (the --compare-dest/--copy-dest/--link-dest
-   transfer links from, never destination content). */
+   basis trees, which the transfer links from and are never destination
+   content). */
+typedef struct {
+  const char* prefix;
+  bool top_level_only;
+} DeleteSkipEntry;
+/* Remove files/dirs under dest_root that are not listed in manifest without
+   ever descending into a protected prefix (see DeleteSkipEntry). */
 bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete,
-                           const char* const* skip_prefixes, int skip_prefix_count);
+                           const DeleteSkipEntry* skips, int skip_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

@@ -1990,7 +1990,11 @@ class TestBasisDestDirs:
     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?)
+    # fixture files: source and basis share the mtime pin, so a basis "match"
+    # is decided purely by content (xxHash).  unchanged.txt is byte-identical;
+    # changed.txt is byte-DIFFERENT but has the SAME SIZE as the source (and
+    # the same pinned mtime), which is what forces the content-hash gate;
+    # added.txt does not exist in the basis at all.
     UNCHANGED = "unchanged.txt"
     CHANGED = "changed.txt"
     ADDED = "added.txt"
@@ -2006,21 +2010,21 @@ class TestBasisDestDirs:
             _pin_mtime(full, self.TS)
         return src
 
-    def _basis_root(self, dest, source):
+    def _seed_basis_file(self, dest, source, basis_dir, rel, content, ts=None):
-        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)
+        _pin_mtime(full, self.TS if ts is None else ts)
-        return base
+        return full
+
+    def _seed_basis(self, dest, source, basis_dir, basis_files):
+        for rel, content in basis_files.items():
+            self._seed_basis_file(dest, source, basis_dir, rel, content)
+        return os.path.join(dest, basis_dir, os.path.relpath(
+            get_dest_received_dir(dest, source), dest))
 
     def _source_tree(self, prefix):
         return {
@@ -2030,13 +2034,38 @@ class TestBasisDestDirs:
         }
 
     def _basis_tree(self, prefix):
-        # unchanged.txt matches the source; changed.txt differs in CONTENT but
+        # unchanged.txt is identical to the source; changed.txt has the SAME
-        # shares size/mtime pinning; added.txt is missing from the basis.
+        # byte size and pinned mtime but a different body (equal size forces
+        # the xxHash gate); added.txt is missing from the basis.
         return {
             self.UNCHANGED: b"stable content v1\n",
-            self.CHANGED: b"ANCIENT DIFFERENT CONTENT!\n",
+            self.CHANGED: b"CHANGED CONTENT NOW\n",
         }
 
+    def test_same_size_different_content_is_not_a_basis_match(self, shared_server):
+        # Core safety property: equal size + pinned mtime but different content
+        # must NEVER be hard-linked or copied from the basis -- the xxHash gate
+        # rejects it and the sender's data is transferred instead.
+        for flag, basis_dir in (("--link-dest", "szlb"), ("--copy-dest", "szcp"),
+                                ("--compare-dest", "szcmp")):
+            source = self._make_source("basis_same_size_src",
+                                       {self.UNCHANGED: b"same length body\n"})
+            dest = os.path.join(TEST_DATA_DIR, f"basis_same_size_dst_{basis_dir}")
+            clean_dir(dest)
+            basis_file = self._seed_basis_file(dest, source, basis_dir, self.UNCHANGED,
+                                               b"SAME LENGTH BODY!")
+            result, _ = run_client(source, dest, flags=[f"{flag}={basis_dir}"],
+                                   port=shared_server.port)
+            assert result.returncode == 0, \
+                f"{flag} same-size mismatch failed: {result.stderr[:300]}"
+            received = get_dest_received_dir(dest, source)
+            dest_file = os.path.join(received, self.UNCHANGED)
+            assert _read_file(dest_file) == b"same length body\n", \
+                f"{flag}: basis content leaked into the destination on a hash mismatch"
+            if flag != "--compare-dest":
+                assert os.stat(dest_file).st_ino != os.stat(basis_file).st_ino, \
+                    f"{flag}: linked/copied from a content-mismatched basis file"
+
     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")
@@ -2087,7 +2116,7 @@ class TestBasisDestDirs:
         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.
+        # Equal-size/different-content basis file falls back to 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)) == \
@@ -2109,7 +2138,8 @@ class TestBasisDestDirs:
         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).
+        # Equal-size/different-content basis file 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
@@ -2181,3 +2211,137 @@ class TestBasisDestDirs:
                 "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
+
+    def test_delay_delete_keeps_nested_staging_named_dir_as_content(self):
+        # The real --delay-updates staging directory is protected from --delete
+        # only as a DIRECT child of the receive root.  A nested destination
+        # directory that merely shares the staging name is ordinary content, so
+        # its extras must still be deleted (regression guard for the walker).
+        source = self._make_source("basis_nested_stage_src",
+                                   {"top.txt": b"top\n", "sub/real.txt": b"real\n"})
+        dest = os.path.join(TEST_DATA_DIR, "basis_nested_stage_dst")
+        clean_dir(dest)
+        self._seed_basis(dest, source, "nstbasis",
+                         {"top.txt": b"top\n", "sub/real.txt": b"real\n"})
+        received = get_dest_received_dir(dest, source)
+        nested = os.path.join(received, "sub", self.STAGING)
+        os.makedirs(nested, exist_ok=True)
+        extra = os.path.join(nested, "extra.txt")
+        with open(extra, "wb") as fh:
+            fh.write(b"nested extra")
+        with ServerManager() as server:
+            server.start(extra_args=["--allow-delete"])
+            result, _ = run_client(source, dest,
+                                   flags=["--link-dest=nstbasis", "--delete",
+                                          "--delay-updates"],
+                                   port=server.port)
+            assert result.returncode == 0, \
+                f"delay-delete nested staging failed: {result.stderr[:300]}"
+            assert not os.path.exists(extra), \
+                "extra inside a nested .fastsync-stage dir was not deleted"
+            assert not os.path.isdir(nested), \
+                "nested .fastsync-stage dir should have been removed after its extra"
+            assert _read_file(os.path.join(received, "sub", "real.txt")) == b"real\n"
+            assert not os.path.isdir(os.path.join(dest, self.STAGING)), \
+                "real delay-updates staging tree was not cleaned up"
+
+    def test_basis_priority_first_match_wins(self, shared_server):
+        # Two link-dest dirs both hold the exact file: the FIRST (command-line
+        # order) basis directory must win and supply the hard link.
+        source = self._make_source("basis_prio_src", {"f.txt": b"content\n"})
+        dest = os.path.join(TEST_DATA_DIR, "basis_prio_dst")
+        clean_dir(dest)
+        first = self._seed_basis_file(dest, source, "b1", "f.txt", b"content\n")
+        self._seed_basis_file(dest, source, "b2", "f.txt", b"content\n")
+        result, _ = run_client(source, dest, flags=["--link-dest=b1", "--link-dest=b2"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"link-dest priority failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        assert os.stat(os.path.join(received, "f.txt")).st_ino == os.stat(first).st_ino, \
+            "first basis dir did not win over the second"
+
+    def test_basis_priority_across_compare_and_link(self, shared_server):
+        # A compare-dest entry listed BEFORE a link-dest entry shadows it (the
+        # exact match is found first and nothing is materialized); reversing the
+        # order lets the link-dest entry win and materialize a hard link.
+        source = self._make_source("basis_prio_mixed_src", {"f.txt": b"content\n"})
+
+        dest = os.path.join(TEST_DATA_DIR, "basis_prio_mixed_dst")
+        clean_dir(dest)
+        self._seed_basis_file(dest, source, "cmpb", "f.txt", b"content\n")
+        self._seed_basis_file(dest, source, "lnb", "f.txt", b"content\n")
+        result, _ = run_client(source, dest,
+                               flags=["--compare-dest=cmpb", "--link-dest=lnb"],
+                               port=shared_server.port)
+        assert result.returncode == 0, \
+            f"mixed priority (compare first) failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        assert not os.path.exists(os.path.join(received, "f.txt")), \
+            "compare-dest matched first, so the file must stay sparse (no link-dest materialize)"
+
+        dest = os.path.join(TEST_DATA_DIR, "basis_prio_mixed_dst2")
+        clean_dir(dest)
+        self._seed_basis_file(dest, source, "cmpb", "f.txt", b"content\n")
+        linkb2 = self._seed_basis_file(dest, source, "lnb", "f.txt", b"content\n")
+        result, _ = run_client(source, dest,
+                               flags=["--link-dest=lnb", "--compare-dest=cmpb"],
+                               port=shared_server.port)
+        assert result.returncode == 0, \
+            f"mixed priority (link first) failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        assert os.stat(os.path.join(received, "f.txt")).st_ino == os.stat(linkb2).st_ino, \
+            "link-dest did not materialize when listed before compare-dest"
+
+    def test_link_dest_size_only_ignores_mtime(self, shared_server):
+        # --size-only drops the mtime leg of the quick check: a basis file with
+        # the SAME content but a DIFFERENT mtime is still an exact match.
+        source = self._make_source("basis_sizeonly_src", {"f.txt": b"content\n"})
+        dest = os.path.join(TEST_DATA_DIR, "basis_sizeonly_dst")
+        clean_dir(dest)
+        basis_file = self._seed_basis_file(dest, source, "sob", "f.txt", b"content\n",
+                                           ts=self.TS + 500)
+        result, _ = run_client(source, dest, flags=["--link-dest=sob", "--size-only"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"size-only link-dest failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        assert os.stat(os.path.join(received, "f.txt")).st_ino == os.stat(basis_file).st_ino, \
+            "--size-only should link a basis file whose mtime differs"
+
+    def test_link_dest_ignore_times_never_links(self, shared_server):
+        # -I/--ignore-times forces every file to be updated, so a basis dir is
+        # never used to hard-link (rsync parity).  The file is transferred and
+        # stored as a fresh inode even though it matches the basis exactly.
+        source = self._make_source("basis_igntimes_src", {"f.txt": b"content\n"})
+        dest = os.path.join(TEST_DATA_DIR, "basis_igntimes_dst")
+        clean_dir(dest)
+        basis_file = self._seed_basis_file(dest, source, "itb", "f.txt", b"content\n")
+        result, _ = run_client(source, dest, flags=["--link-dest=itb", "--ignore-times"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"ignore-times link-dest failed: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        dest_file = os.path.join(received, "f.txt")
+        assert _read_file(dest_file) == b"content\n"
+        assert os.stat(dest_file).st_ino != os.stat(basis_file).st_ino, \
+            "--ignore-times must not hard-link to a basis file"
+
+    def test_basis_refuses_file_above_whole_file_limit(self, shared_server):
+        # Every whole-file payload path in FastSync (basis dirs included) is
+        # bounded by MAX_RECEIVE_WHOLE_FILE_SIZE.  rsync supports basis dirs for
+        # arbitrary sizes; FastSync refuses such a run up front with a clear
+        # diagnostic instead of letting the receiver abort the whole transfer
+        # mid-stream with no client-side explanation.
+        source = self._make_source("basis_oversize_src", {"small.txt": b"ok\n"})
+        big = os.path.join(source, "huge.bin")
+        with open(big, "wb") as fh:
+            os.ftruncate(fh.fileno(), 256 * 1024 * 1024 + 4096)
+        dest = os.path.join(TEST_DATA_DIR, "basis_oversize_dst")
+        clean_dir(dest)
+        result, _ = run_client(source, dest, flags=["--link-dest=nope"],
+                               port=shared_server.port)
+        assert result.returncode != 0, \
+            "basis run with an over-limit file unexpectedly succeeded"
+        assert "larger than" in result.stderr, \
+            f"no clear over-limit diagnostic: {result.stderr[:300]}"
+        received = get_dest_received_dir(dest, source)
+        assert not os.path.exists(received), \
+            "over-limit basis run transferred files before failing"

tests/test_config.c

@@ -528,6 +528,29 @@ static void test_config_basis_wire_rejects_escaping() {
   config_delete(c);
 }
 
+/* Basis-dir paths are canonicalized on the way in: trailing slashes and
+   interior empty / "." components are dropped so validation, the delete-walker
+   prefix and the receiver lookup all agree on one stored form. */
+static void test_config_basis_normalization() {
+  Config* c = config_create();
+  EXPECT_NOT_NULL(c);
+  EXPECT_EQ_INT(config_basis_append(c, BASIS_DEST_LINK, "prior/"), 0);
+  EXPECT_EQ_INT(config_basis_append(c, BASIS_DEST_LINK, "a//b"), 0);
+  EXPECT_EQ_INT(config_basis_append(c, BASIS_DEST_LINK, "./x/./y/"), 0);
+  EXPECT_EQ_INT(c->basis_count, 3);
+  EXPECT_EQ_STR(c->basis_dirs[0].path, "prior");
+  EXPECT_EQ_STR(c->basis_dirs[1].path, "a/b");
+  EXPECT_EQ_STR(c->basis_dirs[2].path, "x/y");
+
+  /* Degenerate values that normalize away to nothing stay rejected. */
+  EXPECT_EQ_INT(config_basis_append(c, BASIS_DEST_LINK, "."), -1);
+  EXPECT_EQ_INT(config_basis_append(c, BASIS_DEST_LINK, ".."), -1);
+  EXPECT_EQ_INT(config_basis_append(c, BASIS_DEST_LINK, "/abs"), -1);
+  EXPECT_EQ_INT(config_basis_append(c, BASIS_DEST_LINK, "a/../b"), -1);
+  EXPECT_EQ_INT(config_basis_append(c, BASIS_DEST_LINK, ""), -1);
+  config_delete(c);
+}
+
 static void test_config_is_remote_dest() {
   /* Valid SSH-style destinations */
   EXPECT_TRUE(config_is_remote_dest("user@host:/path"));
@@ -566,6 +589,7 @@ void test_config() {
     test_config_delay_updates_reserved_backup_rejected();
     test_config_basis_roundtrip();
     test_config_basis_wire_rejects_escaping();
+    test_config_basis_normalization();
   }
   test_config_is_remote_dest();
 }