fix(parity): receiver-side --ignore-existing short-circuit before payload

src/client/client_cli.c

@@ -2349,6 +2349,15 @@ static int cli_finalize_config(Config* config, bool verbose, bool no_delta, bool
       config->preserve_times = true;
   }
 
+  /* --ignore-existing is a receiver-side existence policy: the receiver must
+   * answer "skip" BEFORE the sender transmits any payload, which only the
+   * per-file STATUS_CHECK handshake provides.  Imply --incremental here (after
+   * the auto-preserve capture above, so a bare --ignore-existing does not gain
+   * -p/-t, which rsync likewise does not imply) so an existing destination is
+   * skipped on the wire instead of being streamed and discarded. */
+  if (config->ignore_existing)
+    config->use_incremental = true;
+
   /* Derive the transport bit from the FINAL parsed flags.  Every
    * preservation/ownership option that needs the metadata frame (per-attribute
    * perms/times/owner/group, atimes/crtimes, executability, xattrs/acls,

src/shared/file_receive.c

@@ -1764,6 +1764,7 @@ typedef struct {
   long long check_mtime_nsec;
   uint8_t check_digest[CHECKSUM_MAX_DIGEST_LEN];
   size_t check_digest_len;
+  bool dest_exists; /* any destination entry exists (lstat succeeded) */
   bool has_old_file;
   int old_fd;
   struct stat old_st;
@@ -1860,6 +1861,9 @@ static IncrementalCheckOutcome incremental_check_open_destination(IncrementalChe
   char* leaf = NULL;
   int parent_fd = file_open_secure_parent(full_path, &leaf, false);
   if (parent_fd >= 0) {
+    struct stat dest_st;
+    if (fstatat(parent_fd, leaf, &dest_st, AT_SYMLINK_NOFOLLOW) == 0)
+      state->dest_exists = true;
     /* O_NONBLOCK: an existing FIFO at the destination must not block this
        openat(); the S_ISREG gate below rejects the non-regular entry. */
     state->old_fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW | O_NONBLOCK);
@@ -1903,6 +1907,21 @@ static IncrementalCheckOutcome incremental_check_report_dest_info(IncrementalChe
   return INCREMENTAL_CONTINUE;
 }
 
+/* --ignore-existing short-circuit.  The receiver must answer "skip" (STATUS_OK)
+   BEFORE the sender transmits any payload, otherwise the whole file crosses the
+   wire only to be discarded at write time.  rsync skips an existing destination
+   entry regardless of its content or type, so the reply depends only on the
+   lstat existence probe; the ordinary --ignore-existing checks inside
+   file_receive remain as defense-in-depth for the frame types that have no
+   per-file check (directories/symlinks/specials/hard-links). */
+static IncrementalCheckOutcome incremental_check_ignore_existing(IncrementalCheckState* state) {
+  if (!state->config->ignore_existing || !state->dest_exists)
+    return INCREMENTAL_CONTINUE;
+  if (!send_status(state->fd, STATUS_OK))
+    return INCREMENTAL_ERROR;
+  return INCREMENTAL_SKIP;
+}
+
 /* Metadata-only (and, when --checksum forces it, content) up-to-date decision.
    Loads the old contents only when a checksum comparison or delta needs them. */
 static IncrementalCheckOutcome incremental_check_quick_skip(IncrementalCheckState* state,
@@ -2351,6 +2370,16 @@ File* receive_incremental_check_ex(int fd, const Config* config, bool* skipped,
   if (outcome == INCREMENTAL_ERROR)
     goto done;
 
+  /* --ignore-existing must answer before any data is requested; it takes
+     precedence over the metadata up-to-date check below. */
+  outcome = incremental_check_ignore_existing(&state);
+  if (outcome == INCREMENTAL_ERROR)
+    goto done;
+  if (outcome == INCREMENTAL_SKIP) {
+    *skipped = true;
+    goto done;
+  }
+
   outcome = incremental_check_quick_skip(&state, &try_delta);
   if (outcome == INCREMENTAL_ERROR)
     goto done;