feat: add rsync-compatible update option

src/client/client_cli.c

@@ -146,6 +146,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--backup", NULL, OPT_FLAG, offsetof(Config, backup)},
     {"--stats", NULL, OPT_FLAG, offsetof(Config, stats)},
     {"--partial", NULL, OPT_FLAG, offsetof(Config, partial)},
+    {"--update", "-u", OPT_FLAG, offsetof(Config, update)},
     {"--links", "-l", OPT_FLAG, offsetof(Config, follow_symlinks)},
     {"--copy-links", NULL, OPT_FLAG, offsetof(Config, copy_links)},
     {"--safe-links", NULL, OPT_FLAG, offsetof(Config, safe_links)},
@@ -189,6 +190,8 @@ static int apply_table_option(Config* config, const OptionEntry* entry, const ch
   switch (entry->kind) {
   case OPT_FLAG:
     *(bool*)field = true;
+    if (entry->offset == offsetof(Config, update))
+      config->use_metadata = true;
     return 0;
   case OPT_STRING:
     return set_string_option((char**)field, value, entry->name);

src/client/usage.c

@@ -28,6 +28,7 @@ void print_usage(void) {
   printf("  --max-size <n>      Skip files larger than n bytes\n");
   printf("  --min-size <n>      Skip files smaller than n bytes\n");
   printf("  --incremental       Skip files unchanged since last transfer\n");
+  printf("  -u, --update        Skip files newer than the source on receiver\n");
   printf("  --delta             Delta transfer for changed files (requires --incremental)\n");
   printf("  --delta-block <n>   Delta block size in bytes (default: %d)\n",
          DELTA_BLOCK_SIZE_DEFAULT);

src/shared/file_receive.c

@@ -21,6 +21,17 @@
 #define MAX_SERVER_DELETE_COUNT 100000U
 #define MAX_FILE_DATA_SIZE MAX_RECEIVE_FILE_SIZE
 
+static bool destination_is_newer(const char* path, const FileMetadata* source_metadata) {
+  struct stat destination_stat;
+  if (!source_metadata || !file_stat_secure(path, &destination_stat))
+    return false;
+  time_t destination_sec = destination_stat.st_mtim.tv_sec;
+  long destination_nsec = destination_stat.st_mtim.tv_nsec;
+  return destination_sec > source_metadata->mtime_sec ||
+         (destination_sec == source_metadata->mtime_sec &&
+          destination_nsec > source_metadata->mtime_nsec);
+}
+
 bool file_save_to_disk(const char* root_directory, const File* file, const Config* config) {
   bool backup_enabled = config && config->backup;
   bool inplace = config && config->inplace;
@@ -62,15 +73,11 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
   }
 
   /* --update is receiver-side policy: never replace a newer destination. */
-  if (config && config->update) {
-    struct stat destination_stat;
-    if (file_stat_secure(disk_path, &destination_stat) && file->metadata &&
-        destination_stat.st_mtime > file->metadata->mtime_sec) {
-      free(confined_backup);
-      free(confined_partial);
-      free(disk_path);
-      return true;
-    }
+  if (config && config->update && destination_is_newer(disk_path, file->metadata)) {
+    free(confined_backup);
+    free(confined_partial);
+    free(disk_path);
+    return true;
   }
 
   if (backup_enabled) {