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;
@@ -62,16 +73,12 @@ 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) {
+  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) {
     struct stat backup_stat;

tests/integration/test_features.py

@@ -214,6 +214,40 @@ class TestIncremental:
             assert f.read() == b"hello world\n"
 
 
+class TestUpdate:
+    def test_update_skips_older_destination_and_allows_equal_or_newer_source(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-u"], port=shared_server.port)
+        assert result.returncode == 0
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        source_file = os.path.join(SOURCE_DIR, "small.txt")
+        received_file = os.path.join(received, "small.txt")
+        source_stat = os.stat(source_file)
+
+        with open(received_file, "wb") as f:
+            f.write(b"newer destination\n")
+        os.utime(received_file, (source_stat.st_mtime + 10, source_stat.st_mtime + 10))
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-u"], port=shared_server.port)
+        assert result.returncode == 0
+        with open(received_file, "rb") as f:
+            assert f.read() == b"newer destination\n"
+
+        os.utime(received_file, (source_stat.st_atime, source_stat.st_mtime))
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-u"], port=shared_server.port)
+        assert result.returncode == 0
+        with open(received_file, "rb") as f:
+            assert f.read() == b"hello world\n"
+
+        with open(received_file, "wb") as f:
+            f.write(b"older destination\n")
+        os.utime(received_file, (source_stat.st_atime, source_stat.st_mtime - 10))
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-u"], port=shared_server.port)
+        assert result.returncode == 0
+        with open(received_file, "rb") as f:
+            assert f.read() == b"hello world\n"
+
+
 class TestDelete:
     def test_delete_removes_extra_files(self, shared_server):
         clean_dir(DEST_DIR)

tests/test_client_cli.c

@@ -282,8 +282,6 @@ static void test_parse_args_rejects_unimplemented_options() {
                                         "--list-only",
                                         "-h",
                                         "--human-readable",
-                                        "-u",
-                                        "--update",
                                         "--append",
                                         "--append-verify",
                                         "--delete-excluded",
@@ -323,6 +321,20 @@ static void test_parse_args_rejects_unimplemented_options() {
   }
 }
 
+static void test_parse_args_update() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "-u", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->update);
+  EXPECT_TRUE(cfg->use_metadata);
+  EXPECT_EQ_INT(positional_count, 2);
+
+  config_delete(cfg);
+}
+
 /* Test parse_args with --archive flag */
 static void test_parse_args_archive() {
   Config* cfg = config_create();
@@ -359,5 +371,6 @@ void test_client_cli() {
   test_parse_args_valid_compression_level();
   test_parse_args_unknown_option();
   test_parse_args_rejects_unimplemented_options();
+  test_parse_args_update();
   test_parse_args_archive();
 }