Add rsync-compatible ignore-existing option

src/client/client_cli.c

@@ -153,6 +153,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--sparse", "-S", OPT_FLAG, offsetof(Config, preserve_sparse)},
     {"--inplace", NULL, OPT_FLAG, offsetof(Config, inplace)},
     {"--checksum", NULL, OPT_FLAG, offsetof(Config, checksum)},
+    {"--ignore-existing", NULL, OPT_FLAG, offsetof(Config, ignore_existing)},
 
     {"--source-dir", NULL, OPT_STRING, offsetof(Config, send_directory)},
     {"--dest-dir", NULL, OPT_STRING, offsetof(Config, receive_root_directory)},

src/client/usage.c

@@ -21,6 +21,7 @@ void print_usage(void) {
   printf("  -p <port>           SSH port (default: 22)\n");
   printf("  --progress          Show transfer progress\n");
   printf("  --delete            Delete files on receiver not in source\n");
+  printf("  --ignore-existing  Skip files that already exist on receiver\n");
   printf("  --exclude <pattern> Exclude files matching pattern\n");
   printf("  --include <pattern> Only include files matching pattern\n");
   printf("  --exclude-from <file> Read exclude patterns from file\n");

src/shared/config.c

@@ -68,6 +68,7 @@ static void config_set_defaults(Config* config) {
   config->debug_level = 0;
   config->list_only = false;
   config->human_readable = false;
+  config->ignore_existing = false;
   config->update = false;
   config->inplace = false;
   config->append = false;
@@ -123,12 +124,13 @@ static bool validate_received_config(const Config* config) {
          valid_wire_bool(config->copy_unsafe_links) &&
          valid_wire_bool(config->preserve_hard_links) && valid_wire_bool(config->preserve_acls) &&
          valid_wire_bool(config->preserve_xattrs) && valid_wire_bool(config->preserve_devices) &&
-         valid_wire_bool(config->preserve_sparse) && valid_wire_bool(config->update) &&
-         valid_wire_bool(config->inplace) && valid_wire_bool(config->append) &&
-         valid_wire_bool(config->append_verify) && valid_wire_bool(config->delete_excluded) &&
-         valid_wire_bool(config->delete_after) && valid_wire_bool(config->relative) &&
-         valid_wire_bool(config->prune_empty_dirs) && valid_wire_bool(config->partial) &&
-         valid_wire_bool(config->delete_before) && valid_wire_bool(config->checksum) &&
+         valid_wire_bool(config->preserve_sparse) && valid_wire_bool(config->ignore_existing) &&
+         valid_wire_bool(config->update) && valid_wire_bool(config->inplace) &&
+         valid_wire_bool(config->append) && valid_wire_bool(config->append_verify) &&
+         valid_wire_bool(config->delete_excluded) && valid_wire_bool(config->delete_after) &&
+         valid_wire_bool(config->relative) && valid_wire_bool(config->prune_empty_dirs) &&
+         valid_wire_bool(config->partial) && valid_wire_bool(config->delete_before) &&
+         valid_wire_bool(config->checksum) &&
          (!config->use_compression ||
           (config->compression_level >= 1 && config->compression_level <= 22)) &&
          config->chunk_size > 0 && config->chunk_size <= MAX_CHUNK_SIZE &&
@@ -243,10 +245,11 @@ static bool send_file_options(int fd, const Config* c) {
 }
 
 static bool send_selection_options(int fd, const Config* c) {
-  return send_int(fd, c->update) && send_int(fd, c->inplace) && send_int(fd, c->append) &&
-         send_int(fd, c->append_verify) && send_int(fd, c->delete_excluded) &&
-         send_int(fd, c->delete_after) && send_n_data(fd, &c->max_delete, sizeof(c->max_delete)) &&
-         send_int(fd, c->relative) && send_int(fd, c->prune_empty_dirs);
+  return send_int(fd, c->ignore_existing) && send_int(fd, c->update) && send_int(fd, c->inplace) &&
+         send_int(fd, c->append) && send_int(fd, c->append_verify) &&
+         send_int(fd, c->delete_excluded) && send_int(fd, c->delete_after) &&
+         send_n_data(fd, &c->max_delete, sizeof(c->max_delete)) && send_int(fd, c->relative) &&
+         send_int(fd, c->prune_empty_dirs);
 }
 
 static bool send_resume_options(int fd, const Config* c) {
@@ -304,8 +307,8 @@ static bool receive_file_options(int fd, Config* c) {
 }
 
 static bool receive_selection_options(int fd, Config* c) {
-  bool* flags[] = {&c->update,        &c->inplace,         &c->append,
-                   &c->append_verify, &c->delete_excluded, &c->delete_after};
+  bool* flags[] = {&c->ignore_existing, &c->update,          &c->inplace,     &c->append,
+                   &c->append_verify,   &c->delete_excluded, &c->delete_after};
   for (size_t i = 0; i < sizeof(flags) / sizeof(flags[0]); i++) {
     if (!receive_wire_bool(fd, flags[i]))
       return false;

src/shared/config.h

@@ -76,6 +76,7 @@ typedef struct Config {
   bool human_readable;
 
   // Issue #127: Transfer modes
+  bool ignore_existing;
   bool update;
   bool inplace;
   bool append;

src/shared/file_receive.c

@@ -61,6 +61,21 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
     return false;
   }
 
+  /* --ignore-existing checks the final destination before partial files or
+     overwrite policies can modify it. */
+  if (config && config->ignore_existing) {
+    struct stat destination_stat;
+    char* final_path = path_cat(root_directory, file->path);
+    bool exists = final_path && file_stat_secure(final_path, &destination_stat);
+    free(final_path);
+    if (exists) {
+      free(confined_backup);
+      free(confined_partial);
+      free(disk_path);
+      return true;
+    }
+  }
+
   /* --update is receiver-side policy: never replace a newer destination. */
   if (config && config->update) {
     struct stat destination_stat;

tests/integration/test_features.py

@@ -214,6 +214,29 @@ class TestIncremental:
             assert f.read() == b"hello world\n"
 
 
+class TestIgnoreExisting:
+    def test_ignore_existing_preserves_existing_and_transfers_new(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, port=shared_server.port)
+        assert result.returncode == 0
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        existing_file = os.path.join(received, "small.txt")
+        with open(existing_file, "wb") as f:
+            f.write(b"destination content\n")
+        new_source = os.path.join(SOURCE_DIR, "new.txt")
+        with open(new_source, "wb") as f:
+            f.write(b"new file\n")
+
+        result, _ = run_client(SOURCE_DIR, DEST_DIR,
+                               flags=["--ignore-existing"], port=shared_server.port)
+        assert result.returncode == 0, f"Sync failed: {(result.stderr or result.stdout)[:200]}"
+        with open(existing_file, "rb") as f:
+            assert f.read() == b"destination content\n"
+        with open(os.path.join(received, "new.txt"), "rb") as f:
+            assert f.read() == b"new file\n"
+
+
 class TestDelete:
     def test_delete_removes_extra_files(self, shared_server):
         clean_dir(DEST_DIR)

tests/test_client_cli.c

@@ -181,6 +181,19 @@ static void test_parse_args_valid_port() {
   config_delete(cfg);
 }
 
+static void test_parse_args_ignore_existing() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--ignore-existing", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 4, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, 0);
+  EXPECT_TRUE(cfg->ignore_existing);
+
+  config_delete(cfg);
+}
+
 /* Test parse_args rejects port > 65535 */
 static void test_parse_args_invalid_port() {
   Config* cfg = config_create();
@@ -352,6 +365,7 @@ void test_client_cli() {
   test_parse_args_help();
   test_parse_args_version();
   test_parse_args_valid_port();
+  test_parse_args_ignore_existing();
   test_parse_args_invalid_port();
   test_parse_args_non_numeric_port();
   test_parse_args_invalid_server_port();

tests/test_config.c

@@ -126,6 +126,7 @@ static void test_config_send_receive() {
   send_cfg->use_metadata = true;
   send_cfg->compression_level = 5;
   send_cfg->chunk_size = 1024;
+  send_cfg->ignore_existing = true;
 
   /* Use socketpair for bidirectional communication */
   int p[2];
@@ -160,6 +161,8 @@ static void test_config_send_receive() {
         ok = false;
       if (recv_cfg->chunk_size != 1024)
         ok = false;
+      if (!recv_cfg->ignore_existing)
+        ok = false;
     }
     config_delete(recv_cfg);
     close(p[0]);

tests/test_file.c

@@ -88,6 +88,38 @@ static void test_file_save_to_disk() {
   rmdir("test_save_tmp");
 }
 
+static void test_file_save_to_disk_ignore_existing() {
+  const char* path = "test_ignore_existing_tmp/existing.txt";
+  EXPECT_TRUE(file_write_to_disk(path, "old", 3, false, false));
+
+  File* file = file_create("existing.txt");
+  EXPECT_NOT_NULL(file);
+  file->data->data = malloc(3);
+  EXPECT_NOT_NULL(file->data->data);
+  memcpy(file->data->data, "new", 3);
+  file->data->size = 3;
+
+  Config* config = config_create();
+  EXPECT_NOT_NULL(config);
+  config->ignore_existing = true;
+  EXPECT_TRUE(file_save_to_disk("test_ignore_existing_tmp", file, config));
+
+  FILE* stream = fopen(path, "rb");
+  char content[4] = {0};
+  EXPECT_NOT_NULL(stream);
+  // cppcheck-suppress knownConditionTrueFalse
+  if (stream) {
+    EXPECT_EQ_INT((int)fread(content, 1, 3, stream), 3);
+    fclose(stream);
+  }
+  EXPECT_EQ_STR(content, "old");
+
+  file_destroy(file);
+  config_delete(config);
+  unlink(path);
+  rmdir("test_ignore_existing_tmp");
+}
+
 static void test_file_write_to_disk_basic() {
   const char* content = "Basic file_write_to_disk test";
   EXPECT_TRUE(file_write_to_disk("test_file_write_to_disk_basic.txt", content, strlen(content),
@@ -458,6 +490,7 @@ void test_file() {
   test_file_load_data();
   test_file_load_data_missing_file();
   test_file_save_to_disk();
+  test_file_save_to_disk_ignore_existing();
   test_file_write_to_disk_basic();
   test_file_write_to_disk_creates_dirs();
   test_file_write_to_disk_does_not_follow_symlink();