src/client/client_cli.c

@@ -2612,6 +2612,15 @@ static int cli_finalize_config(Config* config, bool verbose, bool no_delta, bool
   if (config->use_delete && !config->delete_before && !config->delete_during &&
       !config->delete_delay && !config->delete_after)
     config->delete_during = true;
+  /* rsync parity: --partial-dir=DIR chooses where an interrupted transfer's
+     partial file is kept, so it implies --partial.  rsync applies the
+     implication after option parsing, so it wins over an explicit --no-partial
+     regardless of the order the two options appear in (verified on rsync
+     3.4.1).  --inplace is the exception: the destination file is written in
+     place with no partial/temp staging, so the partial machinery is bypassed
+     and the implication is skipped to leave --inplace behavior untouched. */
+  if (config->partial_dir && !config->inplace)
+    config->partial = true;
   if (config->compress_choice) {
     int algo = compression_algo_from_name(config->compress_choice);
     if (algo >= 0) {

src/client/usage.c

@@ -288,7 +288,7 @@ void print_usage(void) {
   printf("  --log-file <path>, --log-file=<path>  Write log messages to file\n");
   printf("  --stderr=MODE       Route logging to stderr: errors or all\n");
   printf("  --partial           Keep partial files on interrupted transfer\n");
-  printf("  --partial-dir <dir> Directory for partial files\n");
+  printf("  --partial-dir <dir> Directory for partial files (implies --partial)\n");
   printf("  -T, --temp-dir <dir>  Scratch dir for temp files before atomic install.\n");
   printf("                      Confined to the receive root: a relative dir resolves below\n");
   printf("                      it and an absolute/traversal dir is rejected.  The dir must\n");

tests/integration/test_features.py

@@ -2275,6 +2275,36 @@ class TestPartialDir:
         partial = os.path.join(dest, ".partial", os.path.relpath(source_file, os.path.sep))
         assert not os.path.exists(partial)
 
+    def test_partial_dir_alone_implies_partial(self, shared_server):
+        """--partial-dir=DIR with no --partial implies --partial, like rsync.
+
+        rsync 3.4.1 retains the staged partial when --partial-dir is given by
+        itself; before the implication was added FastSync discarded it.  The
+        transfer is made to fail deterministically by placing a non-empty
+        directory at the destination path, so the final partial-dir ->
+        destination rename fails and whatever was staged under the partial dir
+        stays on disk."""
+        source = os.path.join(TEST_DATA_DIR, "partial_dir_implied_src")
+        dest = os.path.join(TEST_DATA_DIR, "partial_dir_implied_dst")
+        clean_dir(source)
+        clean_dir(dest)
+        source_file = os.path.join(source, "f.bin")
+        with open(source_file, "wb") as f:
+            f.write(b"partial payload")
+
+        received = get_dest_received_dir(dest, source)
+        os.makedirs(os.path.join(received, "f.bin"))
+        with open(os.path.join(received, "f.bin", "keep"), "wb") as f:
+            f.write(b"keep")
+
+        result, _ = run_client(source, dest, flags=["--partial-dir=.partial"],
+                               port=shared_server.port)
+        assert result.returncode != 0, "expected the blocked install to fail"
+
+        partial = os.path.join(dest, ".partial", os.path.relpath(source_file, os.path.sep))
+        assert os.path.exists(partial), \
+            "--partial-dir alone must imply --partial and retain the partial file"
+
 
 class TestLargeFile:
     def test_transfer_100mb_file(self, shared_server):

tests/test_client_cli.c

@@ -511,6 +511,61 @@ static void test_parse_args_ignore_existing() {
   config_delete(cfg);
 }
 
+/* --partial-dir=DIR implies --partial, matching rsync 3.4.1.  rsync resolves
+ * this after option parsing, so the implication wins over an explicit
+ * --no-partial in either order.  It is skipped under --inplace, where partial
+ * staging is bypassed and the destination is written in place. */
+static void test_parse_args_partial_dir_implies_partial() {
+  {
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--partial-dir=.partial", "/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->partial);
+    config_delete(cfg);
+  }
+  {
+    /* Explicit --no-partial before --partial-dir: --partial-dir still wins. */
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--no-partial", "--partial-dir=.partial", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), 0);
+    EXPECT_TRUE(cfg->partial);
+    config_delete(cfg);
+  }
+  {
+    /* Reversed order must not change the precedence. */
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--partial-dir=.partial", "--no-partial", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), 0);
+    EXPECT_TRUE(cfg->partial);
+    config_delete(cfg);
+  }
+  {
+    /* --inplace bypasses partial staging: the implication must not fire. */
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--inplace", "--partial-dir=.partial", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), 0);
+    EXPECT_FALSE(cfg->partial);
+    config_delete(cfg);
+  }
+  {
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--no-partial", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+    EXPECT_FALSE(cfg->partial);
+    config_delete(cfg);
+  }
+}
+
 static void test_parse_args_executability() {
   Config* cfg = config_create();
   char* argv[] = {"fastsync", "-E", "/src", "/dst"};
@@ -4784,6 +4839,7 @@ void test_client_cli() {
   test_parse_args_valid_port();
   test_parse_args_size_only();
   test_parse_args_ignore_existing();
+  test_parse_args_partial_dir_implies_partial();
   test_parse_args_executability();
   test_parse_args_chmod();
   test_parse_args_numeric_chmod();