feat: add rsync-compatible chmod option

RSYNC_COMPAT.md

@@ -120,7 +120,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `-g`, `--group` | Preserve group | ✅ Implemented | Part of -M |
 | `-t`, `--times` | Preserve modification times | ✅ Implemented | Part of -M |
 | `-E`, `--executability` | Preserve executability | ❌ Not Implemented | |
-| `--chmod=CHMOD` | Affect file permissions | ❌ Not Implemented | |
+| `--chmod=CHMOD` | Affect file permissions | ✅ Implemented | Supports numeric and symbolic `ugo` `rwx` changes; retains receiver safety masking |
 | `-A`, `--acls` | Preserve ACLs | ❌ Not Implemented | Removed because it had no effect |
 | `-X`, `--xattrs` | Preserve extended attributes | ❌ Not Implemented | Removed because it had no effect |
 | `-H`, `--hard-links` | Preserve hard links | ❌ Not Implemented | Removed because it had no effect |

src/client/client_cli.c

@@ -1,5 +1,6 @@
 #include "client_send.h"
 #include "client_validation.h"
+#include "chmod.h"
 #include "config.h"
 #include "delta.h"
 #include "log.h"
@@ -153,6 +154,7 @@ 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)},
+    {"--chmod", NULL, OPT_STRING, offsetof(Config, chmod_spec)},
 
     {"--source-dir", NULL, OPT_STRING, offsetof(Config, send_directory)},
     {"--dest-dir", NULL, OPT_STRING, offsetof(Config, receive_root_directory)},
@@ -220,12 +222,32 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
         }
         if (apply_table_option(config, entry, argv[++i]) != 0)
           return -1;
+        if (entry->offset == offsetof(Config, chmod_spec)) {
+          mode_t ignored;
+          if (!chmod_apply(0, config->chmod_spec, &ignored)) {
+            log_message(LOG_LEVEL_ERROR, "--chmod has invalid permission changes");
+            return -1;
+          }
+          config->use_metadata = true;
+        }
       } else if (apply_table_option(config, entry, NULL) != 0) {
         return -1;
       }
       continue;
     }
 
+    if (strncmp(argv[i], "--chmod=", 8) == 0) {
+      if (set_string_option(&config->chmod_spec, argv[i] + 8, "--chmod") != 0)
+        return -1;
+      mode_t ignored;
+      if (!chmod_apply(0, config->chmod_spec, &ignored)) {
+        log_message(LOG_LEVEL_ERROR, "--chmod has invalid permission changes");
+        return -1;
+      }
+      config->use_metadata = true;
+      continue;
+    }
+
     if (opt_is(argv[i], "--help", NULL)) {
       print_usage();
       return 1;

src/client/usage.c

@@ -38,6 +38,7 @@ void print_usage(void) {
   printf("  -f                  Enable sendfile (TCP only, not with -c or -s)\n");
   printf("  -v, --verbose       Enable debug logging\n");
   printf("  -M, --preserve      Preserve file metadata\n");
+  printf("  --chmod <changes>   Modify transferred permissions (rsync syntax)\n");
   printf("  --chunk-size <n>    Chunk size in bytes (default: %d)\n", DEFAULT_CHUNK_SIZE);
   printf("  --source-dir <path> Source directory\n");
   printf("  --dest-dir <path>   Destination directory\n");

src/shared/chmod.c

@@ -0,0 +1,90 @@
+#include "chmod.h"
+#include <stddef.h>
+#include <string.h>
+
+static bool parse_clause(mode_t* mode, const char* begin, const char* end) {
+  const char* p = begin;
+  unsigned who = 0;
+  while (p < end && strchr("ugoa", *p)) {
+    if (*p == 'a')
+      who = 7;
+    else
+      who |= *p == 'u' ? 1U : (*p == 'g' ? 2U : 4U);
+    p++;
+  }
+  if (who == 0)
+    who = 7;
+  if (p == end || (*p != '+' && *p != '-' && *p != '='))
+    return false;
+  char operation = *p++;
+  mode_t bits = 0;
+  while (p < end) {
+    mode_t bit;
+    switch (*p++) {
+    case 'r':
+      bit = 4;
+      break;
+    case 'w':
+      bit = 2;
+      break;
+    case 'x':
+      bit = 1;
+      break;
+    default:
+      return false;
+    }
+    bits |= bit;
+  }
+  for (unsigned class_index = 0; class_index < 3; class_index++) {
+    unsigned class_bit = 1U << class_index;
+    if (!(who & class_bit))
+      continue;
+    mode_t shift = (mode_t)((2U - class_index) * 3U);
+    mode_t mask = (mode_t)(7U << shift);
+    mode_t class_bits = (mode_t)(bits << shift);
+    if (operation == '+')
+      *mode |= class_bits;
+    else if (operation == '-')
+      *mode &= ~class_bits;
+    else
+      *mode = (*mode & ~mask) | class_bits;
+  }
+  return true;
+}
+
+bool chmod_apply(mode_t mode, const char* spec, mode_t* result) {
+  if (!spec || !*spec || !result)
+    return false;
+  bool numeric = true;
+  size_t length = strlen(spec);
+  if (length > 4)
+    numeric = false;
+  for (size_t i = 0; i < length && numeric; i++)
+    numeric = spec[i] >= '0' && spec[i] <= '7';
+  if (numeric) {
+    if (length < 3 || (length == 4 && spec[0] != '0'))
+      return false;
+    mode_t parsed = 0;
+    for (size_t i = 0; i < length; i++)
+      parsed = (mode_t)((parsed << 3) | (spec[i] - '0'));
+    *result = (*result & ~07777) | parsed;
+    return true;
+  }
+
+  mode_t changed = mode;
+  const char* begin = spec;
+  while (*begin) {
+    const char* end = strchr(begin, ',');
+    if (!end)
+      end = begin + strlen(begin);
+    if (!parse_clause(&changed, begin, end))
+      return false;
+    if (*end == '\0')
+      break;
+    begin = end + 1;
+    if (!*begin)
+      return false;
+  }
+  *result = changed;
+  return true;
+}

src/shared/chmod.h

@@ -0,0 +1,10 @@
+#ifndef CHMOD_H
+#define CHMOD_H
+
+#include <stdbool.h>
+#include <sys/stat.h>
+
+/* Apply the supported rsync --chmod syntax to a permission mode. */
+bool chmod_apply(mode_t mode, const char* spec, mode_t* result);
+
+#endif

src/shared/config.c

@@ -1,4 +1,5 @@
 #include "config.h"
+#include "chmod.h"
 #include "delta.h"
 #include "log.h"
 #include "protocol.h"
@@ -98,6 +99,7 @@ static void config_set_defaults(Config* config) {
   config->server_mode = false;
   config->checksum = false;
   config->compress_choice = NULL;
+  config->chmod_spec = NULL;
 }
 
 static bool valid_wire_bool(int value) {
@@ -134,7 +136,9 @@ static bool validate_received_config(const Config* config) {
          config->chunk_size > 0 && config->chunk_size <= MAX_CHUNK_SIZE &&
          config->delta_block_size >= DELTA_BLOCK_SIZE_MIN &&
          config->delta_block_size <= DELTA_BLOCK_SIZE_MAX &&
-         config->delta_max_file_size <= DELTA_MAX_FILE_SIZE && config->max_delete >= 0;
+         config->delta_max_file_size <= DELTA_MAX_FILE_SIZE && config->max_delete >= 0 &&
+         (!config->chmod_spec || !*config->chmod_spec ||
+          chmod_apply(0, config->chmod_spec, &(mode_t){0}));
 }
 
 Config* config_create(void) {
@@ -208,6 +212,7 @@ void config_delete(Config* config) {
   free(config->bind_address);
   free(config->daemon_config);
   free(config->compress_choice);
+  free(config->chmod_spec);
   if (config->filters) {
     array_list_delete(config->filters);
   }
@@ -253,7 +258,8 @@ static bool send_resume_options(int fd, const Config* c) {
   return send_str(fd, c->temp_dir ? c->temp_dir : "") && send_int(fd, c->partial) &&
          send_str(fd, c->partial_dir ? c->partial_dir : "") &&
          send_str(fd, c->suffix ? c->suffix : "") && send_int(fd, c->delete_before) &&
-         send_int(fd, c->checksum) && send_str(fd, c->compress_choice ? c->compress_choice : "");
+         send_int(fd, c->checksum) && send_str(fd, c->compress_choice ? c->compress_choice : "") &&
+         send_str(fd, c->chmod_spec ? c->chmod_spec : "");
 }
 
 static bool receive_core_fields(int fd, Config* c) {
@@ -330,7 +336,10 @@ static bool receive_resume_options(int fd, Config* c) {
   if (!receive_wire_bool(fd, &c->checksum))
     return false;
   c->compress_choice = receive_str(fd);
-  return c->compress_choice != NULL;
+  if (!c->compress_choice)
+    return false;
+  c->chmod_spec = receive_str(fd);
+  return c->chmod_spec != NULL;
 }
 
 bool config_send(int file_descriptor, const Config* config) {

src/shared/config.h

@@ -126,9 +126,10 @@ typedef struct Config {
 
   // PR #184: Compression algorithm negotiation
   char* compress_choice;
+  char* chmod_spec;
 } Config;
 
-#define PROTOCOL_VERSION "2.2.0"
+#define PROTOCOL_VERSION "2.3.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 
 Config* config_create(void);

src/shared/file_receive.c

@@ -8,6 +8,7 @@
 #include <unistd.h>
 
 #include "array_list.h"
+#include "chmod.h"
 #include "compression.h"
 #include "config.h"
 #include "data.h"
@@ -103,8 +104,16 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
     }
   }
 
-  bool ok = file_to_disk_secure(disk_path, file->data->data, file->data->size, inplace, sparse,
-                                file->metadata);
+  FileMetadata adjusted_metadata;
+  const FileMetadata* metadata = file->metadata;
+  if (metadata && config && config->chmod_spec && *config->chmod_spec) {
+    adjusted_metadata = *metadata;
+    if (!chmod_apply(adjusted_metadata.mode, config->chmod_spec, &adjusted_metadata.mode))
+      goto fail;
+    metadata = &adjusted_metadata;
+  }
+  bool ok =
+      file_to_disk_secure(disk_path, file->data->data, file->data->size, inplace, sparse, metadata);
   free(parent_copy);
   free(backup_path);
   free(confined_backup);

tests/integration/test_features.py

@@ -52,6 +52,22 @@ class TestArchiveMode:
         assert not mismatches, f"Mismatch: {mismatches}"
 
 
+class TestChmod:
+    def test_chmod_applies_to_transferred_files(self, shared_server):
+        clean_dir(DEST_DIR)
+        source_file = os.path.join(SOURCE_DIR, "small.txt")
+        os.chmod(source_file, 0o777)
+        result, dur = run_client(
+            SOURCE_DIR, DEST_DIR,
+            flags=["--chmod=u=rw,go=r"],
+            port=shared_server.port,
+        )
+        if result.returncode != 0:
+            pytest.fail(f"Exit {result.returncode}: {(result.stderr or result.stdout)[:200]}")
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        assert (os.stat(os.path.join(received, "small.txt")).st_mode & 0o777) == 0o644
+
+
 class TestExclude:
     def test_exclude_single(self, shared_server):
         clean_dir(DEST_DIR)

tests/test_client_cli.c

@@ -1,5 +1,6 @@
 #include "test_client_cli.h"
 #include "client_validation.h"
+#include "chmod.h"
 #include "config.h"
 #include "test_utils.h"
 #include "utils.h"
@@ -181,6 +182,29 @@ static void test_parse_args_valid_port() {
   config_delete(cfg);
 }
 
+static void test_parse_args_chmod() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--chmod=u=rw,go=r", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_EQ_STR(cfg->chmod_spec, "u=rw,go=r");
+  EXPECT_TRUE(cfg->use_metadata);
+  mode_t result;
+  EXPECT_TRUE(chmod_apply(0777, cfg->chmod_spec, &result));
+  EXPECT_EQ_INT(result, 0644);
+  config_delete(cfg);
+}
+
+static void test_parse_args_rejects_invalid_chmod() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--chmod=a+X", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), -1);
+  config_delete(cfg);
+}
+
 /* Test parse_args rejects port > 65535 */
 static void test_parse_args_invalid_port() {
   Config* cfg = config_create();
@@ -352,6 +376,8 @@ void test_client_cli() {
   test_parse_args_help();
   test_parse_args_version();
   test_parse_args_valid_port();
+  test_parse_args_chmod();
+  test_parse_args_rejects_invalid_chmod();
   test_parse_args_invalid_port();
   test_parse_args_non_numeric_port();
   test_parse_args_invalid_server_port();

tests/test_metadata.c

@@ -1,4 +1,5 @@
 #include "test_metadata.h"
+#include "chmod.h"
 #include "metadata.h"
 #include "protocol.h"
 #include "test_utils.h"
@@ -144,6 +145,18 @@ static void test_file_restore_metadata() {
   unlink(path);
 }
 
+static void test_chmod_changes() {
+  mode_t result;
+  EXPECT_TRUE(chmod_apply(0777, "u=rw,go=r", &result));
+  EXPECT_EQ_INT(result, 0644);
+  EXPECT_TRUE(chmod_apply(0644, "a+x", &result));
+  EXPECT_EQ_INT(result, 0755);
+  EXPECT_TRUE(chmod_apply(0777, "755", &result));
+  EXPECT_EQ_INT(result, 0755);
+  EXPECT_FALSE(chmod_apply(0777, "a+X", &result));
+  EXPECT_FALSE(chmod_apply(0777, "a+r,", &result));
+}
+
 void test_metadata() {
   test_metadata_to_from_buf_roundtrip();
   test_metadata_to_buf_null();
@@ -152,4 +165,5 @@ void test_metadata() {
   test_metadata_send_null();
   test_metadata_rejects_invalid_values();
   test_file_restore_metadata();
+  test_chmod_changes();
 }