docs: update existing compatibility status

README.md

@@ -102,6 +102,7 @@ partial, alternate, and planned behavior.
 | `--max-size <n>` | Skip files larger than n bytes |
 | `--min-size <n>` | Skip files smaller than n bytes |
 | `--incremental` | Skip files unchanged since last transfer (size + mtime). Auto-enables `--preserve`. Incompatible with `-s`. |
+| `--existing` | Skip files not already present at the destination; update existing files normally. |
 | `--bwlimit <KB/s>` | Bandwidth limit in kilobytes per second |
 | `--chunk-size <n>` | Chunk size in bytes (default: 10485760) |
 | `--timeout <sec>` | I/O timeout in seconds (default: 30) |
@@ -465,7 +466,7 @@ defaults to the current directory. |
 
 ## Protocol and Security
 
-FastSync protocol version `2.2.0` is shared by the client and server. The
+FastSync protocol version `2.3.0` is shared by the client and server. The
 current protocol is sender-driven and includes configuration negotiation,
 incremental checks, checksums, manifests, keep-alives, abort handling, and
 FastSync-native delta messages. Client and server versions must currently

RSYNC_COMPAT.md

@@ -6,11 +6,11 @@ This document maps rsync's full feature set to FastSync's current implementation
 
 | Status | Count | Description |
 |--------|-------|-------------|
-| ✅ Implemented | 34 | Feature works end-to-end |
+| ✅ Implemented | 35 | Feature works end-to-end |
 | 🔀 Alt Arg | 3 | Functionality exists but under different flag/semantics |
-| ⚠️ Partial | 1 | Flag parsed/stored but behavior incomplete |
+| ⚠️ Partial | 3 | Flag parsed/stored but behavior incomplete |
-| ❌ Not Implemented | 98 | Flag not recognized or no behavior |
+| ❌ Not Implemented | 97 | Flag not recognized or no behavior |
-| **Total** | **136** | |
+| **Total** | **138** | |
 
 ---
 
@@ -60,7 +60,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `-I`, `--ignore-times` | Don't skip files matching size+time | ❌ Not Implemented | |
 | `--size-only` | Skip based on size only | ❌ Not Implemented | |
 | `-@`, `--modify-window=NUM` | Mod-time comparison accuracy | ❌ Not Implemented | |
-| `--existing` | Skip creating new files on receiver | ❌ Not Implemented | |
+| `--existing` | Skip creating new files on receiver | ✅ Implemented | Existing destination files continue through normal update handling |
 | `--ignore-existing` | Skip updating existing files | ❌ Not Implemented | |
 | `--remove-source-files` | Sender removes synced files | ❌ Not Implemented | |
 
@@ -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 | ✅ Implemented | Supports numeric and symbolic `ugo` `rwx` changes; retains receiver safety masking |
+| `--chmod=CHMOD` | Affect file permissions | ❌ Not Implemented | |
 | `-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 |
@@ -247,7 +247,7 @@ Ranked by user demand, implementation complexity, and interoperability impact:
 | 1 | `--whole-file` / `-W` | Low | High — users expect opt-out of delta |
 | 2 | `--ignore-times` / `-I` | Low | Medium — useful for forcing re-transfer |
 | 3 | `--size-only` | Low | Medium — common migration scenario |
-| 4 | `--existing` / `--ignore-existing` | Low | Medium — common sync patterns |
+| 4 | `--ignore-existing` | Low | Medium — common sync patterns |
 | 5 | `--remove-source-files` | Low | High — common for moves/backup |
 | 6 | `--delete-during` | Medium | High — performance improvement |
 | 7 | `--delay-updates` | Medium | High — atomic updates |

src/client/client_cli.c

@@ -1,6 +1,5 @@
 #include "client_send.h"
 #include "client_validation.h"
-#include "chmod.h"
 #include "config.h"
 #include "delta.h"
 #include "log.h"
@@ -154,7 +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)},
-    {"--chmod", NULL, OPT_STRING, offsetof(Config, chmod_spec)},
+    {"--existing", NULL, OPT_FLAG, offsetof(Config, existing)},
 
     {"--source-dir", NULL, OPT_STRING, offsetof(Config, send_directory)},
     {"--dest-dir", NULL, OPT_STRING, offsetof(Config, receive_root_directory)},
@@ -222,32 +221,12 @@ 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

@@ -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("  --existing          Skip files not already present at destination\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);
@@ -38,7 +39,6 @@ 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

@@ -1,90 +0,0 @@
-#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 == 0 || length > 4)
-      return false;
-    mode_t parsed = 0;
-    for (size_t i = 0; i < length; i++)
-      parsed = (mode_t)((parsed << 3) | (spec[i] - '0'));
-    *result = 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

@@ -1,10 +0,0 @@
-#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

@@ -1,5 +1,4 @@
 #include "config.h"
-#include "chmod.h"
 #include "delta.h"
 #include "log.h"
 #include "protocol.h"
@@ -69,6 +68,7 @@ static void config_set_defaults(Config* config) {
   config->debug_level = 0;
   config->list_only = false;
   config->human_readable = false;
+  config->existing = false;
   config->update = false;
   config->inplace = false;
   config->append = false;
@@ -99,7 +99,6 @@ 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) {
@@ -125,20 +124,19 @@ 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->preserve_sparse) && valid_wire_bool(config->existing) &&
-         valid_wire_bool(config->inplace) && valid_wire_bool(config->append) &&
+         valid_wire_bool(config->update) && valid_wire_bool(config->inplace) &&
-         valid_wire_bool(config->append_verify) && valid_wire_bool(config->delete_excluded) &&
+         valid_wire_bool(config->append) && valid_wire_bool(config->append_verify) &&
-         valid_wire_bool(config->delete_after) && valid_wire_bool(config->relative) &&
+         valid_wire_bool(config->delete_excluded) && valid_wire_bool(config->delete_after) &&
-         valid_wire_bool(config->prune_empty_dirs) && valid_wire_bool(config->partial) &&
+         valid_wire_bool(config->relative) && valid_wire_bool(config->prune_empty_dirs) &&
-         valid_wire_bool(config->delete_before) && valid_wire_bool(config->checksum) &&
+         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 &&
          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) {
@@ -212,7 +210,6 @@ 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);
   }
@@ -248,18 +245,18 @@ 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) &&
+  return send_int(fd, c->existing) && send_int(fd, c->update) && send_int(fd, c->inplace) &&
-         send_int(fd, c->append_verify) && send_int(fd, c->delete_excluded) &&
+         send_int(fd, c->append) && send_int(fd, c->append_verify) &&
-         send_int(fd, c->delete_after) && send_n_data(fd, &c->max_delete, sizeof(c->max_delete)) &&
+         send_int(fd, c->delete_excluded) && send_int(fd, c->delete_after) &&
-         send_int(fd, c->relative) && send_int(fd, c->prune_empty_dirs);
+         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) {
   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) {
@@ -310,7 +307,7 @@ 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,
+  bool* flags[] = {&c->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]))
@@ -336,10 +333,7 @@ static bool receive_resume_options(int fd, Config* c) {
   if (!receive_wire_bool(fd, &c->checksum))
     return false;
   c->compress_choice = receive_str(fd);
-  if (!c->compress_choice)
+  return c->compress_choice != NULL;
-    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

@@ -76,6 +76,7 @@ typedef struct Config {
   bool human_readable;
 
   // Issue #127: Transfer modes
+  bool existing;
   bool update;
   bool inplace;
   bool append;
@@ -126,7 +127,6 @@ typedef struct Config {
 
   // PR #184: Compression algorithm negotiation
   char* compress_choice;
-  char* chmod_spec;
 } Config;
 
 #define PROTOCOL_VERSION "2.3.0"

src/shared/file_receive.c

@@ -8,7 +8,6 @@
 #include <unistd.h>
 
 #include "array_list.h"
-#include "chmod.h"
 #include "compression.h"
 #include "config.h"
 #include "data.h"
@@ -30,6 +29,7 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
   const char* backup_dir = (config && config->backup_dir) ? config->backup_dir : NULL;
   const char* partial_dir = (config && config->partial_dir) ? config->partial_dir : NULL;
   char *confined_backup = NULL, *confined_partial = NULL, *disk_path = NULL;
+  char* destination_path = NULL;
   char *backup_path = NULL, *parent_copy = NULL;
 
   if (!file || !file->path || !file->data || (file->data->size != 0 && !file->data->data) ||
@@ -55,13 +55,27 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
 
   const char* actual_root =
       (partial_dir && config && config->partial) ? confined_partial : root_directory;
+  destination_path = path_cat(root_directory, file->path);
   disk_path = path_cat(actual_root, file->path);
-  if (disk_path == NULL) {
+  if (destination_path == NULL || disk_path == NULL) {
     free(confined_backup);
     free(confined_partial);
+    free(destination_path);
+    free(disk_path);
     return false;
   }
 
+  /* --existing checks the final destination, not a temporary partial path. */
+  if (config && config->existing && !file_path_exists_secure(destination_path)) {
+    free(confined_backup);
+    free(confined_partial);
+    free(destination_path);
+    free(disk_path);
+    return true;
+  }
+  free(destination_path);
+  destination_path = NULL;
+
   /* --update is receiver-side policy: never replace a newer destination. */
   if (config && config->update) {
     struct stat destination_stat;
@@ -69,6 +83,7 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
         destination_stat.st_mtime > file->metadata->mtime_sec) {
       free(confined_backup);
       free(confined_partial);
+      free(destination_path);
       free(disk_path);
       return true;
     }
@@ -104,20 +119,13 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
     }
   }
 
-  FileMetadata adjusted_metadata;
+  bool ok = file_to_disk_secure(disk_path, file->data->data, file->data->size, inplace, sparse,
-  const FileMetadata* metadata = file->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);
   free(confined_partial);
+  free(destination_path);
   free(disk_path);
   return ok;
 
@@ -126,6 +134,7 @@ fail:
   free(backup_path);
   free(confined_backup);
   free(confined_partial);
+  free(destination_path);
   free(disk_path);
   return false;
 }

tests/integration/test_features.py

@@ -52,22 +52,6 @@ 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)
@@ -230,6 +214,34 @@ class TestIncremental:
             assert f.read() == b"hello world\n"
 
 
+class TestExisting:
+    def test_existing_updates_existing_and_skips_new(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0, f"Initial sync failed: {(result.stderr or result.stdout)[:200]}"
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        source_file = os.path.join(SOURCE_DIR, "small.txt")
+        new_source_file = os.path.join(SOURCE_DIR, "new-existing-test.txt")
+        with open(source_file, "wb") as f:
+            f.write(b"updated existing content\n")
+        with open(new_source_file, "wb") as f:
+            f.write(b"this file must not be created\n")
+
+        try:
+            result, _ = run_client(SOURCE_DIR, DEST_DIR,
+                                   flags=["-M", "--existing"], port=shared_server.port)
+            assert result.returncode == 0, f"--existing sync failed: {(result.stderr or result.stdout)[:200]}"
+
+            with open(os.path.join(received, "small.txt"), "rb") as f:
+                assert f.read() == b"updated existing content\n"
+            assert not os.path.exists(os.path.join(received, "new-existing-test.txt"))
+        finally:
+            os.unlink(new_source_file)
+            with open(source_file, "wb") as f:
+                f.write(b"hello world\n")
+
+
 class TestDelete:
     def test_delete_removes_extra_files(self, shared_server):
         clean_dir(DEST_DIR)

tests/test_client_cli.c

@@ -1,6 +1,5 @@
 #include "test_client_cli.h"
 #include "client_validation.h"
-#include "chmod.h"
 #include "config.h"
 #include "test_utils.h"
 #include "utils.h"
@@ -182,40 +181,6 @@ 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_numeric_chmod() {
-  Config* cfg = config_create();
-  char* argv[] = {"fastsync", "--chmod", "7777", "/src", "/dst"};
-  int positional_args[2];
-  int positional_count = 0;
-  EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), 0);
-  EXPECT_EQ_STR(cfg->chmod_spec, "7777");
-  EXPECT_TRUE(cfg->use_metadata);
-  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();
@@ -374,6 +339,17 @@ static void test_parse_args_archive() {
   config_delete(cfg);
 }
 
+static void test_parse_args_existing() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--existing", "/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->existing);
+  config_delete(cfg);
+}
+
 void test_client_cli() {
   test_validate_config_required_paths();
   test_validate_config_incompatible_options();
@@ -387,9 +363,6 @@ void test_client_cli() {
   test_parse_args_help();
   test_parse_args_version();
   test_parse_args_valid_port();
-  test_parse_args_chmod();
-  test_parse_args_numeric_chmod();
-  test_parse_args_rejects_invalid_chmod();
   test_parse_args_invalid_port();
   test_parse_args_non_numeric_port();
   test_parse_args_invalid_server_port();
@@ -398,4 +371,5 @@ void test_client_cli() {
   test_parse_args_unknown_option();
   test_parse_args_rejects_unimplemented_options();
   test_parse_args_archive();
+  test_parse_args_existing();
 }

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->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->existing)
+        ok = false;
     }
     config_delete(recv_cfg);
     close(p[0]);

tests/test_file.c

@@ -88,6 +88,50 @@ static void test_file_save_to_disk() {
   rmdir("test_save_tmp");
 }
 
+static void test_file_save_to_disk_existing() {
+  const char* root = "test_existing_tmp";
+  const char* existing_path = "test_existing_tmp/existing.txt";
+  const char* missing_path = "test_existing_tmp/missing.txt";
+  EXPECT_TRUE(file_write_to_disk(existing_path, "old", 3, false, false));
+
+  Config* cfg = config_create();
+  EXPECT_NOT_NULL(cfg);
+  cfg->existing = true;
+
+  File* existing = file_create("existing.txt");
+  EXPECT_NOT_NULL(existing);
+  existing->data->data = malloc(3);
+  EXPECT_NOT_NULL(existing->data->data);
+  memcpy(existing->data->data, "new", 3);
+  existing->data->size = 3;
+  EXPECT_TRUE(file_save_to_disk(root, existing, cfg));
+  file_destroy(existing);
+
+  File* missing = file_create("missing.txt");
+  EXPECT_NOT_NULL(missing);
+  missing->data->data = malloc(7);
+  EXPECT_NOT_NULL(missing->data->data);
+  memcpy(missing->data->data, "skipped", 7);
+  missing->data->size = 7;
+  EXPECT_TRUE(file_save_to_disk(root, missing, cfg));
+  file_destroy(missing);
+
+  FILE* fp = fopen(existing_path, "rb");
+  char content[4] = {0};
+  EXPECT_NOT_NULL(fp);
+  // cppcheck-suppress knownConditionTrueFalse
+  if (fp) {
+    EXPECT_EQ_INT((int)fread(content, 1, 3, fp), 3);
+    fclose(fp);
+  }
+  EXPECT_EQ_STR(content, "new");
+  EXPECT_EQ_INT(access(missing_path, F_OK), -1);
+
+  config_delete(cfg);
+  unlink(existing_path);
+  rmdir(root);
+}
+
 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 +502,7 @@ void test_file() {
   test_file_load_data();
   test_file_load_data_missing_file();
   test_file_save_to_disk();
+  test_file_save_to_disk_existing();
   test_file_write_to_disk_basic();
   test_file_write_to_disk_creates_dirs();
   test_file_write_to_disk_does_not_follow_symlink();

tests/test_metadata.c

@@ -1,5 +1,4 @@
 #include "test_metadata.h"
-#include "chmod.h"
 #include "metadata.h"
 #include "protocol.h"
 #include "test_utils.h"
@@ -145,27 +144,6 @@ 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);
-  result = 0777;
-  EXPECT_TRUE(chmod_apply(0777, "0000", &result));
-  EXPECT_EQ_INT(result, 0000);
-  result = 0777;
-  EXPECT_TRUE(chmod_apply(0777, "7777", &result));
-  EXPECT_EQ_INT(result, 07777);
-  result = 0777;
-  EXPECT_TRUE(chmod_apply(0777, "755", &result));
-  EXPECT_EQ_INT(result, 0755);
-  EXPECT_FALSE(chmod_apply(0777, "888", &result));
-  EXPECT_FALSE(chmod_apply(0777, "10000", &result));
-  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();
@@ -174,5 +152,4 @@ void test_metadata() {
   test_metadata_send_null();
   test_metadata_rejects_invalid_values();
   test_file_restore_metadata();
-  test_chmod_changes();
 }