feat(dry-run): server-contacting --dry-run (protocol 2.21.0)

--dry-run now handshakes with a remote/daemon receiver and reports what
WOULD transfer/skip based on receiver state, mutating nothing on either
side.

- Serialize Config.dry_run into the wire config frame and append
  STATUS_DRY_RUN_TRANSFER to the status enum (no renumbering); bump
  PROTOCOL_VERSION/CMake VERSION/CHANGELOG/golden wire to 2.21.0.
- Receiver: receive_incremental_check_ex runs the normal read-only
  decision and answers STATUS_OK (skip) or STATUS_DRY_RUN_TRANSFER
  (would transfer) with no basis materialization/append/delta/full
  transfer.  All mutation sites are guarded by !dry_run: file store,
  manifest deletes, --mkpath root creation, --delay-updates staging,
  publication, directory-time application, and outcome acks.
- Client: send_dry_run_remote connects, sends the config, checks each
  regular file and prints the would-transfer set + trailer; no file data
  or delete manifest is sent.  Plain local destinations keep the
  client-side manifest.

src/client/client_cli.c

@@ -1389,6 +1389,7 @@ static int set_server_port_option(Config* config, const char* value, const char*
     return -1;
   }
   config->server_port = port;
+  config->server_port_set = true;
   return 0;
 }
 

src/client/client_send.c

@@ -481,6 +481,21 @@ static void disconnect_transfer_client(Client* client) {
   client_delete(client);
 }
 
+/* True when --dry-run should contact a receiver rather than running the
+ * client-side local manifest.  A remote (SSH host:path), daemon
+ * (host::module/path), or an explicit --server-port/--port selects the
+ * server-contacting path; a plain local destination keeps the original
+ * client-side behavior (which never dials the default 127.0.0.1:8080). */
+static bool dry_run_targets_server(const Config* config) {
+  if (!config)
+    return false;
+  if (config->transport == TRANSPORT_SSH)
+    return true;
+  if (config->module && config->module[0] != '\0')
+    return true;
+  return config->server_port_set;
+}
+
 static bool add_chunk_to_manifest(ArrayList* manifest, const Chunk* chunk) {
   if (!manifest)
     return true;
@@ -1024,6 +1039,12 @@ static int incremental_check(Client* client, File* file, const Config* config,
       *resume_offset = offset;
     return 3;
   }
+  /* Server-contacting --dry-run: the receiver decided the file is not up to
+     date and answered "would transfer" WITHOUT expecting any data.  The caller
+     only uses this in the dry-run path; a non-dry-run sender never receives it
+     because the receiver only emits it when the wire config sets dry_run. */
+  if (s == STATUS_DRY_RUN_TRANSFER)
+    return 4;
   if (s != STATUS_NEXT) {
     log_message(LOG_LEVEL_ERROR, "Unexpected server status");
     send_status(client->file_descriptor, STATUS_ERROR);
@@ -1163,6 +1184,174 @@ static int send_append(const Client* client, File* file, Config* config,
   return ok ? 0 : -1;
 }
 
+/* Server-contacting --dry-run.  Connects to the configured remote/daemon and
+ * runs the normal per-file incremental decision WITHOUT transmitting any file
+ * data: the receiver (which also sees dry_run=true on the wire) answers
+ * STATUS_OK for an up-to-date file and STATUS_DRY_RUN_TRANSFER for a file it
+ * would otherwise write, mutating nothing on either side.  The would-transfer
+ * set and the same trailer as the local dry-run are printed.  A
+ * --compare-dest exact basis hit with no destination copy is reported as a
+ * skip by the receiver.
+ *
+ * Only regular files take the receiver-consulted check; directory / symlink /
+ * special / hard-link-sibling entries have no per-file content check, so they
+ * are reported conservatively as would-transfer and their frames are never
+ * sent (which is what keeps the receiver mutation-free).  --delete* is
+ * deliberately NOT transmitted in dry-run, so no deletion can occur; the
+ * would-delete manifest report is a documented follow-up.
+ *
+ * Returns 0 on success, 1 on error. */
+static int send_dry_run_remote(Config* config) {
+  int from_skipped = 0;
+  ArrayList* missing_args = NULL;
+  if (config->delete_missing_args) {
+    missing_args = array_list_create(free);
+    if (!missing_args)
+      return 1;
+  }
+  if (!files_from_list_check(config, missing_args, &from_skipped)) {
+    if (missing_args)
+      array_list_delete(missing_args);
+    return 1;
+  }
+  if (missing_args)
+    array_list_delete(missing_args);
+  /* Alternate basis dirs force the whole-file per-file check on the real
+     receiver; refuse an oversize source up front exactly as send_files does so
+     dry-run reports the same clear diagnostic instead of aborting mid-stream. */
+  if (config_has_basis(config) && !basis_oversize_preflight(config))
+    return 1;
+  /* Would-delete reporting requires a receiver-side read-only extras walk that
+     is not implemented yet; be explicit that --delete is a no-op in dry-run
+     rather than silently ignoring it. */
+  if ((config->use_delete || config->delete_missing_args) && !config->quiet)
+    log_message(LOG_LEVEL_WARNING,
+                "--dry-run: would-delete reporting is not available in this release; nothing is "
+                "deleted");
+
+  /* A live session may follow, so arm graceful abort handling. */
+  client_set_abort_armed(true);
+  Client* client = connect_transfer_client(config);
+  if (!client) {
+    if (config->transport == TRANSPORT_TCP)
+      log_message(LOG_LEVEL_ERROR, "could not connect to server%s",
+                  config->use_tls ? " via TLS" : "");
+    client_set_abort_armed(false);
+    return 1;
+  }
+  ProtocolSession session;
+  protocol_session_init(&session, client->file_descriptor, client->file_descriptor);
+  protocol_session_set_io_timeout(&session, config->timeout);
+  protocol_session_set_ssl(&session, (SSL*)client->ssl);
+  protocol_session_bind(&session);
+
+  int ret = 1;
+  PreparedScanner prepared;
+  memset(&prepared, 0, sizeof(prepared));
+  DirectoryScanner* scanner = NULL;
+  if (!config_send(client->file_descriptor, config))
+    goto dry_fail;
+  receive_daemon_motd(client, config);
+  if (!prepare_scanner(config, 0, &prepared))
+    goto dry_fail;
+  scanner = directory_scanner_create_with_options(config->send_directory, &prepared.options);
+  if (!scanner)
+    goto dry_fail;
+
+  int file_count = 0;
+  unsigned long long total_bytes = 0;
+  char size_buffer[32];
+  if (!config->quiet)
+    printf("Dry run: files to be transferred\n");
+  Chunk* chunk;
+  while ((chunk = directory_scanner_next(scanner)) != NULL) {
+    for (int i = 0; i < chunk->element_count; i++) {
+      File* f = chunk->items[i];
+      if (!f)
+        continue;
+      unsigned long long fsize = f->data ? f->data->size : 0;
+      bool would;
+      if (f->is_dir || f->is_symlink || f->is_special ||
+          (f->link_group != 0 && !f->link_first && f->hardlink_target != NULL)) {
+        /* No receiver-side content check exists for these frame types; a real
+           run would (re)create them, so report would-transfer and send no
+           frame (the receiver must stay mutation-free). */
+        would = true;
+      } else if (fsize > MAX_RECEIVE_WHOLE_FILE_SIZE && !config->use_incremental &&
+                 !config_has_basis(config)) {
+        /* A non-incremental run streams a >whole-file-limit source without the
+           STATUS_CHECK handshake, so no read-only receiver decision is possible
+           (and none is needed: a real run would transfer it). */
+        would = true;
+      } else {
+        DeltaSignature* sig = NULL;
+        unsigned long long resume_offset = 0;
+        int rc = incremental_check(client, f, config, &sig, &resume_offset);
+        delta_signature_destroy(sig);
+        if (rc < 0) {
+          chunk_destroy(chunk);
+          goto dry_fail;
+        }
+        if (rc == 1)
+          continue; /* up to date; nothing to report */
+        if (rc != 4) {
+          log_message(LOG_LEVEL_ERROR, "Unexpected receiver reply during dry-run");
+          chunk_destroy(chunk);
+          goto dry_fail;
+        }
+        would = true;
+      }
+      if (would) {
+        if (!config->quiet) {
+          char* escaped_path = output_escape(file_wire_path(f), config->eight_bit_output);
+          if (!escaped_path) {
+            chunk_destroy(chunk);
+            goto dry_fail;
+          }
+          if (config->human_readable)
+            printf("  %s (%s)\n", escaped_path,
+                   display_bytes(fsize, true, size_buffer, sizeof(size_buffer)));
+          else
+            printf("  %s (%llu bytes)\n", escaped_path, fsize);
+          free(escaped_path);
+        }
+        total_bytes += fsize;
+        file_count++;
+      }
+    }
+    chunk_destroy(chunk);
+  }
+  bool io_error = directory_scanner_had_io_error(scanner);
+  if (directory_scanner_failed(scanner))
+    goto dry_fail;
+  if (io_error)
+    log_message(LOG_LEVEL_WARNING, "source scan hit an unreadable directory");
+  /* Terminate the stream so the receiver emits its success frame; no data
+     frame and no delete manifest are ever sent in dry-run. */
+  if (!send_status(client->file_descriptor, STATUS_FINISHED))
+    goto dry_fail;
+  Status status;
+  if (!receive_status(client->file_descriptor, &status) || status != STATUS_OK)
+    goto dry_fail;
+  if (!config->quiet) {
+    if (config->human_readable)
+      printf("Total: %d files, %s\n", file_count,
+             display_bytes(total_bytes, true, size_buffer, sizeof(size_buffer)));
+    else
+      printf("Total: %d files, %.1f MB\n", file_count, (double)total_bytes / (double)BYTES_PER_MIB);
+  }
+  ret = io_error ? 1 : 0;
+
+dry_fail:
+  if (scanner)
+    directory_scanner_destroy(scanner);
+  prepared_scanner_destroy(&prepared);
+  disconnect_transfer_client(client);
+  protocol_session_unbind();
+  client_set_abort_armed(false);
+  return ret;
+}
+
 // Send a single file directly (non-incremental path).
 static bool send_file_direct(File* file, int fd, bool use_metadata, int compression_level,
                              const Config* config) {
@@ -1931,7 +2120,8 @@ int send_files(Config* config) {
   if (config->list_only)
     return send_list_only(config);
   if (config->dry_run)
-    return send_dry_run_manifest(config);
+    return dry_run_targets_server(config) ? send_dry_run_remote(config)
+                                          : send_dry_run_manifest(config);
   ArrayList* missing_args = NULL;
   int skipped = 0;
   if (config->delete_missing_args) {
@@ -2243,7 +2433,8 @@ int send_files_multithreaded(Config** config_ptr) {
   if (config->list_only)
     return send_list_only(config);
   if (config->dry_run)
-    return send_dry_run_manifest(config);
+    return dry_run_targets_server(config) ? send_dry_run_remote(config)
+                                          : send_dry_run_manifest(config);
   ArrayList* missing_args = NULL;
   int skipped = 0;
   if (config->delete_missing_args) {

src/server/receiver.c

@@ -288,10 +288,19 @@ int receiver_process_pending(Config* config, int file_descriptor, const Receiver
       goto fail;
     }
     if (status == STATUS_CHECK) {
-      bool skipped;
-      File* file = receive_incremental_check(file_descriptor, config, &skipped);
-      if (!skipped && (!file || !sink->store_file(file, sink->context)))
+      bool skipped = false;
+      bool would_transfer = false;
+      File* file = receive_incremental_check_ex(file_descriptor, config, &skipped, &would_transfer);
+      if (config->dry_run) {
+        /* Server-contacting --dry-run: the reply has already been sent
+           (STATUS_OK = up to date, STATUS_DRY_RUN_TRANSFER = would transfer) and
+           nothing may be stored.  Both flags false means a genuine protocol
+           error (STATUS_ERROR already sent or sent by receive_error below). */
+        if (!skipped && !would_transfer)
+          goto receive_error;
+      } else if (!skipped && (!file || !sink->store_file(file, sink->context))) {
         goto receive_error;
+      }
     } else if (status == STATUS_CHUNK) {
       Chunk* chunk = receive_chunk_data(file_descriptor, config);
       if (!chunk || !receiver_process_chunk(chunk, sink))
@@ -323,6 +332,15 @@ int receiver_process_pending(Config* config, int file_descriptor, const Receiver
       DeleteManifest* manifest = receive_manifest_entries(file_descriptor);
       if (!manifest)
         goto fail; /* receive_manifest_entries already sent STATUS_ERROR */
+      if (config->dry_run) {
+        /* Server-contacting --dry-run mutates nothing, so a keep-set manifest
+           is consumed and discarded.  The early-delete mode still needs its ACK
+           so a sender blocked on the delete handshake is not left hanging. */
+        delete_manifest_free(manifest);
+        if (early_delete && !send_status(file_descriptor, STATUS_OK))
+          goto fail;
+        goto next_status;
+      }
       if (early_delete) {
         /* --delete-before / --delete-during: the manifest is authoritative the
            moment it arrives, before any file data.  Delete now and acknowledge
@@ -441,7 +459,11 @@ typedef struct {
 static bool receiver_save_file(File* file, void* context_pointer) {
   ReceiverSaveContext* context = context_pointer;
   FileSaveResult result = FILE_SAVE_ERROR;
-  if (!context->config->save_to_disk) {
+  if (context->config->dry_run) {
+    /* Defense in depth: a dry-run receiver mutates nothing even if a data
+       frame reaches the sink (the sender is not supposed to send one). */
+    result = FILE_SAVE_SKIPPED;
+  } else if (!context->config->save_to_disk) {
     /* Nothing is stored; report the file as not-written so a
        --remove-source-files sender keeps its source. */
     result = FILE_SAVE_SKIPPED;
@@ -469,6 +491,10 @@ static bool receiver_save_file(File* file, void* context_pointer) {
 
 static bool receiver_send_success_frame(int fd, void* context_pointer) {
   ReceiverSaveContext* context = context_pointer;
+  /* Server-contacting --dry-run: nothing was staged or written, so there is
+     nothing to publish and no directory times to stamp. */
+  if (context->config->dry_run)
+    return receiver_send_final_success(fd, context->config, &context->outcomes);
   /* --delay-updates: the whole protocol stream (including manifest/delete
      handling, which ran inside receiver_process) has succeeded and every
      staged file was fully written.  Publish them atomically now, before the

src/server/receiver_pipeline.c

@@ -196,7 +196,11 @@ int write_thread(void* pipeline_context) {
     }
     size_t file_bytes = file->data ? file->data->size : 0;
     FileSaveResult result = FILE_SAVE_SKIPPED;
-    if (save_to_disk) {
+    /* Server-contacting --dry-run: never write.  The receiver thread does not
+       enqueue anything on the dry-run path, but this keeps the writer thread
+       provably mutation-free if a data frame ever reached it. */
+    bool dry_run = context->config->dry_run;
+    if (save_to_disk && !dry_run) {
       result = file_save_to_disk_full(root_directory, file, context->config);
       if (result == FILE_SAVE_ERROR) {
         file_destroy(file);
@@ -215,7 +219,7 @@ int write_thread(void* pipeline_context) {
     /* P7 Wave D: a directory's times are never applied inline (a later child
        write would clobber them); accumulate the metadata here and let the
        caller apply it once every writer has drained. */
-    if (result != FILE_SAVE_ERROR && file->is_dir && file->metadata &&
+    if (!dry_run && result != FILE_SAVE_ERROR && file->is_dir && file->metadata &&
         dir_times_should_capture(context->config) &&
         !dir_time_list_add(&context->dir_times, file->path, file->metadata)) {
       file_destroy(file);
@@ -234,8 +238,8 @@ int write_thread(void* pipeline_context) {
        which sources were actually written versus skipped on the receiver.
        Explicit directory entries and recreated device/special nodes have no
        source and are never acknowledged (mirrors receiver.c). */
-    if (context->config->remove_source_files && !file->is_dir && !file->is_special && !file->skip &&
-        !receiver_outcomes_append(&context->outcomes, (unsigned char)result)) {
+    if (!dry_run && context->config->remove_source_files && !file->is_dir && !file->is_special &&
+        !file->skip && !receiver_outcomes_append(&context->outcomes, (unsigned char)result)) {
       file_destroy(file);
       pipeline_context_receiver_note_bytes_released(context, file_bytes);
       mtx_lock(&context->mutex);

src/server/server.c

@@ -756,9 +756,12 @@ void handler(int file_descriptor) {
      skipped via its implied --ignore-missing-args, but nothing is deleted). */
   config->delete_missing_args = config->delete_missing_args && allow_delete;
   /* --mkpath: create the destination root (and its missing leading components)
-     before anything else; without it the root must pre-exist.  A failure here
-     aborts the connection cleanly before any file data is exchanged. */
-  if (!ensure_receive_root(config)) {
+   * before anything else; without it the root must pre-exist.  A failure here
+   * aborts the connection cleanly before any file data is exchanged.  A
+   * server-contacting --dry-run must NOT create anything: the root is only
+   * read for the would-transfer/skip decision (an absent root simply means
+   * "everything would transfer"). */
+  if (!config->dry_run && !ensure_receive_root(config)) {
     char* escaped_root = output_escape(config->receive_root_directory, log_get_8_bit_output());
     log_message(LOG_LEVEL_ERROR, "destination root is not available: %s",
                 escaped_root ? escaped_root : "<allocation failed>");
@@ -767,8 +770,9 @@ void handler(int file_descriptor) {
   }
   /* A --delay-updates transfer stages under a private 0700 directory inside
      the receive root.  Create it up front (wiping leftovers of any previously
-     interrupted delayed transfer) so a fully-skipped run also starts clean. */
-  if (config->delay_updates) {
+     interrupted delayed transfer) so a fully-skipped run also starts clean.
+     A dry-run stages nothing, so the staging tree is never created. */
+  if (config->delay_updates && !config->dry_run) {
     config->delay_context = delay_updates_context_create(config->receive_root_directory);
     if (!config->delay_context || !delay_updates_prepare(config->delay_context)) {
       log_message(LOG_LEVEL_ERROR, "Failed to initialize --delay-updates staging area");
@@ -864,12 +868,13 @@ void handler(int file_descriptor) {
     thrd_join(receiver, &receiver_result);
     thrd_join(writer, &writer_result);
     bool transfer_ok = receiver_result == thrd_success && writer_result == thrd_success;
-    if (transfer_ok) {
+    if (transfer_ok && !config->dry_run) {
       /* Commit-style (late) deletion: receive_thread handed the keep-set
          manifest here instead of deleting while write_thread might still be
          draining, so by now every file is on disk and the whole transfer is
          known to have succeeded.  Remove the extras before publishing a
-         --delay-updates run; the walker skips the staging directory. */
+         --delay-updates run; the walker skips the staging directory.  A
+         server-contacting --dry-run deletes nothing (no manifest is sent). */
       if (context->deferred_manifest) {
         if (!manifest_delete_all(config, context->deferred_manifest)) {
           transfer_ok = false;
@@ -878,7 +883,7 @@ void handler(int file_descriptor) {
         context->deferred_manifest = NULL;
       }
     }
-    if (transfer_ok) {
+    if (transfer_ok && !config->dry_run) {
       /* --delay-updates: receive_thread has finished the whole protocol stream
          (including manifest/delete handling) and write_thread has drained its
          queue, so every staged file is complete.  Publish atomically before the

src/shared/config.c

@@ -21,7 +21,6 @@ static void config_set_defaults(Config* config) {
   config->scanner_threads = 0;
   config->metadata_explicitly_disabled = false;
   config->show_progress = false;
-  config->dry_run = false;
   config->compression_threads = 0;
   config->ssh_port = 22;
   config->transport = TRANSPORT_TCP;
@@ -42,6 +41,7 @@ static void config_set_defaults(Config* config) {
   config->tls_ca = NULL;
   config->server_host = str_dup("127.0.0.1");
   config->server_port = 8080;
+  config->server_port_set = false;
   /* 0 means "--timeout not given": the transport keeps its own built-in 30 s
    * socket timeout (tcp_set_timeouts ignores non-positive values) and the
    * protocol layer keeps its built-in 60 s per-message deadline.  A positive
@@ -190,11 +190,11 @@ static bool validate_received_config(const Config* config) {
          valid_wire_bool(config->delay_updates) && valid_wire_bool(config->mkpath) &&
          valid_wire_bool(config->partial) && valid_wire_bool(config->delete_before) &&
          valid_wire_bool(config->checksum) && valid_wire_bool(config->eight_bit_output) &&
-         checksum_algo_valid(config->checksum_algo) && identity_wire_valid(config) &&
-         valid_wire_bool(config->preserve_atimes) && valid_wire_bool(config->preserve_crtimes) &&
-         valid_wire_bool(config->omit_dir_times) && valid_wire_bool(config->omit_link_times) &&
-         valid_wire_bool(config->munge_links) && valid_wire_bool(config->keep_dirlinks) &&
-         valid_wire_bool(config->fake_super) &&
+         valid_wire_bool(config->dry_run) && checksum_algo_valid(config->checksum_algo) &&
+         identity_wire_valid(config) && valid_wire_bool(config->preserve_atimes) &&
+         valid_wire_bool(config->preserve_crtimes) && valid_wire_bool(config->omit_dir_times) &&
+         valid_wire_bool(config->omit_link_times) && valid_wire_bool(config->munge_links) &&
+         valid_wire_bool(config->keep_dirlinks) && valid_wire_bool(config->fake_super) &&
          (!config->copy_as_set || (config->copy_as_uid >= 0 && config->copy_as_gid >= 0)) &&
          (!config->use_compression ||
           (config->compression_level >= 1 && config->compression_level <= 22)) &&

src/shared/config.h

@@ -76,7 +76,7 @@ typedef struct {
 typedef enum SuperMode { SUPER_MODE_AUTO = 0, SUPER_MODE_ON = 1, SUPER_MODE_OFF = 2 } SuperMode;
 
 /* ===========================================================================
- * Config wire-field table (single source of truth for protocol 2.20.0).
+ * Config wire-field table (single source of truth for protocol 2.21.0).
  *
  * Every field below crosses the wire.  The table is the ONLY place a
  * serialized field is named: config.h expands CONFIG_WIRE_FIELDS() to declare
@@ -109,6 +109,11 @@ typedef enum SuperMode { SUPER_MODE_AUTO = 0, SUPER_MODE_ON = 1, SUPER_MODE_OFF
  * =========================================================================== */
 #define CONFIG_WIRE_HEADER_FIELDS(X) X(version, char*, str_dup(PROTOCOL_VERSION), STR)
 
+/* dry_run (--dry-run) is CLIENT-INTENT that now CROSSES the wire (protocol
+ * 2.21.0): the receiver needs it to answer what WOULD transfer/skip without
+ * touching disk.  The client-only launch behavior (no server contact for a
+ * local destination) is decided separately in client_send.c before the frame
+ * is ever sent. */
 #define CONFIG_WIRE_CORE_FIELDS(X)                                                                 \
   X(eight_bit_output, bool, false, BOOL_8BIT)                                                      \
   X(max_alloc, unsigned long long, DEFAULT_MAX_ALLOC, RAW_MAXALLOC)                                \
@@ -122,7 +127,8 @@ typedef enum SuperMode { SUPER_MODE_AUTO = 0, SUPER_MODE_ON = 1, SUPER_MODE_OFF
   X(use_executability, bool, false, BOOL)                                                          \
   X(compression_level, int, 5, INT)                                                                \
   X(chunk_size, unsigned long long, DEFAULT_CHUNK_SIZE, RAW)                                       \
-  X(use_sendfile, bool, false, BOOL)
+  X(use_sendfile, bool, false, BOOL)                                                               \
+  X(dry_run, bool, false, BOOL)
 
 #define CONFIG_WIRE_DELTA_FIELDS(X)                                                                \
   X(use_delete, bool, false, BOOL)                                                                 \
@@ -261,7 +267,6 @@ typedef struct Config {
   int scanner_threads;
   bool metadata_explicitly_disabled;
   bool show_progress;
-  bool dry_run;
   int compression_threads;
   int ssh_port;
   TransportType transport;
@@ -281,6 +286,12 @@ typedef struct Config {
   bool use_tls;
   char* server_host;
   int server_port;
+  /* True when --server-port/--port was explicitly given.  CLIENT-ONLY (never
+   * serialized): --dry-run uses it to decide whether a real server handshake
+   * was requested, so a plain local destination (no explicit port) keeps the
+   * existing client-side dry-run behavior instead of dialing the default
+   * 127.0.0.1:8080. */
+  bool server_port_set;
   char* tls_cert;
   char* tls_key;
   char* tls_ca;
@@ -756,8 +767,23 @@ typedef struct Config {
  * The bump is therefore a deliberate lockstep-release marker, not a
  * desynchronization fix — the strict same-version handshake still rejects a
  * mixed 2.19/2.20 deployment.  The chunk codec, which already used the packed
- * metadata_to_buf()/metadata_from_buf() form, is unchanged. */
-#define PROTOCOL_VERSION "2.20.0"
+ * metadata_to_buf()/metadata_from_buf() form, is unchanged.
+ *
+ * Server-contacting Dry-run Wave: 2.20.0 -> 2.21.0.
+ *
+ * WHY the bump, grounded in the wire: this wave makes --dry-run contact the
+ * receiver and report exactly what WOULD change.  The binary config frame
+ * gains one serialized bool (Config->dry_run) appended to CONFIG_WIRE_CORE_
+ * FIELDS after use_sendfile, and the frame stream gains one terminal status
+ * (STATUS_DRY_RUN_TRANSFER) sent in reply to a per-file STATUS_CHECK when the
+ * file is not already up to date.  The receiver performs the normal read-only
+ * incremental decision but no mutation; the sender then skips the data.  Any
+ * config-frame layout or frame-sequence change must bump the protocol version:
+ * a 2.20 peer would desynchronize on the extra trailing byte and the unknown
+ * status, and the strict same-version handshake (config_receive rejects a
+ * mismatched version before parsing anything else) is what keeps a 2.21 client
+ * and a 2.20 server from ever reaching that state. */
+#define PROTOCOL_VERSION "2.21.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 /* Upper bound on total basis-dir entries (rsync caps --link-dest at 20). */
 #define MAX_BASIS_DIRS 64

src/shared/file_receive.c

@@ -1647,7 +1647,14 @@ static File* receive_full_file(int fd, const Config* config, const char* path) {
   return file;
 }
 
-File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
+/* Core implementation.  `would_transfer` (may be NULL) is set true only on the
+ * server-contacting --dry-run path, when the file is not up to date and the
+ * receiver answered STATUS_DRY_RUN_TRANSFER; the caller then knows no File is
+ * returned and nothing was stored. */
+File* receive_incremental_check_ex(int fd, const Config* config, bool* skipped,
+                                   bool* would_transfer) {
+  if (would_transfer)
+    *would_transfer = false;
   if (!config || !skipped) {
     send_status(fd, STATUS_ERROR);
     return NULL;
@@ -1799,6 +1806,44 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
     return NULL;
   }
 
+  /* ---- Server-contacting --dry-run ----
+   * The destination does not already hold this file.  In dry-run the receiver
+   * must NOT materialize anything (no basis link/copy, no append/delta/full
+   * transfer) and the sender must NOT send any data, so answer
+   * STATUS_DRY_RUN_TRANSFER and return immediately.  The one exception is a
+   * --compare-dest exact hit with no destination copy: a real run would
+   * suppress the data without changing the destination, so it reports as a
+   * skip (STATUS_OK) exactly as the full path below would.  Everything read
+   * here (destination file, basis candidates) is read-only. */
+  if (config->dry_run) {
+    bool skip_via_compare = false;
+    if (config_has_basis(config) && !config->ignore_times) {
+      BasisMatch basis;
+      basis_match_find(config, check_path, check_size, (time_t)check_mtime, (long)check_mtime_nsec,
+                       check_digest, check_digest_len, false, &basis);
+      if (basis.hit && basis.type == BASIS_DEST_COMPARE && !has_old_file)
+        skip_via_compare = true;
+      basis_match_free(&basis);
+    }
+    Status reply = skip_via_compare ? STATUS_OK : STATUS_DRY_RUN_TRANSFER;
+    if (!send_status(fd, reply)) {
+      free(old_data);
+      close(old_fd);
+      free(full_path);
+      free(check_path);
+      return NULL;
+    }
+    if (skip_via_compare)
+      *skipped = true;
+    else if (would_transfer)
+      *would_transfer = true;
+    free(old_data);
+    close(old_fd);
+    free(full_path);
+    free(check_path);
+    return NULL;
+  }
+
   /* ---- Alternate basis directories ---- */
   if (config_has_basis(config)) {
     BasisMatch basis;
@@ -2181,6 +2226,10 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   return file;
 }
 
+File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
+  return receive_incremental_check_ex(fd, config, skipped, NULL);
+}
+
 File* file_receive(const Config* config, int file_descriptor) {
   char* path = receive_wire_str(file_descriptor);
   if (path == NULL)

src/shared/file_receive.h

@@ -24,6 +24,13 @@ File* file_receive_symlink(int file_descriptor, const Config* config);
 File* file_receive_special(int file_descriptor);
 bool file_special_rdev_valid(int32_t major, int32_t minor, mode_t mode);
 File* receive_incremental_check(int fd, const Config* config, bool* skipped);
+/* Extended variant used by the receiver.  `would_transfer` (may be NULL) is set
+ * true only on the server-contacting --dry-run path when the file is not up to
+ * date: the receiver has already sent STATUS_DRY_RUN_TRANSFER and returns NULL
+ * without storing anything.  On that path `*skipped` is true for an up-to-date
+ * (STATUS_OK) file and both flags are false for a genuine error. */
+File* receive_incremental_check_ex(int fd, const Config* config, bool* skipped,
+                                   bool* would_transfer);
 
 /* P7 Wave D directory-time accumulator.  The receiver collects the metadata of
  * every directory it creates/receives (STATUS_MKDIR with metadata and/or the

src/shared/protocol.c

@@ -446,6 +446,8 @@ static const char* status_to_string(Status status) {
     return "AUTH_OK";
   case STATUS_AUTH_FAILED:
     return "AUTH_FAILED";
+  case STATUS_DRY_RUN_TRANSFER:
+    return "DRY_RUN_TRANSFER";
   default:
     return "UNKNOWN";
   }

src/shared/protocol.h

@@ -132,7 +132,15 @@ enum NET_STATUS {
   STATUS_AUTH_CHALLENGE,
   STATUS_AUTH_RESPONSE,
   STATUS_AUTH_OK,
-  STATUS_AUTH_FAILED
+  STATUS_AUTH_FAILED,
+  /* Server-contacting --dry-run (protocol 2.21.0).  Sent by the receiver in
+   * response to a per-file STATUS_CHECK when the wire config carries
+   * dry_run=true and the file is NOT already up to date: it tells the sender
+   * the file WOULD be transferred, and the sender must NOT transmit any data
+   * (the receiver reads none in dry-run).  STATUS_OK keeps its meaning in this
+   * path ("already up to date / nothing to do").  Appended after
+   * STATUS_AUTH_FAILED so no existing status is renumbered. */
+  STATUS_DRY_RUN_TRANSFER
 };
 
 void io_set_fds(int read_fd, int write_fd);