Merge feat/p7-devices: Phase 7 Wave C (device/special statuses; --copy-devices --sendfile non-regular fallback)

README.md

@@ -63,11 +63,15 @@ replacement for every rsync feature or protocol mode.
 - Archive mode does not yet provide all of rsync's `-rlptgoD` behavior.
 - Symlink transfer is incomplete; link targets are not yet recreated in all
   modes.
-- Owner/group, ACL, xattr, hard-link, device, and special-file handling is
+- Owner/group, ACL, xattr, and hard-link handling is incomplete or
-  incomplete or unavailable.
+  unavailable.
+- Device and special-file preservation is implemented with documented
+  divergences: recreated device nodes require `CAP_MKNOD` on the receiver (a
+  non-root receiver skips the entry), and sockets cannot be recreated (FIFOs
+  are).
 - Sparse-file hole preservation (`-S`, `--sparse`) is implemented receiver-side:
   long all-zero runs are written as holes (no wire change; the full file image
-  is already in memory). 
+  is already in memory).
 - `--partial`, `--partial-dir`, `-P`, `--append`, and `--append-verify` keep
   the write atomic (temp + rename). With `--partial`, a failed/interrupted write
   now retains the already-written temp at the destination path (best-effort) so

RSYNC_COMPAT.md

@@ -249,10 +249,10 @@ why plain `--append` works on the normal atomic path, not only with `--inplace`.
 | `-X`, `--xattrs` | Preserve extended attributes | ✅ Implemented | Preserves unprivileged `user.*` extended attributes (Linux `listxattr`/`getxattr` on capture, `fsetxattr` on the written destination fd). Both capture (sender) and application (receiver) are restricted to the `user.*` namespace and the two POSIX ACL xattrs, so a client can **never** force a `security.*`/`trusted.*`/privileged attribute onto the destination; the receiver independently re-validates every incoming name against this whitelist and rejects anything else. Payloads are bounded (per-name ≤255B, per-value ≤1MiB, per-file count ≤256 total bytes ≤4MiB) on both ends, and an oversized/malformed frame is a clean protocol rejection (no OOM). Applied fd-relative to the exact written file. Implies metadata transmission. Incompatible with `-s` (chunk serialization), rejected up front (see the notes); a `--link-dest`/`-H` hard-link copy fallback re-applies the attributes so they are not dropped when a link is refused |
 | `-H`, `--hard-links` | Preserve hard links | ✅ Implemented | Files on the source that share an inode (`st_dev`+`st_ino`, e.g. a `cp -al` tree) are re-created as hard links to one another on the destination, so duplicate links stay deduplicated and only the first member's data is sent (later members are transmitted as payload-less `STATUS_HARDLINK` frames). The receiver links each sibling to the first member's installed file with an atomic link + rename; on `link()` failure it falls back to a byte-identical local copy of the first member, never a partial/corrupt file. Requires the sequential scan for ordering (the first member is always emitted and installed before any sibling is linked). Works single-threaded and under `-j`/`--threads`, `--inplace`, `--delay-updates` (links staged and published by rename) and `--partial`. Crosses the wire (`preserve_hard_links` bool; `PROTOCOL_VERSION` bumped **2.11.0 → 2.12.0**, peers must match). Incompatible with `-s` (chunk serialization) and `--append`/`--append-verify`, rejected up front with a distinct error. See the Phase-4 hard-links notes below |
 | `-D` | Same as --devices --specials | ✅ Implemented | Implies `--devices --specials`. `-D` was unassigned in FastSync (verified: no collision), so it is free to imply both device-node and special-file preservation. See the `--devices`/`--specials` rows and the Phase-4 devices notes below |
-| `--devices` | Preserve device files | ⚠️ Partial | Recreates char/block device nodes on the destination via `mknod` instead of transferring content. Type + rdev are validated strictly (S_IFMT from the transmitted mode; major/minor range-checked, non-negative), and creation is **privilege-gated**: `mknod` needs `CAP_MKNOD`, so a non-root receiver (CI runs via setpriv as non-root) logs a warning and **skips the device entry safely** — the whole transfer never aborts just because the node could not be made. The node is created fd-relative below the receive root (`mknodat` on the confined secure parent), so it can never be placed outside the authorized root, never follows a symlink, and never replaces an existing directory. Only a char/block mode is honored. Crosses the wire (a new `STATUS_SPECIAL` frame carries the path + metadata mode + rdev; `PROTOCOL_VERSION` bumped **2.12.0 → 2.13.0**). Divergence: per-entry skip (not a hard error) when the receiver lacks `CAP_MKNOD`, documented in the Phase-4 devices notes |
+| `--devices` | Preserve device files | ✅ Implemented | Recreates char/block device nodes on the destination via `mknod` instead of transferring content. Type + rdev are validated strictly (S_IFMT from the transmitted mode; major/minor range-checked, non-negative), and creation is **privilege-gated**: `mknod` needs `CAP_MKNOD`, so a non-root receiver (CI runs via setpriv as non-root) logs a warning and **skips the device entry safely** — the whole transfer never aborts just because the node could not be made. The node is created fd-relative below the receive root (`mknodat` on the confined secure parent), so it can never be placed outside the authorized root, never follows a symlink, and never replaces an existing directory. Only a char/block mode is honored. Crosses the wire (a new `STATUS_SPECIAL` frame carries the path + metadata mode + rdev; `PROTOCOL_VERSION` bumped **2.12.0 → 2.13.0**). Divergence: per-entry skip (not a hard error) when the receiver lacks `CAP_MKNOD`, documented in the Phase-4 devices notes |
-| `--specials` | Preserve special files | ⚠️ Partial | Recreates **FIFOs** on the destination via `mkfifo` (unprivileged, so this is a real, assertable behavior under CI). Sockets cannot be recreated by any standard filesystem call and are skipped with an explicit note (best-effort / unsupported, matching the plan). FIFO creation is privileged-gated only in the sense of graceful skip on any permission failure. Node creation is confined below the receive root (`mkfifoat` on the secure fd-relative parent; no `..`, no symlink follow). Crosses the wire like `--devices` (the `STATUS_SPECIAL` frame; `PROTOCOL_VERSION` bumped **2.12.0 → 2.13.0**). See the Phase-4 devices notes |
+| `--specials` | Preserve special files | ⛔ Impossible/Divergence | **FIFO recreation works**: FIFOs are recreated on the destination via `mkfifo` (unprivileged, so this is a real, assertable behavior under CI). **Only socket recreation is impossible**: a socket entry can be created only by `bind(2)` on a live socket, not by any filesystem call, so a source socket is skipped with an explicit note. That one unsupported node kind is why the flag is classified Impossible/Divergence even though FIFO recreation itself works; its normal path is otherwise complete. FIFO creation is privileged-gated only in the sense of graceful skip on any permission failure. Node creation is confined below the receive root (`mkfifoat` on the secure fd-relative parent; no `..`, no symlink follow). Crosses the wire like `--devices` (the `STATUS_SPECIAL` frame; `PROTOCOL_VERSION` bumped **2.12.0 → 2.13.0**). See the Phase-4 devices notes |
-| `--copy-devices` | Copy device contents as file | ⚠️ Partial | Copy a device's CONTENT into an ordinary regular file on the destination instead of recreating the node — non-privileged and safe. FastSync scans a device/FIFO as a regular file: its reported size (`st_size`, typically 0 for char devices and FIFOs) is copied, so a FIFO or a non-readable device becomes an empty (or size-bounded) regular file without ever blocking or reading unbounded pseudo-device streams. The run always succeeds and never crashes on such input. **Deliberate, safe divergence from rsync's dd-like unbounded device read.** See the Phase-4 devices notes |
+| `--copy-devices` | Copy device contents as file | ✅ Implemented | Copy a device's CONTENT into an ordinary regular file on the destination instead of recreating the node — non-privileged and safe. FastSync scans a device/FIFO as a regular file: its reported size (`st_size`, typically 0 for char devices and FIFOs) is copied, so a FIFO or a non-readable device becomes an empty (or size-bounded) regular file. The default data path is size-bounded and never blocks (it sends exactly `st_size` bytes, never an unbounded pseudo-device stream); with `--sendfile`, a non-regular source (FIFO/device) is detected from its `stat` mode and falls back to that same buffered read, so `--copy-devices --sendfile` cannot hang either. The run always succeeds and never crashes on such input. **Deliberate, safe divergence from rsync's dd-like unbounded device read.** See the Phase-4 devices notes |
-| `--write-devices` | Write to devices as files | ⚠️ Partial | Write the received data directly into an **existing** device node on the destination instead of creating a regular file. Restricted and best-effort: the destination must already exist and be a char/block device (opened only under the confined receive root, with `O_NOFOLLOW` + `O_NONBLOCK`); a missing, symlinked, FIFO-with-no-reader (`ENXIO`), non-device destination, or any write failure is **skipped with a warning** rather than allowed, so a run can never clobber the system, never blocks on a special-file target, and never aborts on an unusable target. See the Phase-4 devices notes |
+| `--write-devices` | Write to devices as files | ✅ Implemented | Write the received data directly into an **existing** device node on the destination instead of creating a regular file. Restricted and best-effort: the destination must already exist and be a char/block device (opened only under the confined receive root, with `O_NOFOLLOW` + `O_NONBLOCK`); a missing, symlinked, FIFO-with-no-reader (`ENXIO`), non-device destination, or any write failure is **skipped with a warning** rather than allowed, so a run can never clobber the system, never blocks on a special-file target, and never aborts on an unusable target. See the Phase-4 devices notes |
 | `-U`, `--atimes` | Preserve access times | ✅ Implemented | Captures the source access time (from the scanner's pre-read stat, so it is not clobbered by reading the file for transfer) and transmits it over the wire; the receiver restores it together with the mtime via `futimens`/`utimensat`. Implies metadata transmission (the times travel inside the `-M` metadata payload), but does not enable ownership application (that stays opt-in via the identity flags). Wire: new `atime` fields on the metadata frame + a `preserve_atimes` config boolean; `PROTOCOL_VERSION` bumped **2.11.0 → 2.12.0** |
 | `-N`, `--crtimes` | Preserve create times | ⛔ Impossible/Divergence | Birth-times cannot be set by any portable filesystem call (`utimensat`/`futimens` only set atime/mtime), so this row is an explicit **Impossible/Divergence** (Phase 7 Wave B). Capture + transmit stays: `statx(STATX_BTIME)` on Linux records the source birth time as a wire field; the receiver logs a debug note that it cannot be applied and continues — never failing the transfer and never pretending it worked. On platforms without `statx` it parses as a documented no-op (flag accepted; nothing is captured). Implies metadata transmission. Wire: new `crtime` fields + a `preserve_crtimes` config boolean; `PROTOCOL_VERSION` bumped **2.11.0 → 2.12.0** (see the Phase-4 metadata-time notes) |
 | `-O`, `--omit-dir-times` | Omit dirs from --times | 🔄 Compatibility No-op | Accepted and parsed for CLI compatibility, and the config boolean crosses the wire, but it has **no effect**: FastSync never preserves directory mtimes in the first place (directories are created via `mkdir` with no metadata, a documented divergence under `-d`/recursive), so there is nothing for an "omit" to suppress. It never breaks a normal run |
@@ -814,7 +814,7 @@ These are the last compatibility items and the closing phase toward rsync flag p
 
 **Wave B — Output & filesystem completion (✅ implemented).** `-S`/`--sparse` (`⚠️→✅`): real hole preservation — a sparse-aware writer (`write_all_sparse`) skips all-zero runs ≥ 4096 bytes with `lseek(SEEK_CUR)` and `ftruncate`s the final size, wired into both the atomic temp+rename store and `--inplace` receiver-side with **no wire change** (the full file image is already in memory; the ftruncate presize is kept). `-P` (`⚠️→✅`): interrupted-write retention — on a save failure after data reached the temp fd, `--partial` now renames the already-written temp to the destination path (best-effort; falls through to the normal unlink on failure, never retains when `--partial` is off) so a later `--append`/`--append-verify` run can resume. `--block-size=SIZE` (`⚠️→✅`): promoted after verification — `--block-size` is now an alias for `--delta-block`, both set `config->delta_block_size`, which the delta engine already honored end-to-end (`delta_signature_create_seeded` + `delta_apply`); out-of-range values keep the default. `--fake-super` (`⚠️→✅`): added `fake_super_restore_fd` to parse and re-apply the recorded `user.fastsync.stat` record fd-relative (fchown best-effort/non-root skipped, fchmod, futimens); a save under `--fake-super` now re-applies the recorded attrs instead of only recording them, with the recording format unchanged. `--stderr=client` (`⚠️→⛔ Impossible/Divergence`): FastSync has no rsync client-message channel, and `client` is rejected at CLI parse — the rejection is the documented behavior (unit-tested). `-N`/`--crtimes` (`⚠️→⛔ Impossible/Divergence`): birth-times cannot be set by any portable fs call (`utimensat` sets only atime/mtime); capture/transmit stays, setting is impossible, the flag is accepted and safely inert. Review-hardening (post-eval): fake-super replay applies the mode through the same sanitization as the normal metadata path (group/other write bits are never granted); `--sparse` takes precedence over `--preallocate` (posix_fallocate skipped so holes survive); `--partial` retention is disabled for `--no_replace` (ignore/existing) and only marks a write-attempt after the actual write begins; `--block-size=SIZE`/`--delta-block=SIZE` inline forms are accepted.
 
-**Wave C — Devices & special files (finalize statuses + tests).** `--devices`, `--specials`, `--copy-devices`, `--write-devices` (`⚠️`) are already functionally implemented with documented, safety-driven divergences (CAP_MKNOD per-entry skip; FIFO-recreate-with-no-socket; size-bounded content copy; confined best-effort device write). During this wave each is promoted to its final status with coverage tests: `--specials` **sockets** cannot be recreated by any standard filesystem call → mark **Impossible/Divergence**; the rest are complete → **✅**.
+**Wave C — Devices & special files (finalize statuses + tests) (✅ implemented).** The four special-file rows are finalized with coverage tests. `--devices`, `--copy-devices`, and `--write-devices` are **✅ Implemented**, each with a documented, safety-driven divergence: device-node creation is privilege-gated, so a receiver without `CAP_MKNOD` skips that entry with a warning (a per-entry skip, never a transfer failure); `--copy-devices` copies a device/FIFO's reported size into an ordinary regular file (a size-bounded safe divergence from rsync's unbounded dd-like read); `--write-devices` writes only into an existing char/block node under the confined receive root and skips every unusable target rather than clobbering or aborting. `--specials` is classified **⛔ Impossible/Divergence** for one reason only: **FIFO recreation works** (unprivileged `mkfifo`, asserted under CI), but **sockets cannot be recreated by any standard filesystem call**, so a source socket is skipped with an explicit note. Tests assert FIFO recreation, the safe socket skip, the regular-file result of `--copy-devices`, the skipped/missing and non-device `--write-devices` targets, and (root-gated) real device-node creation; a root runner additionally drops the receiver to an unprivileged user to assert the `CAP_MKNOD` skip is graceful.
 
 **Wave D — Times superstructure & arg-protection no-ops (`🔄`).** `-O`/`--omit-dir-times`, `-J`/`--omit-link-times` (`🔄`) are no-ops *only because* FastSync never preserves directory/symlink times in the first place. To make them real (honoring "all possible flags"): **add directory-mtime and symlink-mtime/owner preservation**, so `-O`/`-J` become meaningful modifiers — an intentional behavior addition that reverses the old "never preserves dir times" divergence. If instead this is judged out of scope at Wave-D time, mark both **Impossible/Divergence**. `--secluded-args` (`🔄→Impossible/Divergence`): a true arg-send protocol is large and FastSync already builds remote SSH argv securely (single-quote-escaped shell words, injection-safe), so there is no argument-leak to close; document the already-safe behavior.
 

src/client/client_send.c

@@ -1259,6 +1259,19 @@ static int send_single_file(Client* client, File* file, Config* config, bool use
   return 0;
 }
 
+/* Sendfile calls a blocking open() on the source (file_send_sendfile_with_skip
+ * -> file_open_for_read), which never returns for a FIFO/device with no writer.
+ * Only a regular file may take the zero-copy sendfile path; a non-regular source
+ * (FIFO/device copied by --copy-devices) must use the buffered, size-bounded
+ * read path instead.  `stat` follows symlinks, so a dereferenced symlink to a
+ * regular file keeps the sendfile fast path. */
+static bool source_is_regular_file(const File* file) {
+  if (!file || !file->path)
+    return false;
+  struct stat st;
+  return stat(file->path, &st) == 0 && S_ISREG(st.st_mode);
+}
+
 static int send_chunk_with_removal(Client* client, Chunk* chunk, Config* config,
                                    ArrayList* remove_sources) {
   if (config->use_chunk_serialization) {
@@ -1337,8 +1350,9 @@ static int send_chunk_with_removal(Client* client, Chunk* chunk, Config* config,
       continue;
     }
     bool stream = f->data->data == NULL && f->data->size > 0;
-    bool use_sendfile =
+    bool use_sendfile = ((config->use_sendfile && !config->use_compression) ||
-        (config->use_sendfile && !config->use_compression) || (stream && !config->use_compression);
+                         (stream && !config->use_compression)) &&
+                        source_is_regular_file(f);
     SourceFile* source = remove_sources ? source_file_create(f) : NULL;
     int rc = send_single_file(client, f, config, config->use_incremental, use_sendfile);
     if (rc == 1) {

tests/integration/test_features.py

@@ -3,6 +3,7 @@ import filecmp
 import os
 import random
 import shutil
+import socket
 import stat
 import subprocess
 import sys
@@ -14,7 +15,8 @@ from common import (
     PROJECT_ROOT, BUILD_DIR, TEST_DATA_DIR,
     run_client, CountingProxy,
     generate_test_files, verify_transfer, clean_dir, make_result,
-    get_dest_received_dir, CLIENT_CMD, ServerManager,
+    get_dest_received_dir, CLIENT_CMD, SERVER_CMD, ServerManager,
+    _find_free_port, _wait_for_port, _wait_proc,
 )
 
 SOURCE_DIR = os.path.join(TEST_DATA_DIR, "feature_source")
@@ -23,6 +25,31 @@ DEVICE_SOURCE = os.path.join(TEST_DATA_DIR, "device_source")
 DEVICE_DEST = os.path.join(TEST_DATA_DIR, "device_dest")
 
 
+def _start_captured_server(prefix=None, extra_args=None):
+    """Start a plain-TCP server with captured stdout/stderr for one test.
+
+    Returns (proc, port).  The caller owns `proc` and must terminate it via
+    `_wait_proc` so a server that ignores SIGTERM is killed instead of leaving
+    a zombie or raising TimeoutExpired.  The shared session server discards its
+    output, so tests that lock in a receiver-side warning need their own.  The
+    server's SIGTERM handler exits via `_exit`, which does not flush stdio, so
+    `stdbuf -oL` keeps stdout line-buffered and the warning observable."""
+    port = _find_free_port()
+    cmd = ["stdbuf", "-oL"] + (prefix or []) + SERVER_CMD + ["-p", str(port), "--allow-unauthenticated"]
+    if extra_args:
+        cmd += extra_args
+    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True)
+    _wait_for_port(port)
+    return proc, port
+
+
+def _stop_captured_server(proc):
+    """Terminate a captured server and return its (stdout, stderr) text."""
+    proc.terminate()
+    _wait_proc(proc)
+    return proc.communicate()
+
+
 class TestDeviceSpecial:
     """Phase 4: --devices / --specials / -D / --copy-devices / --write-devices.
 
@@ -64,14 +91,56 @@ class TestDeviceSpecial:
         received = get_dest_received_dir(DEVICE_DEST, DEVICE_SOURCE)
         assert stat.S_ISFIFO(os.stat(os.path.join(received, "pipe.fifo")).st_mode)
 
-    def test_copy_devices_non_crash(self, shared_server):
+    @pytest.mark.ci
-        """--copy-devices treats a special/device source as a regular-file copy;
+    def test_specials_socket_source_skipped_safely(self):
-        a FIFO (st_size 0) must transfer without hanging or crashing."""
+        """A socket cannot be recreated by any standard filesystem call, so
+        --specials must skip it with a note and still complete the run (the
+        adjacent regular file transfers normally; no socket node appears)."""
+        self._setup()
+        sock_path = os.path.join(DEVICE_SOURCE, "source.sock")
+        s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        server, port = _start_captured_server()
+        try:
+            s.bind(sock_path)
+            result, _ = run_client(DEVICE_SOURCE, DEVICE_DEST,
+                                   flags=["--specials"], port=port)
+        finally:
+            s.close()
+            out, err = _stop_captured_server(server)
+        assert result.returncode == 0, f"Exit {result.returncode}: {result.stderr[:200]}"
+        received = get_dest_received_dir(DEVICE_DEST, DEVICE_SOURCE)
+        with open(os.path.join(received, "plain.txt")) as f:
+            assert f.read() == "regular content\n"
+        assert not os.path.lexists(os.path.join(received, "source.sock")), (
+            "socket source must be skipped, not materialized"
+        )
+        assert "socket not recreated" in (out + err), (
+            f"receiver did not log the documented socket skip: out={out!r} err={err!r}"
+        )
+
+    @pytest.mark.ci
+    @pytest.mark.parametrize("flags", [["--copy-devices"], ["--copy-devices", "--sendfile"]])
+    def test_copy_devices_fifo_becomes_regular_file(self, shared_server, flags):
+        """--copy-devices treats a special source as an ordinary regular-file
+        copy: a FIFO (st_size 0) becomes a zero-length REGULAR file on the
+        destination (never a FIFO, never a hang), and the run succeeds.  The
+        --sendfile variant previously blocked forever in the sendfile open();
+        the non-regular source now falls back to the buffered read path, so it
+        must complete within the bounded-time assertion below."""
         self._setup()
         os.mkfifo(os.path.join(DEVICE_SOURCE, "device_copy.fifo"))
-        result, _ = run_client(DEVICE_SOURCE, DEVICE_DEST,
+        result, dur = run_client(DEVICE_SOURCE, DEVICE_DEST,
-                               flags=["--copy-devices"], port=shared_server.port)
+                                 flags=flags, port=shared_server.port)
         assert result.returncode == 0, f"Exit {result.returncode}: {result.stderr[:200]}"
+        received = get_dest_received_dir(DEVICE_DEST, DEVICE_SOURCE)
+        copied = os.path.join(received, "device_copy.fifo")
+        assert os.path.lexists(copied), "copy-devices source was not transferred"
+        st = os.lstat(copied)
+        assert stat.S_ISREG(st.st_mode), (
+            f"copy-devices must produce a regular file, got mode {oct(st.st_mode)}"
+        )
+        assert st.st_size == 0, f"expected a size-bounded 0-byte copy, got {st.st_size}"
+        assert dur < 60, f"{' '.join(flags)} hung on a FIFO source"
 
     def test_write_devices_non_crash(self, shared_server):
         """--write-devices writes into an existing device only; when the
@@ -85,6 +154,56 @@ class TestDeviceSpecial:
                                flags=["--write-devices"], port=shared_server.port)
         assert result.returncode == 0, f"Exit {result.returncode}: {result.stderr[:200]}"
 
+    @pytest.mark.ci
+    def test_write_devices_regular_file_target_skipped(self, shared_server):
+        """--write-devices only ever writes into an existing char/block node: a
+        pre-existing REGULAR file at the destination path is left byte-identical
+        (not clobbered) and the run still succeeds."""
+        self._setup()
+        received = get_dest_received_dir(DEVICE_DEST, DEVICE_SOURCE)
+        os.makedirs(received, exist_ok=True)
+        target = os.path.join(received, "plain.txt")
+        with open(target, "wb") as f:
+            f.write(b"pre-existing local content\n")
+        result, _ = run_client(DEVICE_SOURCE, DEVICE_DEST,
+                               flags=["--write-devices"], port=shared_server.port)
+        assert result.returncode == 0, f"Exit {result.returncode}: {result.stderr[:200]}"
+        with open(target, "rb") as f:
+            assert f.read() == b"pre-existing local content\n", (
+                "write-devices clobbered a non-device destination"
+            )
+
+    @pytest.mark.setpriv
+    def test_devices_nonroot_receiver_skips_safely(self):
+        """A receiver without CAP_MKNOD must skip a device entry with a warning
+        and never abort.  A root runner drops the receiver (server) to nobody
+        via setpriv; on a non-root runner (or without setpriv) the test skips."""
+        if os.geteuid() != 0 or shutil.which("setpriv") is None:
+            pytest.skip("requires root + setpriv to run the receiver unprivileged")
+        self._setup()
+        os.mknod(os.path.join(DEVICE_SOURCE, "chardev"), stat.S_IFCHR | 0o666,
+                 os.makedev(1, 3))
+        # The unprivileged receiver must be able to create the destination tree.
+        os.makedirs(DEVICE_DEST, exist_ok=True)
+        os.chmod(DEVICE_DEST, 0o777)
+        server, port = _start_captured_server(
+            prefix=["setpriv", "--reuid=65534", "--regid=65534", "--clear-groups"])
+        try:
+            result, _ = run_client(DEVICE_SOURCE, DEVICE_DEST,
+                                   flags=["--devices"], port=port)
+        finally:
+            out, err = _stop_captured_server(server)
+        assert result.returncode == 0, f"Exit {result.returncode}: {result.stderr[:300]}"
+        received = get_dest_received_dir(DEVICE_DEST, DEVICE_SOURCE)
+        with open(os.path.join(received, "plain.txt")) as f:
+            assert f.read() == "regular content\n"
+        assert not os.path.lexists(os.path.join(received, "chardev")), (
+            "a receiver without CAP_MKNOD must skip the device node, not create it"
+        )
+        assert "cannot create device node" in (out + err), (
+            f"receiver did not log the documented CAP_MKNOD skip: out={out!r} err={err!r}"
+        )
+
     @pytest.mark.skipif(os.geteuid() != 0, reason="requires root to create device nodes")
     def test_devices_recreates_real_char_device(self, shared_server):
         """Root-only: a source char device node is recreated on the destination

tests/test_file.c

@@ -37,6 +37,7 @@ static void test_file_special_rdev_valid() {
   mode_t fake_char = S_IFCHR | 0600;
   mode_t fake_blk = S_IFBLK | 0600;
   mode_t fake_fifo = S_IFIFO | 0600;
+  mode_t fake_sock = S_IFSOCK | 0600;
   /* char/block devices: accept a legal pair, reject negative / oversized. */
   EXPECT_TRUE(file_special_rdev_valid(1, 3, fake_char));
   EXPECT_TRUE(file_special_rdev_valid(0xffff, 0x00ffffff, fake_blk));
@@ -47,6 +48,8 @@ static void test_file_special_rdev_valid() {
   /* FIFOs/sockets must carry an empty rdev. */
   EXPECT_TRUE(file_special_rdev_valid(0, 0, fake_fifo));
   EXPECT_FALSE(file_special_rdev_valid(1, 0, fake_fifo));
+  EXPECT_TRUE(file_special_rdev_valid(0, 0, fake_sock));
+  EXPECT_FALSE(file_special_rdev_valid(0, 1, fake_sock));
   EXPECT_FALSE(file_special_rdev_valid(0, 0, (mode_t)(S_IFREG | 0600)));
 }