fix: handle short reads from ffmpeg stdout in capture loop

io.BufferedReader.read(n) is documented to return "up to" n bytes, and
a subprocess pipe will return a short read whenever the producer closes
the pipe mid-frame — even on a blocking pipe. The existing assignment

    frame_buffer[:] = ffmpeg_process.stdout.read(frame_size)

then raises ValueError on the slice assignment, and the surrounding
``except`` only breaks out of the capture loop when ``poll()`` reports
the process has already exited. If ffmpeg is still alive (e.g. an RTSP
source that dropped a chunk and recovered), the loop hits ``continue``
and the next read picks up data from the middle of the next frame,
putting capture out of sync with frame boundaries until the watchdog
later forces a restart.

Read into the pre-allocated buffer in a loop and treat any short read
as the end of the stream, so the watchdog can respawn ffmpeg from a
clean state rather than continuing at the wrong offset. As a side
effect, ``readinto`` writes directly into the shared-memory frame
buffer, avoiding the per-frame Python ``bytes`` allocation and copy
that the previous slice assignment performed.

frigate/test/test_video.py

@@ -1,3 +1,4 @@
+import io
 import unittest
 
 import cv2
@@ -13,6 +14,7 @@ from frigate.util.object import (
     get_region_from_grid,
     reduce_detections,
 )
+from frigate.video.ffmpeg import _read_frame_into
 
 
 def draw_box(frame, box, color=(255, 0, 0), thickness=2):
@@ -352,3 +354,57 @@ class TestRegionGrid(unittest.TestCase):
 
         region = get_region_from_grid(frame_shape, box, 320, region_grid)
         assert region[2] - region[0] > 320
+
+
+class _ChunkedPipe(io.RawIOBase):
+    """Stub stdout that returns the queued chunks one readinto at a time."""
+
+    def __init__(self, chunks):
+        self._chunks = [bytes(c) for c in chunks]
+
+    def readable(self) -> bool:
+        return True
+
+    def readinto(self, b) -> int:
+        if not self._chunks:
+            return 0
+        chunk = self._chunks.pop(0)
+        n = min(len(b), len(chunk))
+        b[:n] = chunk[:n]
+        if n < len(chunk):
+            self._chunks.insert(0, chunk[n:])
+        return n
+
+
+class TestReadFrameInto(unittest.TestCase):
+    """Cover the partial-read path of the capture loop helper."""
+
+    def test_full_frame_in_one_call(self):
+        buf = bytearray(8)
+        n = _read_frame_into(_ChunkedPipe([b"\x01\x02\x03\x04\x05\x06\x07\x08"]), buf)
+        assert n == 8
+        assert bytes(buf) == b"\x01\x02\x03\x04\x05\x06\x07\x08"
+
+    def test_short_reads_are_reassembled(self):
+        buf = bytearray(8)
+        pipe = _ChunkedPipe([b"\x01\x02\x03", b"\x04\x05", b"\x06\x07\x08"])
+        n = _read_frame_into(pipe, buf)
+        assert n == 8
+        assert bytes(buf) == b"\x01\x02\x03\x04\x05\x06\x07\x08"
+
+    def test_eof_before_full_buffer_returns_partial_count(self):
+        buf = bytearray(8)
+        n = _read_frame_into(_ChunkedPipe([b"\x01\x02\x03"]), buf)
+        assert n == 3
+
+    def test_immediate_eof_returns_zero(self):
+        buf = bytearray(8)
+        n = _read_frame_into(_ChunkedPipe([]), buf)
+        assert n == 0
+
+    def test_writes_into_existing_memoryview(self):
+        backing = bytearray(8)
+        view = memoryview(backing)
+        n = _read_frame_into(_ChunkedPipe([b"abcdefgh"]), view)
+        assert n == 8
+        assert bytes(backing) == b"abcdefgh"

frigate/video/ffmpeg.py

@@ -52,6 +52,32 @@ def _get_record_segment_time(config: CameraConfig) -> int:
         return DEFAULT_RECORD_SEGMENT_TIME
 
 
+def _read_frame_into(stream, buffer) -> int:
+    """Read len(buffer) bytes from stream directly into buffer.
+
+    Returns the number of bytes actually read. A return value less than
+    len(buffer) indicates the stream reached EOF (or the pipe was closed)
+    before the buffer could be filled — the contents of buffer past the
+    returned offset must be treated as undefined.
+
+    BufferedReader.read(n) is documented to return "up to" n bytes, and a
+    subprocess pipe will hand back a short read if the producer closes
+    mid-frame even on a blocking pipe. Reading directly into the
+    pre-allocated frame buffer also avoids the extra Python bytes
+    allocation and copy that ``buffer[:] = stream.read(n)`` performs on
+    every frame.
+    """
+    target = len(buffer)
+    view = memoryview(buffer)
+    pos = 0
+    while pos < target:
+        n = stream.readinto(view[pos:])
+        if not n:
+            return pos
+        pos += n
+    return pos
+
+
 def capture_frames(
     ffmpeg_process: sp.Popen[Any],
     config: CameraConfig,
@@ -92,7 +118,7 @@ def capture_frames(
             frame_name = f"{config.name}_frame{frame_index}"
             frame_buffer = frame_manager.write(frame_name)
             try:
-                frame_buffer[:] = ffmpeg_process.stdout.read(frame_size)
+                bytes_read = _read_frame_into(ffmpeg_process.stdout, frame_buffer)
             except Exception:
                 # shutdown has been initiated
                 if stop_event.is_set():
@@ -110,6 +136,23 @@ def capture_frames(
 
                 continue
 
+            if bytes_read < frame_size:
+                # ffmpeg's stdout pipe was closed (or returned EOF) before
+                # the full frame could be received. The buffer holds a
+                # partial frame, so don't pass it to detection: a
+                # half-frame in shared memory will produce garbage
+                # detections, and continuing to read would resume in the
+                # middle of the next frame, putting capture out of sync
+                # with frame boundaries until the next watchdog restart.
+                if stop_event.is_set():
+                    break
+
+                logger.error(
+                    f"{config.name}: ffmpeg returned an incomplete frame "
+                    f"({bytes_read}/{frame_size} bytes); exiting capture thread..."
+                )
+                break
+
             frame_rate.update()
 
             # don't lock the queue to check, just try since it should rarely be full