Fix: If a file remains stable but has zero size after the stability window has passed, drop it from tracking (#14047)

A slow writer might create a zero byte file, then take longer than the window to
finish the write.  We would then queue a zero byte file for consumption and race the writer
to most likely fail due to still being empty.  Instead, drop zero size files at yeild time.
The slow writer may or may not finish, but if it does, there will be a Change.modified
event fired again

src/documents/management/commands/document_consumer.py

@@ -156,6 +156,15 @@ class FileStabilityTracker:
                     logger.debug(f"File disappeared during stability check: {path}")
                 continue
 
+            # Stable, but empty: some scanners create a zero byte placeholder
+            # and only write the page some time later. Consuming it now can
+            # only fail  so drop it and let the writer's next event
+            # (or the periodic rescan) bring it back once it has content
+            if not tracked.last_size:
+                to_remove.append(path)
+                logger.debug("Ignoring stable but empty file: %s", path)
+                continue
+
             # File is stable, we can return it
             to_yield.append(path)
             logger.info(f"File is stable: {path}")

src/documents/tests/test_management_consumer.py

@@ -136,6 +136,23 @@ def wait_for_mock_call(
     return False
 
 
+def sleep_past_stability(
+    owner: FileStabilityTracker | ConsumerThread,
+    *,
+    windows: float = 1.5,
+) -> None:
+    """
+    Block until a tracked file's stability window has certainly elapsed.
+
+    Args:
+        owner: The tracker, or the consumer thread running one, whose
+               configured stability delay sets the wait.
+        windows: How many stability windows to wait, giving slop for a slow
+                 or loaded test runner.
+    """
+    sleep(owner.stability_delay * windows)
+
+
 class TestTrackedFile:
     """Tests for the TrackedFile dataclass."""
 
@@ -261,6 +278,56 @@ class TestFileStabilityTracker:
         assert len(stable) == 0
         assert stability_tracker.pending_count == 1
 
+    def test_get_stable_files_skips_empty_file(
+        self,
+        stability_tracker: FileStabilityTracker,
+        tmp_path: Path,
+    ) -> None:
+        """
+        GIVEN:
+            - A zero byte file, tracked and past its stability delay
+        WHEN:
+            - Stable files are collected
+        THEN:
+            - The file is not yielded for consumption
+            - The file is dropped from tracking rather than held, so an
+              abandoned placeholder does not keep the watch loop awake
+        """
+        empty = tmp_path / "scan.pdf"
+        empty.write_bytes(b"")
+        stability_tracker.track(empty, Change.added)
+        sleep_past_stability(stability_tracker)
+
+        stable = list(stability_tracker.get_stable_files())
+
+        assert stable == []
+        assert stability_tracker.pending_count == 0
+
+    def test_empty_file_is_yielded_once_content_arrives(
+        self,
+        stability_tracker: FileStabilityTracker,
+        tmp_path: Path,
+    ) -> None:
+        """
+        GIVEN:
+            - A zero byte file which was dropped from tracking while empty
+        WHEN:
+            - The writer fills the file and a new event re-tracks it
+        THEN:
+            - The file is yielded for consumption once it is stable
+        """
+        target = tmp_path / "scan.pdf"
+        target.write_bytes(b"")
+        stability_tracker.track(target, Change.added)
+        sleep_past_stability(stability_tracker)
+        assert list(stability_tracker.get_stable_files()) == []
+
+        target.write_bytes(b"%PDF-1.4 content")
+        stability_tracker.track(target, Change.modified)
+        sleep_past_stability(stability_tracker)
+
+        assert list(stability_tracker.get_stable_files()) == [target]
+
     def test_get_stable_files_deleted_during_check(self, temp_file: Path) -> None:
         """Test deleted file is not returned during stability check."""
         tracker = FileStabilityTracker(stability_delay=0.1)
@@ -879,6 +946,51 @@ class TestCommandWatch:
 
         mock_consume_file_delay.apply_async.assert_called()
 
+    def test_scanner_placeholder_is_not_consumed_while_empty(
+        self,
+        consumption_dir: Path,
+        sample_pdf: Path,
+        mock_consume_file_delay: MagicMock,
+        start_consumer: Callable[..., ConsumerThread],
+    ) -> None:
+        """
+        GIVEN:
+            - A scanner which creates a zero byte placeholder and only writes
+              the page some time later (GH discussion #13969)
+        WHEN:
+            - The placeholder sits untouched well past the stability delay
+            - The scanner then writes the real content
+        THEN:
+            - The empty placeholder is never queued, as it could only fail
+              with "Unsupported mime type inode/x-empty"
+            - The file is queued exactly once, when the content lands
+        """
+        thread = start_consumer(stability_delay=0.2)
+
+        target = consumption_dir / "scan.pdf"
+        target.write_bytes(b"")  # the scanner's placeholder
+
+        # Well past the stability delay: the old behaviour queued it here.
+        sleep_past_stability(thread, windows=5)
+        if thread.exception:
+            raise thread.exception
+        assert mock_consume_file_delay.apply_async.call_count == 0
+
+        shutil.copy(sample_pdf, target)  # the scanner finishes the page
+
+        assert wait_for_mock_call(
+            mock_consume_file_delay.apply_async,
+            timeout_s=5.0,
+        )
+        if thread.exception:
+            raise thread.exception
+
+        assert mock_consume_file_delay.apply_async.call_count == 1
+        queued_doc = mock_consume_file_delay.apply_async.call_args.kwargs["kwargs"][
+            "input_doc"
+        ]
+        assert queued_doc.original_file.name == "scan.pdf"
+
     def test_ignores_macos_files(
         self,
         consumption_dir: Path,