Integrity check: don't quarantine valid streamed FLAC as 'zero-length' (#756)

HiFi assembles FLAC from HLS segments and demuxes with `ffmpeg -c copy`,
which preserves total_samples=0 in the STREAMINFO of Tidal's fragmented/
streamed FLAC. Every audio frame is present and the file plays fine, but
mutagen computes length = total_samples / sample_rate = 0, so the integrity
check rejected it as 'zero-length audio' and quarantined nearly every HiFi
download. Users confirmed the quarantined files play normally once restored.

Length 0 is not proof of corruption at that point: the file already passed
the size gate, was identified as a real audio format, and has a valid info
block — a genuinely empty/truncated/stub file fails one of those earlier
checks instead. Treat length 0 as 'length unknown': accept the file and skip
the duration cross-check we can't perform without a length. mutagen never
decoded/validated frame data anyway, so this doesn't weaken real corruption
detection — size, parse, format, info-block, and duration-drift guards all
remain.

Tests: a large valid-parse length-0 file (streamed-FLAC signature) is now
accepted; a tiny length-0 stub still fails (size gate fires first).

core/imports/file_integrity.py

@@ -183,11 +183,29 @@ def check_audio_integrity(
     checks["actual_length_s"] = actual_length_s
 
     if actual_length_s <= 0:
+        # Length 0 is NOT proof of corruption here: the file already passed the
+        # size gate, was identified as a real audio format, and has a valid
+        # info block. A genuinely empty/truncated/stub file fails one of those
+        # earlier checks instead. The real cause of a clean-but-zero-length
+        # parse is "length unknown" — fragmented / streamed FLAC carries
+        # total_samples=0 in its STREAMINFO even though every audio frame is
+        # present and the file plays fine. HiFi is the common trigger: it
+        # assembles FLAC from HLS segments and demuxes with `ffmpeg -c copy`,
+        # which preserves total_samples=0, so mutagen computes length 0 and the
+        # file was wrongly quarantined (#756). Treat it as unknown length:
+        # accept the file and skip the duration cross-check we can't perform
+        # without a length. mutagen never decoded/validated frame data anyway,
+        # so accepting here doesn't weaken real corruption detection.
+        logger.warning(
+            "[Integrity] %s parsed cleanly (%d bytes, format=%s) but reports "
+            "length 0 — treating as unknown length (likely streamed/fragmented "
+            "FLAC), not rejecting",
+            os.path.basename(file_path), size, type(audio).__name__,
+        )
         return IntegrityResult(
-            ok=False,
-            reason="Mutagen reports zero-length audio — file has no playable "
-                   "audio data",
-            checks={**checks, "mutagen_parse": "zero_length"},
+            ok=True,
+            checks={**checks, "mutagen_parse": "zero_length_unknown",
+                    "length_check": "skipped_unknown_length"},
         )
 
     # --- Check 3: duration agreement (optional) ---

tests/imports/test_file_integrity.py

@@ -124,6 +124,52 @@ def test_accepts_valid_wav_with_no_expected_duration(tmp_path: Path) -> None:
     assert result.checks["length_check"] == "skipped"
 
 
+# ---------------------------------------------------------------------------
+# Zero-length-but-valid parse (#756): streamed / fragmented FLAC
+# ---------------------------------------------------------------------------
+
+
+def test_accepts_zero_length_when_parse_is_otherwise_valid(tmp_path: Path, monkeypatch) -> None:
+    """#756: HiFi assembles FLAC from HLS segments and demuxes with
+    `ffmpeg -c copy`, leaving STREAMINFO total_samples=0. Mutagen then
+    reports length 0 even though every audio frame is present and the file
+    plays fine. Such a file — large, identifiable format, valid info block —
+    must be ACCEPTED (unknown length), not quarantined as 'zero-length'."""
+    import mutagen
+
+    f = tmp_path / "streamed.flac"
+    f.write_bytes(b"\x00" * (50 * 1024))  # clears size gate; bytes irrelevant (mutagen mocked)
+
+    # Valid parse, valid info block, but length 0 — the streamed-FLAC signature.
+    fake_audio = SimpleNamespace(info=SimpleNamespace(length=0))
+    monkeypatch.setattr(mutagen, "File", lambda *a, **k: fake_audio)
+
+    # Even with an expected duration present, it must accept (can't compare to
+    # an unknown length) rather than reject.
+    result = file_integrity.check_audio_integrity(str(f), expected_duration_ms=200_000)
+
+    assert result.ok is True
+    assert result.checks["mutagen_parse"] == "zero_length_unknown"
+    assert result.checks["length_check"] == "skipped_unknown_length"
+
+
+def test_zero_length_still_rejects_when_too_small(tmp_path: Path, monkeypatch) -> None:
+    """A genuinely empty/stub file with length 0 must still fail — the size
+    gate fires before parse, so the relaxation can't let real stubs through."""
+    import mutagen
+
+    f = tmp_path / "stub.flac"
+    f.write_bytes(b"\x00" * 200)  # below the 10KB size gate
+
+    fake_audio = SimpleNamespace(info=SimpleNamespace(length=0))
+    monkeypatch.setattr(mutagen, "File", lambda *a, **k: fake_audio)
+
+    result = file_integrity.check_audio_integrity(str(f))
+
+    assert result.ok is False
+    assert "too small" in result.reason.lower()
+
+
 # ---------------------------------------------------------------------------
 # Duration agreement check
 # ---------------------------------------------------------------------------