fix(plex): a codec the GPU cannot decode is decoded on the CPU and still encoded on the GPU

The prod iGPU has no VC-1 Advanced decode ("No support for codec vc1 profile 3"), so every VC-1
session died on the GPU and fell back to libx264, which needs ~17 s for the three warm-up segments:
a start or a seek took ~19.5 s. VC-1, XviD and DivX now take CPU decode -> hwupload -> h264_vaapi
(three segments in 5.5 s for a 1080p VC-1 remux, measured on prod hardware with this argv).

The transcode_hw facet now says whether the GPU encodes: VC-1/XviD/DivX count, and so does an HDR
film (the OpenCL tonemap runs on the GPU; the facet said software). Migration 0119 rewrites the
stored facet and re-queues 10-bit items for the HDR probe.

backend/tests/test_plex_playback_class.py

@@ -32,15 +32,25 @@ class TestTranscodeHwCapable:
         assert sync.transcode_hw_capable("transcode", "hevc") is True
         assert sync.transcode_hw_capable("transcode", "vp9", bit_depth=8) is True
 
-    def test_non_decodable_codec_is_sw(self):
-        # AV1 / MPEG-4 ASP (XviD) can't be HW-decoded on Gen 9.5 → software regardless of bit depth.
+    def test_a_codec_neither_path_knows_is_sw(self):
         assert sync.transcode_hw_capable("transcode", "av1") is False
-        assert sync.transcode_hw_capable("transcode", "mpeg4", bit_depth=8) is False
+        assert sync.transcode_hw_capable("transcode", "av1", bit_depth=8) is False
 
-    def test_10bit_or_hdr_hevc_is_sw(self):
-        # A HW-decodable codec that is 10-bit or HDR still breaks the VAAPI nv12 pipeline → software.
+    def test_a_cpu_decoded_8bit_codec_is_encoded_on_the_gpu(self):
+        # VC-1 (the iGPU has no VC-1 Advanced decode), XviD, DivX: the CPU decodes, the GPU encodes.
+        for codec in ("vc1", "mpeg4", "msmpeg4v3"):
+            assert sync.transcode_hw_capable("transcode", codec) is True
+        assert sync.transcode_hw_capable("transcode", "mpeg4", bit_depth=10) is False
+
+    def test_10bit_sdr_hevc_is_sw(self):
+        # 10-bit SDR: an exact downconvert on the CPU (libx264), never the HDR tone curve.
         assert sync.transcode_hw_capable("transcode", "hevc", bit_depth=10) is False
-        assert sync.transcode_hw_capable("transcode", "hevc", bit_depth=8, hdr=True) is False
+
+    def test_hdr_hevc_is_encoded_on_the_gpu(self):
+        # HDR goes through the OpenCL tonemap between VAAPI decode and encode (the prod image ships the
+        # runtime); the facet said software for it while every such session ran on the GPU.
+        assert sync.transcode_hw_capable("transcode", "hevc", bit_depth=10, hdr=True) is True
+        assert sync.transcode_hw_capable("transcode", "hevc", bit_depth=8, hdr=True) is True
 
 
 class TestVideoStreamProps:
@@ -87,6 +97,9 @@ class TestSyncDerivation:
         row2 = PlexItem(rating_key="y", kind="movie")
         sync._apply_item(row2, lib_id=1, kind="movie", meta=self._meta("av1"))
         assert row2.transcode_hw is False  # av1 not HW-decodable
+        row3 = PlexItem(rating_key="z", kind="movie")
+        sync._apply_item(row3, lib_id=1, kind="movie", meta=self._meta("vc1"))
+        assert row3.transcode_hw is True  # CPU decode, GPU encode
 
     def test_probed_value_survives_resync_when_codec_unchanged(self):
         row = PlexItem(rating_key="x", kind="movie")
@@ -124,21 +137,24 @@ class TestSyncDerivation:
         assert row.video_bit_depth is None and row.media_probed_at is None  # re-probe scheduled
 
 
-def test_migration_codec_set_matches_hw_decodable_codecs():
-    # Migration 0077 hardcodes the HW-decodable codec set in raw SQL and its comment mandates it stay
-    # identical to plex.sync.HW_DECODABLE_CODECS. A comment isn't enforcement (house rule) — pin the
-    # equality so adding/removing a codec from the frozenset without editing the migration fails here
-    # (else already-migrated prod rows keep a wrong transcode_hw the runtime derivation wouldn't catch).
+def test_migration_codec_sets_match_the_sync_sets():
+    # The newest migration that rewrites `transcode_hw` hardcodes both codec sets in raw SQL. Pin them
+    # to plex.sync, so a codec added to or removed from a frozenset without a migration that rewrites
+    # the stored rows fails here (else prod rows keep a facet the runtime choice disagrees with).
     mig = (
         Path(__file__).resolve().parent.parent
         / "alembic"
         / "versions"
-        / "0077_plex_playback_class.py"
+        / "0119_plex_transcode_gpu.py"
     )
-    m = re.search(r"lower\(codec_video\)\s+IN\s+\(([^)]*)\)", mig.read_text())
-    assert m, "could not find the codec IN(...) list in migration 0077"
-    codecs = {c.strip().strip("'\"").lower() for c in m.group(1).split(",")}
-    assert codecs == {c.lower() for c in sync.HW_DECODABLE_CODECS}
+    src = mig.read_text()
+    sets = {}
+    for name in ("HW_DECODABLE_CODECS", "CPU_DECODE_CODECS"):
+        m = re.search(name + r"\s*=\s*\(([^)]*)\)", src)
+        assert m, f"could not find {name} in migration 0119"
+        sets[name] = {c.strip().strip("'\"").lower() for c in m.group(1).split(",") if c.strip()}
+    assert sets["HW_DECODABLE_CODECS"] == {c.lower() for c in sync.HW_DECODABLE_CODECS}
+    assert sets["CPU_DECODE_CODECS"] == {c.lower() for c in sync.CPU_DECODE_CODECS}
 
 
 class _FakePlex:
@@ -182,22 +198,42 @@ def test_probe_media_batch_refines_hw_to_sw_for_10bit(db):
         codec_video="mpeg4",
         transcode_hw=False,
     )
-    db.add_all([hevc8, hevc10, xvid])
+    hdr10 = PlexItem(
+        rating_key="hdr",
+        library_id=lib.id,
+        kind="movie",
+        title="hdr",
+        playable="transcode",
+        codec_video="hevc",
+        transcode_hw=True,
+    )
+    db.add_all([hevc8, hevc10, xvid, hdr10])
     db.commit()
     plex = _FakePlex(
         {
             "h8": {"Media": [{"Part": [{"Stream": [{"streamType": 1, "bitDepth": 8}]}]}]},
             "h10": {"Media": [{"Part": [{"Stream": [{"streamType": 1, "bitDepth": 10}]}]}]},
+            "hdr": {
+                "Media": [
+                    {
+                        "Part": [
+                            {"Stream": [{"streamType": 1, "bitDepth": 10, "colorTrc": "smpte2084"}]}
+                        ]
+                    }
+                ]
+            },
         }
     )
     probed = sync._probe_media_batch(db, plex)
     db.refresh(hevc8)
     db.refresh(hevc10)
     db.refresh(xvid)
-    # Only the two HW-decodable HEVC items are probed; the XviD is SW-by-codec and never fetched.
-    assert probed == 2
+    db.refresh(hdr10)
+    # Only the HW-decodable HEVC items are probed; the XviD's class follows from its codec alone.
+    assert probed == 3
     assert hevc8.video_bit_depth == 8 and hevc8.transcode_hw is True and hevc8.media_probed_at
-    assert hevc10.video_bit_depth == 10 and hevc10.transcode_hw is False  # downgraded to software
+    assert hevc10.video_bit_depth == 10 and hevc10.transcode_hw is False  # 10-bit SDR: software
+    assert hdr10.video_bit_depth == 10 and hdr10.transcode_hw is True  # HDR: the GPU tonemap
     assert xvid.media_probed_at is None  # not in the work-list
     # A second run has nothing left to do (watermarks set).
     assert sync._probe_media_batch(db, plex) == 0