[Bugfix] Avoid blocking model launching when no system ffmpeg available for TorchCodec (#47888)

Signed-off-by: Isotr0py <Isotr0py@outlook.com>

vllm/multimodal/video.py

@@ -15,7 +15,7 @@ import torch
 
 from vllm import envs
 from vllm.logger import init_logger
-from vllm.utils.import_utils import PlaceholderModule
+from vllm.utils.import_utils import PlaceholderModule, check_torchcodec_available
 from vllm.utils.mem_constants import MiB_bytes
 from vllm.utils.registry import ExtensionManager
 
@@ -33,7 +33,7 @@ except ImportError:
 
 try:
     from torchcodec.decoders import VideoDecoder
-except ImportError:
+except (ImportError, RuntimeError):
     VideoDecoder = PlaceholderModule("torchcodec").placeholder_attr(  # type: ignore[assignment]
         "decoders.VideoDecoder"
     )
@@ -956,6 +956,7 @@ class VideoBackend(
             assert not frame_recovery, (
                 "frame_recovery is only available for `opencv` backend"
             )
+            check_torchcodec_available()
             decoder = cls.make_torchcodec_decoder(
                 data,
                 num_ffmpeg_threads=num_ffmpeg_threads,

vllm/utils/import_utils.py

@@ -552,3 +552,21 @@ def has_cutedsl() -> bool:
 def has_humming() -> bool:
     """Whether the optional `humming` package is available."""
     return _has_module("humming")
+
+
+def check_torchcodec_available():
+    """Whether the optional `torchcodec` package is available."""
+    try:
+        import torchcodec  # noqa: F401
+    except RuntimeError as e:
+        # torchcodec will raise RuntimeError during import instead
+        # of ImportError when system ffmpeg unavailable, with a
+        # message that can leak sensitive system information.
+        # Trim it down to avoid it.
+        marker = (
+            "The following exceptions were raised as we tried to load libtorchcodec:"
+        )
+        message = str(e)
+        if marker in message:
+            raise RuntimeError(message.split(marker, 1)[0].rstrip()) from None
+        raise e