[Bugfix][MM] Fix OpenPangu video backend crash on num_frames=-1

[hungnnvidia]

Purpose

OpenCVDynamicOpenPanguVideoBackend.compute_frames_index_to_sample never normalizes the num_frames = -1 ("no cap / all frames") sentinel that every other video backend honors via if num_frames > 0. The backend's own load_bytes default is num_frames=-1, so with the default the fps > 0 cap branch condition num_frames >= int(total_duration * fps) + 1 evaluates to -1 >= (>=1) → False, leaving num_frames == -1, which then hits:

np.linspace(0, total_duration, num_frames, dtype=float)  # num_frames == -1

→ ValueError: Number of samples, -1, must be non-negative. The fps == -1 ("no fps limit") path crashes the same way.

Fix

Treat num_frames < 0 as "no cap":

• fps > 0: use the fps-derived count (int(total_duration * fps) + 1).
• fps == -1: sample every frame (total_frames_num).

Positive num_frames (within or above the fps cap) and the invalid-fps ValueError are unchanged.

Reproduction (from the real code)

Before (origin/main):

>>> OpenCVDynamicOpenPanguVideoBackend.compute_frames_index_to_sample(
...     VideoSourceMetadata(total_frames_num=100, original_fps=30, duration=100/30),
...     VideoTargetMetadata(num_frames=-1, fps=2, max_duration=300))
ValueError: Number of samples, -1, must be non-negative.

After:

num_frames=-1, fps=2   -> 7 indices,  range=[0,90], sorted, in-range   (was CRASH)
num_frames=-1, fps=-1  -> 100 indices, range=[0,99], sorted, in-range  (was CRASH)
num_frames=8,  fps=2   -> capped to fps count (unchanged)
num_frames=10000, fps=2-> capped to fps count (unchanged)
fps=0                  -> ValueError: requires dataset fps is -1 or greater than 0 (unchanged)

Test Plan / Result

• Added test_openpangu_num_frames_sentinel (parametrized fps=2 and fps=-1) to tests/multimodal/test_video.py.
• pytest tests/multimodal/test_video.py -k openpangu_num_frames_sentinel → 2 passed.
• pre-commit run ruff-check --files vllm/multimodal/video.py tests/multimodal/test_video.py → Passed.

Not a duplicate

Searched open PRs on vllm-project/vllm (openpangu num_frames, OpenPangu video sampling); none address this. This is distinct from the recent zero-fps guard PRs (#54390 Qwen3-VL, #54396 GLM-GA), which fix a different ZeroDivisionError, not the num_frames=-1 sentinel.

Notes

The openpangu backend is opt-in (registered without a video_processor auto-mapping), but its own default parameter value crashes it, so any explicit selection without also overriding num_frames hits this. AI assistance was used to locate, reproduce, fix, and test this bug; a human has reviewed the change.

[claude]

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