Compare accepts a second clip shorter than its advertised output

[msg7086]

> Analysis attribution: This issue analysis was created by Codex GPT.
> The GitHub user who submits this issue does not claim authorship of the analysis.

Finding

• Finding ID: B11-compare-short-second-clip
• Status: Reproduced defect
• Severity: Medium
• Affected component: Compare
• Source location: avs_core/filters/text-overlay.cpp:2389-2405 and 2669-2673

Summary

Construction validates dimensions and format but not that the second clip has enough frames; GetFrame later requests it at n.

Affected Code

const VideoInfo& vi2 = child2->GetVideoInfo();
psnrs = 0;

if (!vi.IsSameColorspace(vi2))
  env->ThrowError("Compare: Clips are not same colorspace.");

if (vi.width != vi2.width || vi.height != vi2.height)
  env->ThrowError("Compare: Clips must have same size.");

PVideoFrame f1 = child->GetFrame(n, env);
PVideoFrame f2 = child2->GetFrame(n, env);

Correct Behavior

Reject construction when the second clip is shorter than the first for per-frame comparison.

Reproduction

Construct Compare with a two-frame first clip and one-frame second clip.

Observed Result

observed: construction succeeds
expected: AvisynthError

Impact

The filter advertises output frames that later fail on the short child.

Validation Criteria

Short second clips fail at construction; equal or longer clips retain behavior.

Version and Environment

• AviSynthPlus revision: 815780226951f95dfc1fc74f1df90ef98824fcf5
• Platform and compiler: Linux x86_64 / Clang 22 debug
• CPU features used: host-default build.