DoubleWeaveFields and DoubleWeaveFrames request out-of-bounds child frame at output tail

[msg7086]

Analysis attribution: This issue analysis was created by Gemini 3.7 Flash.
The GitHub user who submits this issue does not claim authorship of the analysis.

Finding

• Finding ID: F24-doubleweave-tail-frame-out-of-bounds
• Status: Reproduced defect
• Severity: Medium
• Affected component: DoubleWeaveFields and DoubleWeaveFrames
• Source location: avs_core/filters/field.cpp:866-879 and 930-940

Summary

DoubleWeaveFields and DoubleWeaveFrames advertise output frame counts up to their tail frames, but unconditionally request frame n + 1 or (n + 1) >> 1 from the child clip, which requests 1 frame past the end of the child clip when evaluating the final output frame.

Affected Code

PVideoFrame DoubleWeaveFields::GetFrame(int n, IScriptEnvironment* env)
{
  PVideoFrame a = child->GetFrame(n, env);
  PVideoFrame b = child->GetFrame(n+1, env);

  PVideoFrame result = env->NewVideoFrameP(vi, &a);

  const bool parity = child->GetParity(n);

  copy_field(result, a, vi.IsYUV() || vi.IsYUVA(), vi.IsPlanarRGB() || vi.IsPlanarRGBA(), parity, env);
  copy_field(result, b, vi.IsYUV() || vi.IsYUVA(), vi.IsPlanarRGB() || vi.IsPlanarRGBA(), !parity, env);

  return result;
}

PVideoFrame DoubleWeaveFrames::GetFrame(int n, IScriptEnvironment* env)
{
  if (!(n&1))
  {
    return child->GetFrame(n>>1, env);
  }
  else {
    PVideoFrame a = child->GetFrame(n>>1, env);
    PVideoFrame b = child->GetFrame((n+1)>>1, env);
    bool parity = this->GetParity(n);
...

Correct Behavior

Clamp the second child frame index to the child clip's last available frame (std::min(n + 1, child->GetVideoInfo().num_frames - 1) or std::min((n + 1) >> 1, child->GetVideoInfo().num_frames - 1)) so that evaluating the last advertised frame does not exceed child bounds.

Reproduction

Construct DoubleWeaveFields or DoubleWeaveFrames on a finite child clip of length $N$, then call GetFrame(vi.num_frames - 1).

Observed Result

observed: filter requests frame N from child clip, throwing out_of_range or crashing on finite sources
expected: filter successfully serves the last frame without requesting past-end frames from child

Impact

Consumers requesting the final advertised frame of a double-weave stream fail with out-of-range exceptions when backed by strict or finite frame sources.

Validation Criteria

Evaluating GetFrame(vi.num_frames - 1) succeeds and strictly requests only child frames in the range [0, child_num_frames - 1].

Version and Environment

• AviSynthPlus revision: 80da03a5ee437c2b103dbebc9fab43480a9d97c6
• Platform and compiler: Linux x86_64 / Windows x64 GCC/MSVC
• CPU features used: host-default build.

[msg7086]

AVS+ would silently return last frame when out of range frame is requested. Not reproducible by avs script, but it's still a bug.