GitHub actions may fail with refs/pull/<number>/merge not found when PR is merged immediately after changing title

[FliegendeWurst]

Describe the bug

See e.g.

• https://github.com/NixOS/nixpkgs/actions/runs/12339697729
• https://github.com/NixOS/nixpkgs/actions/runs/12339713317
• https://github.com/NixOS/nixpkgs/actions/runs/12339713307
• https://github.com/NixOS/nixpkgs/actions/runs/12339713316

Steps To Reproduce

See issue title

Expected behavior

GitHub action either uses the merged commit or skips execution.

Additional context

Metadata

Notify maintainers

For Codeowners specifically: @infinisil

It affects other checks too.

---

Note for maintainers: Please tag this issue in your PR.

---

Add a 👍 reaction to issues you find important.

[wolfgangwalther]

I don't think it's related to changing the title - I had this error repeatedly as well the last couple of weeks, without changing any.

[FliegendeWurst]

It is related - changing the title will re-trigger CI.

[infinisil]

Only the first link is an instance of the problem with refs/pull/<number>/merge, and that is fixed on master with #364338! So that just needs a backport :)

The other three failed because they ran on an intermediate commit (4f9ec4f) that didn't have the ffmpeg argument which you later force pushed.

[infinisil]

#365673