422 Update is not a fast forward

[ehuss]

I've been lucky enough to hit "422 Update is not a fast forward" twice in the past two weeks. Not sure if it makes sense to file an issue here (feel free to close). Cross referencing:

• Bors: 422 Update is not a fast forward rust#43535: Report on rust-lang
• Intermittent "Test was successful, but fast-forwarding failed: 422 Update is not a fast forward" servo/homu#24: Report on servo's homu.
• retry if 422 Update is not a fast forward servo/homu#134: Attempt to add a retry

My recent encounters:

• Update cargo, books. rust#65941 (comment)
• Reject feature flags in a virtual workspace. cargo#7507 (comment)

Doesn't seem to happen too often: https://github.com/rust-lang/rust/pulls?utf8=%E2%9C%93&q=is%3Apr+%22update+is+not+a+fast+forward%22+

From what little I've read, I see other projects having similar problems, and some people concluding that the GitHub API can take a while to update (up to 30 seconds?) due to caching. I'm not too familiar with how homu works, but maybe a longer retry might help?

[RalfJung]

It happened again.

[RalfJung]

And again.

[RalfJung]

It now happened in Miri for the first time as well.

[RalfJung]

Looking at the commit it was trying to push to Miri, the error is completely right -- this is not a fast-forward. When bors created the merge commit here, it picked the wrong parent to merge in. It failed to update to the fact that a previous PR had just been merged.

[Mark-Simulacrum]

Interesting! That seems like a potential across the board cause...