PostFetchRefresh's refresh is the only background refresh carrying a
Then callback, and Then callbacks are not generation-guarded: when the
background fetch's refresh crossed a repo switch, the callback still
ran — in the new repo — and auto-forwarded the new repo's branches
because the old repo's fetch had completed. That was harmless in
practice (the update-ref call compares against the expected old value,
and it only does what the next fetch's auto-forward would do anyway),
but mutating refs in a repo whose fetch never happened is not an action
the user took. Skip the auto-forward when the repo generation changed
since the fetch started.
The generation is captured by the fetch's callers before the fetch
runs, not by PostFetchRefresh itself: the background fetch doesn't
block repo switching and is a network call, so by the time
PostFetchRefresh runs a switch may already have happened — a capture
there (or the one the refresh itself takes) would compare against the
new repo's generation and let the auto-forward through. For the manual
fetch the capture point makes no difference, since a foreground
operation blocks repo switching for its entire duration.
This deliberately guards only this call site rather than making Then
callbacks generation-guarded in general: a Then is an arbitrary
callback, and whether it is safe to skip on a repo switch is a decision
for the author of the call site.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Stefan HallerandClaude Fable 5
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