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chore: don't cache errors in file cache#18555

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fromlilac/dont-cache-errors

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@aslilacaslilac commentedJun 24, 2025
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By design, concurrent calls toAcquire in the file cache all share a single database fetch. This is by design, so that everyone can share in the success of whoever asked for the file first. That's kind of what caches do!

but one problem with the current implementation is that errors are also shared. This is mostly fine, because once all of the references are dropped, the cache entry will be freed, and the nextAcquire will trigger a new fetch. However, if enough people are trying to load the same file at once, you could imagine how they might keep retrying and the reference count neverquite hits zero.

To combat this, just immediately and forcibly remove errors from the cache, even if they still have references. Whoever is the first to retry afterwards will trigger a new fetch (like we want), which can then again be shared by others who retry.


Related, one opportunity to reduce the potential for errors we have is to usecontext.Background() for the database fetch so that a canceled request context cannot disrupt others who may be waiting for the file. We can then manually check the context outside of theLoad, just like we already do with authorization.

@aslilacaslilac requested a review fromEmyrkJune 24, 2025 22:50
@aslilacaslilac marked this pull request as ready for reviewJune 24, 2025 22:50
Comment on lines 161 to 163
// Check if the caller's context was canceled
if err := ctx.Err(); err != nil {
return nil, err
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Why do we need to check this? It will fail theAuthorize if it is cancelled

And if we do this check, do we need to close?

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// Check if the caller's context was canceled
iferr:=ctx.Err();err!=nil {
returnnil,err
// Check if the caller's context was canceled
iferr:=ctx.Err();err!=nil {
e.close()
returnnil,err

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I just thought it felt nice to check explicitly 🤷‍♀️

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aslilac commentedJun 25, 2025
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btw@Emyrk, the test as you originally wrote it assumed thatany second caller would refetch, regardless of timing. but we discussed loosening it a bit so that any callerafter the actual errored load would refetch, which is much more timing dependent. I can't really think of a good way to definitively test this behavior, because waiting until after the first fetch errors to run the second fetch means we're also waiting until the refcount would hit zero, which would clear it regardless of error state anyway. but if we call any earlier, most of the time the second caller gets the error, rarely taking long enough to trigger a refetch.

maybe we could add some method to "leak" a reference for testing purposes to ensure that the file is refetched anyway, but I'm never a fan of adding extra complexity just to make something testable.

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