Do not retry permanent backend failures #4803
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An individual flag for each change of the backend error handling would be too finegrained. Thus, add a generic flag.
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This is currently gated behind a feature flag as some unexpected interactions might show up in the wild.
These backends tend to use a large variety of server implementations. Some of those implementations might prove problematic with the new checks.
If a file exhausts its retry attempts, then it is likely not accessible the next time. Thus, immediately fail all load calls for that file to avoid useless retries.
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This was referenced May 25, 2024
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What does this PR change? What problem does it solve?
Restic currently retries attempts to access a non-existing file or out of bound reads for truncated files. These errors are however permanent and cannot be fixed by retrying. This PR adds the necessary infrastructure to let the RetryBackend detect these cases and stop the useless retries.
In addition, the RetryBackend now implements a circuit breaker for inaccessible files. If accessing a file exhausts its retries, then the file is considered broken and won't be accessed again for one hour. This also benefits #4800 , as the fallback paths will immediately fail for such broken files.
Was the change previously discussed in an issue or on the forum?
Not really, but this is another prerequisite (and hopefully the last one) for #4784 .
Checklist
[ ] I have added documentation for relevant changes (in the manual).changelog/unreleased/that describes the changes for our users (see template).gofmton the code in all commits.