fetchGit transient failure not retried, Empty reply from server

[roberth]

Describe the bug

Git fetching should retry if the fetcher fails after a git fetch/git pull failure.

Steps To Reproduce

1. Have an unreliable git host
2. Call fetchGit with a rev that hasn't been fetched yet. (in my case, ref was also specified, and I used an http url to a private github.com repo)
3. Observe a log such as:

fetching Git repository 'https://github.com/ORG/REPO.git'
fatal: unable to access 'https://github.com/ORG/REPO.git/': Empty reply from server
warning: could not update local clone of Git repository 'https://github.com/ORG/REPO.git'; continuing with the most recent version

This was a transient failure. The next evaluation fetched it correctly.

Expected behavior

Perform a retry of the entire fetcher if a possible network error was detected.
Approximately:

1. check the cache for rev (if we don't care about testing that a rev still exists - idk, configurable?)
2. git fetch or similar. Retry this step, because we know that we need it to work. Don't retry a missing ref or permission error.
3. proceed with the fetched git data as usual

nix-env --version output

2.11

Additional context

Priorities

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[roberth]

Here's another one

fetching Git repository 'https://github.com/hercules-ci/hercules-ci-effects.git'
fatal: unable to access 'https://github.com/hercules-ci/hercules-ci-effects.git/': Recv failure: Connection reset by peer
warning: could not update local clone of Git repository 'https://github.com/hercules-ci/hercules-ci-effects.git'; continuing with the most recent version

[roberth]

Or this:

fetching Git repository 'https://github.com/nixified-ai/flake.git'
error: RPC failed; curl 55 Failed sending data to the peer
fatal: expected flush after ref listing
warning: could not update local clone of Git repository 'https://github.com/nixified-ai/flake.git'; continuing with the most recent version