[rush](package-deps-hash) Avoid defect to be produced when spawned "git hash-object" process stdin fails while computing repo-state

[antoine-coulon]

Summary

Fixes #4475

As explained in #3517, symlinked directories can't be hashed by git hash-object. Whatever the underlying reason is for that, as I already stated in the issue, executed commands should not crash due to this problem and just emit a warning.

Note that we might consider this to be a workaround while we fix the root cause which is discarding symlinked directories from the analysis. However I feel like having error handlers there will be worth it anyway as other errors could occur in the future and we definitely don't want to crash the process at this step which is not mandatory for Rush to properly work (file diffing).

Details

I solved the problem by simply attaching an "error" handler on the underlying child process' stdin stream to avoid EPIPE errors (fatal) to occur. This error happens because the git child process closes because of the mentioned errors, but the stdin Readable keeps pushing file paths while the Writable is closed.

How it was tested

Mainly through manual testing using https://github.com/antoine-coulon/rushelp and the Docker container created from it. The issue can easily be re-created, just run the container, move to /build and run rush my-bulk-command, EPIPE will occur. This is because git status includes /bin and /lib symlinked directories so as I said git child process will emit errors.

Impacted documentation

None

[dmichon-msft]

Updating the wording of the comments for clarity about the exact issue being fixed and how the change mitigates it.