Make "NAR info file is corrupt" messages more informative

[wentasah]

Motivation

Recently, I encountered the NAR info file 'xxxx' is corrupt error with my binary cache. The message is not helpful in determining, which kind of corruption happened. The file, fetched with curl, looked reasonably.

This commit adds more information to the error message, which should allow debugging and hopefully fixing the problem. In my case, there was an extra empty line at the end of the narinfo file so the error reported in my case was: NAR info file 'xxxx' is corrupt: expecting ':' at line 8.

Context

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[thufschmitt]

Thanks, that could be useful indeed :)

Do you think you could also add some line numbers to the error message (I assume that just increasing a counter at each iteration would be enough)? That way messages like expecting ':' would really be meaningful and point to the right line

[wentasah]

Good idea. I've added line numbers and also updated the NarSize missing or zero message to better reflect the situation.

Tested with intentionally broken nix-serve.

[thufschmitt]

Thanks :)