Check interrupts even when download stalled

[layus]

Motivation

Under bad network, downloads can stall forever. This leaves a running nix-daemon fork behind, even when the client disconnects. Even worse, that daemon keeps a lock in the path, preventing other attempts at downloading it.

/cc @thufschmitt

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
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

[edolstra]

Looks good to me, but it seems to cause a test failure in fetchurl.sh ("coroutine has finished"). Another fix is to move the checkInterrupt() into the inner while loop, though the inner loop does seem superfluous.

[layus]

Yes, I think it stops writing to the sink, and the sink ends up with a partial, incomplete download.
I wonder if there is a proper way to signal that the download failed, or if this is exactly what the FIXME was about.

[layus]

Oh, right, the "coroutine has finished" is the error that happens when the sink ends abruptly.
So some code path fails to catch it, or I need a cleaner close.

[layus]

This definitely deserves a squash before merge, but I like how the diff looks now. An unrolled while loop :-D.

[layus]

I also do not want to checkInterrupt() while holding the lock. I understand that it throws, and the lock should be properly released, but... Well, I do not think it changes anything in the end.

[layus]

I think this is ready for merge/review