Add FOD hash-mismatch errors to the build log

[thufschmitt]

Fixes #8166.

The implementation in local-derivation-goal.cc is a bit too convoluted for my taste, but I don't know whether there'a an easier way for dumping an error to a Sink, so 🤷

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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[picnoir]

Thanks a lot for tackling this <3

The hash mismatch message does appear in nix log with this PR, however, it does at the very top of the log:

/tmp/tmp.KRKQZ67Z6f » nix log /nix/store/hxj5rv8170q15j36xz4x0sj9jllvpagg-repro-0.0.drv > test                                                                                                          ninjatrappeur@framework

/tmp/tmp.KRKQZ67Z6f » cat test                                                                                                                                                                          ninjatrappeur@framework
error: hash mismatch in fixed-output derivation '/nix/store/hxj5rv8170q15j36xz4x0sj9jllvpagg-repro-0.0.drv':
         specified: sha256-0000000000000000000000000000000000000000000=
            got:    sha256-WJG1tSLV3whtD/CxEPvZ0hu0/HFjrzTQgoai6Eb2vgM=@nix { "action": "setPhase", "phase": "unpackPhase" }
unpacking sources
unpacking source archive /nix/store/qcjc1p279ric98fs6vqnyd76yd8vjk6p-tmp.KRKQZ67Z6f
source root is tmp.KRKQZ67Z6f
@nix { "action": "setPhase", "phase": "patchPhase" }
patching sources
@nix { "action": "setPhase", "phase": "configurePhase" }
configuring
no configure script, doing nothing
@nix { "action": "setPhase", "phase": "buildPhase" }
building
@nix { "action": "setPhase", "phase": "installPhase" }
installing
no Makefile or custom installPhase, doing nothing
@nix { "action": "setPhase", "phase": "fixupPhase" }
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0
checking for references to /build/ in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0...
patching script interpreter paths in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0

I guess the logsink is populated down the road in derivation-goal.cc (this part is quite dense and hard to dive in!)

[thufschmitt]

however, it does at the very top of the log:

Uh my bad, I've used the wrong method for writing the log. The writing is buffered, but the method I've used (.write()) bypasses that buffer. This means that the hash-mismatch exception is interleaved in the middle of the log. If you try with a log big-enough to not fit in the buffer you'll see that it falls somewhere in the middle. I've just fixed that, it should be properly at the end now

[picnoir]

Ooof, I was looking in the wrong direction 😅 Thanks for the explanation.

---

Much better, works as expected now!

/tmp/tmp.sveykX6nvu » ~/code-root/github.com/NixOS/nix/outputs/out/bin/nix-build test.nix                             ninjatrappeur@trantor
this derivation will be built:
  /nix/store/4537qi5m02dysfpz9yinw13cspjzzqn1-repro-0.0.drv
building '/nix/store/4537qi5m02dysfpz9yinw13cspjzzqn1-repro-0.0.drv'...
unpacking sources
unpacking source archive /nix/store/x78f23szjgnpp40xzjvqfb5zxjryayjh-tmp.sveykX6nvu
source root is tmp.sveykX6nvu
patching sources
configuring
no configure script, doing nothing
building
installing
no Makefile or custom installPhase, doing nothing
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0
checking for references to /build/ in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0...
patching script interpreter paths in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0
error: hash mismatch in fixed-output derivation '/nix/store/4537qi5m02dysfpz9yinw13cspjzzqn1-repro-0.0.drv':
         specified: sha256-0000000000000000000000000000000000000000000=
            got:    sha256-WJG1tSLV3whtD/CxEPvZ0hu0/HFjrzTQgoai6Eb2vgM=

/tmp/tmp.sveykX6nvu » nix log /nix/store/4537qi5m02dysfpz9yinw13cspjzzqn1-repro-0.0.drv > out                         ninjatrappeur@trantor

/tmp/tmp.sveykX6nvu » cat out                                                                                         ninjatrappeur@trantor
@nix { "action": "setPhase", "phase": "unpackPhase" }
unpacking sources
unpacking source archive /nix/store/x78f23szjgnpp40xzjvqfb5zxjryayjh-tmp.sveykX6nvu
source root is tmp.sveykX6nvu
@nix { "action": "setPhase", "phase": "patchPhase" }
patching sources
@nix { "action": "setPhase", "phase": "configurePhase" }
configuring
no configure script, doing nothing
@nix { "action": "setPhase", "phase": "buildPhase" }
building
@nix { "action": "setPhase", "phase": "installPhase" }
installing
no Makefile or custom installPhase, doing nothing
@nix { "action": "setPhase", "phase": "fixupPhase" }
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0
checking for references to /build/ in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0...
patching script interpreter paths in /nix/store/nkrm4gmd6zaxkly9dhqz4cw7lz0sjwjd-repro-0.0
error: hash mismatch in fixed-output derivation '/nix/store/4537qi5m02dysfpz9yinw13cspjzzqn1-repro-0.0.drv':
         specified: sha256-0000000000000000000000000000000000000000000=
            got:    sha256-WJG1tSLV3whtD/CxEPvZ0hu0/HFjrzTQgoai6Eb2vgM=%