How do I inspect build artifacts when a build fails?

[doyougnu]

Is your feature request related to a problem? Please describe.

• I'm on a nixOS machine, using nix flakes
• I'm trying to package llvm_git and have to apply some patches. These patches fail:

  [I] ╭─doyougnu at 7thChamber in /home/doyougnu/programming/ghc.nix (master↑8 ✚3…1)
  ╰─λ nix flake update; nix develop .#ghcjs-shell                                                                                                                                                                                                                        1 (16.741s)
  warning: Git tree '/store/Programming/ghc.nix' is dirty
  warning: Git tree '/home/doyougnu/programming/nix/nixpkgs' is dirty
  warning: updating lock file '/store/Programming/ghc.nix/flake.lock':
  • Updated input 'nixpkgs-unstable':
  'git+file:///home/doyougnu/programming/nix/nixpkgs?ref=refs%2fheads%2fmaster&rev=31de1747811d4b18c464310f1ac71220aeb7c859' (2022-07-22)
  → 'git+file:///home/doyougnu/programming/nix/nixpkgs' (2022-07-22)
  warning: Git tree '/store/Programming/ghc.nix' is dirty
  warning: Git tree '/store/Programming/ghc.nix' is dirty
  trace: checking if /hadrian/hadrian.cabal is present:  no
  error: builder for '/nix/store/dw46zk0xm1ippmlyc8cj97gp8w3mzgdc-clang-unstable-2022-07-07.drv' failed with exit code 1;
     last 8 log lines:
     > unpacking sources
     > unpacking source archive /nix/store/7bhp3454ncjk519ix5dq7ri58v1c8bhf-clang-src-unstable-2022-07-07
     > source root is clang-src-unstable-2022-07-07/clang
     > patching sources
     > applying patch /nix/store/34zhl915l308dpa7v2786z3xgbvy6398-purity.patch
     > patching file lib/Driver/ToolChains/Gnu.cpp
     > Hunk #1 FAILED at 487.
     > 1 out of 1 hunk FAILED -- saving rejects to file lib/Driver/ToolChains/Gnu.cpp.rej
     For full logs, run 'nix log /nix/store/dw46zk0xm1ippmlyc8cj97gp8w3mzgdc-clang-unstable-2022-07-07.drv'.
  error: 1 dependencies of derivation '/nix/store/vz35xp3zx3cfdj3dn4qgzn1r8l0m64d2-clang-tools-unstable-2022-07-07.drv' failed to build
  error: 1 dependencies of derivation '/nix/store/vj3iz4biq5rbnw9pbr4bqdnc6q3q7ykl-mesa-22.1.1.drv' failed to build
  error: 1 dependencies of derivation '/nix/store/84qfvgaqyb5wiplssrqna7gvrg6qki0m-ghc-shell-for-ghc-buildenv-9.3-0-env.drv' failed to build

and I want to inspect the *.rej files. Where are they? How can I do that? I see it suggests running nix log ... so let's do that:

╰─λ nix log /nix/store/dw46zk0xm1ippmlyc8cj97gp8w3mzgdc-clang-unstable-2022-07-07.drv                                                                                                                                                                                  1 (13.049s)
@nix { "action": "setPhase", "phase": "unpackPhase" }
unpacking sources
unpacking source archive /nix/store/7bhp3454ncjk519ix5dq7ri58v1c8bhf-clang-src-unstable-2022-07-07
source root is clang-src-unstable-2022-07-07/clang
@nix { "action": "setPhase", "phase": "patchPhase" }
patching sources
applying patch /nix/store/34zhl915l308dpa7v2786z3xgbvy6398-purity.patch
patching file lib/Driver/ToolChains/Gnu.cpp
Hunk #1 FAILED at 487.
1 out of 1 hunk FAILED -- saving rejects to file lib/Driver/ToolChains/Gnu.cpp.rej

Ok I see a nix-store path, lets check that:

╰─λ  ls /nix/store/7bhp3454ncjk519ix5dq7ri58v1c8bhf-clang-src-unstable-2022-07-07                                                                                                                                                                                       0 (0.001s)
clang/  cmake/

Ok great let's check the file:

╰─λ find /nix/store/7bhp3454ncjk519ix5dq7ri58v1c8bhf-clang-src-unstable-2022-07-07/ -name '*.rej' 
[I] ╭─doyougnu at 7thChamber in /home/doyougnu/programming/ghc.nix (master↑8 ✚3…1)
╰─λ  

Nothing.

Describe the solution you'd like Perhaps this is a documentation issue, but if it is I want documentation that says:

1. Run a build ... nix build ....
2. If it fails you can always check the working directory ...
3. Or better yet, run nix log --something or nix develop --something which saves the working directory for inspection to /tmp/something

Describe alternatives you've considered I've considered doing cloning llvm manually just to inspect the patch error. But what is the point of using nix if I have to do this when I run into a build problem?

[cole-h]

Have you tried using the --keep-failed flag? After the build fails, it should output something along the lines of "keeping /tmp/....". Usually /tmp/drvname-0, I believe.

[doyougnu]

Yes this is exactly what I was missing. I cannot find a reference to this flag in either the man page for nix build or the nix manual. Am I missing something or is this just undocumented?

[cole-h]

nix --help | grep keep-failed shows it for me.

[doyougnu]

Closing. Thanks for the help @cole-h