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debug nix-build in a nix-shell

• run the build phases explicitly
• modify the build phases (they are stored in *.sh files)
• continue running build phases from a specified line number

working prototype

what works

• start a debug shell, for examplenix-build-debug '<nixpkgs>' -A hello
• use therunPhase function to run phases, for examplerunPhase unpackPhase
• auto-completion for therunPhase function.withrunPhase [Tab][Tab] all build phases are listed in order
• stop and continue calls to therunPhase function.this works by using abash compiled with--disable-job-control.sending Ctrl-Z (SIGTSTP) torunPhase will stop the whole debug shell.see alsodoc/bash-trap-exit-try-catch.md

what is not tested

• continue phase from line N.example:runPhase buildPhase 123 to continuebuildPhase from line 123

usingnix-shell to debug a failing nix build is not ideal

nix-build runs the build as a bash script withset -eso the build stops on the first error

nix-shell starts an interactive bash shell withset +eso that errors dont exit the shell.but withset +e, the build does not stop on the first error,and continues executing commands after the error

example

$nix_expr='  with import <nixpkgs> {};  stdenv.mkDerivation {    name = "x";    buildCommand = "echo buildCommand; false; echo still running";  }'$nix-build -E"$nix_expr"buildCommanderror: builder for '/nix/store/29zpgfmmsvf81m49piy26daxljpnli0s-x.drv' failed with exit code 1;$nix-shell -E"$nix_expr"$runPhase buildCommandRunning phase: buildCommandbuildCommandstill running

nix-build-debug solves this problemby running the phase functions in subshells

runPhase() {  curPhase=$1  (set -e;$curPhase )# run $curPhase in subshell (...)}

in these subshells,there isset -e to stop the script on the first error.now, when a build phase fails, the subshell is terminated,but therunPhase function keeps running, and the debug shell keeps running

nix-build-debug also allows tocontinue running a build phase from a certain line in the build phase

example:thebuildPhase fails on a command on line 10.now we can modify thebuildPhase.sh scriptand continue running thebuildPhase from line 10

nix-build-debug '<nixpkgs>' -A hello

runPhase [Tab][Tab]

runPhase unpackPhaserunPhase configurePhaserunPhase buildPhaserunPhase installPhase

installPhase will install the build result in$NIX_BUILD_TOP/result*

when a phase fails, fix the phase script

nano $NIX_BUILD_TOP/.nix-build-debug/lib/buildPhase.sh

then continue running the phase, for example from line 123

runPhase buildPhase 123

• https://unix.stackexchange.com/questions/498435/how-do-i-diagnose-a-failing-nix-build
• https://github.com/NixOS/nixpkgs/blob/master/doc/stdenv/stdenv.chapter.md#building-a-stdenv-package-in-nix-shell-sec-building-stdenv-package-in-nix-shell
• https://discourse.nixos.org/t/nix-build-phases-run-nix-build-phases-interactively/36090
• https://nixos.wiki/wiki/Development_environment_with_nix-shell#stdenv.mkDerivation
• https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/generic/setup.sh

• nix-shell
  • nix-shell with build dependencies of derivation
  • nix develop
  • rewrite nix-shell in bash
    • generate rcfile for bash
    • get-env.sh
    • dump nix-shell environment to json file
    • declare bash variables
    • declare bash functions
    • bashFunctions
• nix-build
  • what would nix-build do
  • debug build phases of nix-build
  • debug a failing nix-build
  • debug a failing nix build
• nix
  • nixos
  • nixpkgs