breakpointHook: add for debugging failing builds

Usuage: Add breakpointHook to your `buildInputs` like this:

  stdenv.mkDerivation rec {
    # ...
    buildInputs = [ breakpointHook ];
  });

When the build fails as show in this example:

  pkgs.hello.overrideAttrs (old: {
    buildInputs = [ breakpointHook ];
    postPatch = ''
      false
    '';
  });

It will halt execution printing the following message:

build failed in patchPhase with exit code 1
To attach to this build run the following command as root:

   cntr attach -t command cntr-/nix/store/ynyb4n82x2r7sldd58pbb405jdqh5f00-hello-2.10

Installing cntr and running the command will provide shell access to the
build sandbox of failed build:

sudo cntr attach -t command cntr-/nix/store/ynyb4n82x2r7sldd58pbb405jdqh5f00-hello-2.10
WARNING: bad ownership on /nix/var/nix/profiles/per-user/root, should be 1000
[nixbld@localhost:/var/lib/cntr]$

At /var/lib/cntr the sandbox filesystem is mounted. All commands and
files of the system are still accessible within the shell.
To execute commands from the sandbox use the `cntr exec` subcommand.

1 files changed, 9 insertions, 0 deletions

diff --git a/pkgs/build-support/setup-hooks/breakpoint-hook.sh b/pkgs/build-support/setup-hooks/breakpoint-hook.sh
new file mode 100644
index 000000000000..6bef786ac3ac
--- /dev/null
+++ b/pkgs/build-support/setup-hooks/breakpoint-hook.sh
@@ -0,0 +1,9 @@
+breakpointHook() {
+    local red='\033[0;31m'
+    local no_color='\033[0m'
+
+    echo -e "${red}build failed in ${curPhase} with exit code ${exitCode}${no_color}"
+    printf "To attach install cntr and run the following command as root:\n\n"
+    sh -c "echo '   cntr attach -t command cntr-${out}'; while true; do sleep 99999999; done"
+}
+failureHooks+=(breakpointHook)