breakpointHook: re-implement using nsenter

breakpointHook is amazing and has been a daily driver for me. Though it has been buggy several times in the past and recently it stopped working on my system, as the approach using cntr seems to be a bit sensitive to the environment and which shell is used etc.

After speaking with @mic92, it turned out there is an alternative approach using `nsenter`. After playing with that for a while, I think we should replace the current implementation of breakpointHook.

The new output when a build fails will look like this:
```
build failed in configurePhase with exit code 1
To attach, run the following command:
    sudo /nix/store/2kzihv08w9vsyi6zhs0pi2i9117v4q1v-attach/bin/attach 28715971284
```

Benefits of the new implementation over the old one:
- no need to install cntr tool on the local machine first, the copy pasted command will just work
- no need to execute `cntr exec ...` manually to enter the namespace
- interactive shell by default
- dropped into correct working directory by default
- $out available by default
- possibility to override the shell with zfs or fish, etc. by defining `debugShell` in the derivation to debug
- probably less prone to bugs as it only uses nsenter

cc @mic92

pkgs/by-name/br/breakpointHook/attach.sh

@@ -0,0 +1,46 @@
+#! /usr/bin/env bash
+
+set -eu -o pipefail
+
+id="$1"
+pids="$(pgrep -f "sleep $id" || :)"
+if [ -z "$pids" ]; then
+  echo "Error: No process found for 'sleep $id'. The build must still be running in order to attach. Also make sure it's not on a remote builder." >&2
+  exit 1
+elif [ "$(echo "$pids" | wc -l)" -ne 1 ]; then
+  echo "Error: Multiple processes found matching 'sleep $id'" >&2
+  exit 1
+fi
+pid="$(echo "$pids" | head -n1)"
+
+# helper to extract variables from the build env
+getVar(){
+  while IFS= read -r -d $'\0' line; do
+    case "$line" in
+      *"$1="* )
+        echo "$line"
+        ;;
+    esac
+  done < /proc/$pid/environ \
+  | cut -d "=" -f 2
+}
+
+# bash is needed to load the env vars, as we do not know the syntax of the debug shell.
+# bashInteractive is used instead of bash, as we depend on it anyways, due to it being
+#   the default debug shell
+bashInteractive="$(getVar bashInteractive)"
+# the debug shell will be started as interactive shell after loading the env vars
+debugShell="$(getVar debugShell)"
+# to drop the user into the working directory at the point of failure
+pwd="$(readlink /proc/$pid/cwd)"
+
+# enter the namespace of the failed build
+exec nsenter --mount --net --target "$pid" "$bashInteractive" -c "
+  set -eu -o pipefail
+  while IFS= read -r -d \$'\0' line; do
+    export \"\$line\"
+  done <&3
+  exec 3>&-
+  cd \"$pwd\"
+  exec \"$debugShell\"
+" 3< /proc/$pid/environ

pkgs/by-name/br/breakpointHook/breakpoint-hook.sh

@@ -0,0 +1,20 @@
+breakpointHook() {
+    local red='\033[0;31m'
+    local cyan='\033[0;36m'
+    local green='\033[0;32m'
+    local no_color='\033[0m'
+
+    # provide the user with an interactive shell for better experience
+    export bashInteractive="@bashInteractive@"
+    if [ -z "$debugShell" ]; then
+        export debugShell="@bashInteractive@"
+    fi
+
+    local id
+    id="$(shuf -i 999999-9999999 -n1)"
+    echo -e "${red}build for ${cyan}${name:-unknown}${red} failed in ${curPhase:-unknown} with exit code ${exitCode:-unknown}${no_color}"
+    echo -e "${green}To attach, run the following command:${no_color}"
+    echo -e "${green}    sudo @attach@ $id${no_color}"
+    sleep "$id"
+}
+failureHooks+=(breakpointHook)

pkgs/by-name/br/breakpointHook/package.nix

@@ -0,0 +1,38 @@
+{
+  lib,
+  stdenv,
+
+  bash,
+  bashInteractive,
+  coreutils,
+  makeSetupHook,
+  procps,
+  util-linux,
+  writeShellScriptBin,
+}:
+
+let
+  attach = writeShellScriptBin "attach" ''
+    export PATH="${
+      lib.makeBinPath [
+        bash
+        coreutils
+        procps # needed for pgrep
+        util-linux # needed for nsenter
+      ]
+    }"
+    exec bash ${./attach.sh} "$@"
+  '';
+in
+
+makeSetupHook {
+  name = "breakpoint-hook";
+  meta.broken = !stdenv.buildPlatform.isLinux;
+  substitutions = {
+    attach = "${attach}/bin/attach";
+    # The default interactive shell in case $debugShell is not set in the derivation.
+    # Can be overridden to zsh or fish, etc.
+    # This shell is also used to load the env variables before the $debugShell is started.
+    bashInteractive = lib.getExe bashInteractive;
+  };
+} ./breakpoint-hook.sh