guix: Record precious directories and add guix-clean

Many users have reported problems that stem from having an unclean
working tree. To that end, I've written a guix-clean script which should
help reset the working tree while respecting user-specified precious
directories.

Precious directories, such as:

- SOURCES_PATH
- BASE_CACHE
- SDK_PATH
- OUTDIR

Should be preserved when cleaning the working tree, and are thus
recorded in ./contrib/guix/var/precious_dirs.

The ./contrib/guix/guix-clean script is able to parse that file and make
sure to avoid them when cleaning out the working tree.

contrib/guix/guix-clean

@@ -0,0 +1,83 @@
+#!/usr/bin/env bash
+export LC_ALL=C
+set -e -o pipefail
+
+# Source the common prelude, which:
+#   1. Checks if we're at the top directory of the Bitcoin Core repository
+#   2. Defines a few common functions and variables
+#
+# shellcheck source=libexec/prelude.bash
+source "$(dirname "${BASH_SOURCE[0]}")/libexec/prelude.bash"
+
+
+###################
+## Sanity Checks ##
+###################
+
+################
+# Required non-builtin commands should be invokable
+################
+
+check_tools cat mkdir make git guix
+
+
+#############
+##  Clean  ##
+#############
+
+# Usage: under_dir MAYBE_PARENT MAYBE_CHILD
+#
+# If MAYBE_CHILD is a subdirectory of MAYBE_PARENT, print the relative path
+# from MAYBE_PARENT to MAYBE_CHILD. Otherwise, return 1 as the error code.
+#
+# NOTE: This does not perform any symlink-resolving or path canonicalization.
+#
+under_dir() {
+    local path_residue
+    path_residue="${2##${1}}"
+    if [ -z "$path_residue" ] || [ "$path_residue" = "$2" ]; then
+        return 1
+    else
+        echo "$path_residue"
+    fi
+}
+
+# Usage: dir_under_git_root MAYBE_CHILD
+#
+# If MAYBE_CHILD is under the current git repository and exists, print the
+# relative path from the git repository's top-level directory to MAYBE_CHILD,
+# otherwise, exit with an error code.
+#
+dir_under_git_root() {
+    local rv
+    rv="$(under_dir "$(git_root)" "$1")"
+    [ -n "$rv" ] && echo "$rv"
+}
+
+shopt -s nullglob
+found_precious_dirs_files=( "${version_base_prefix}"*/"${var_base_basename}/precious_dirs" ) # This expands to an array of directories...
+shopt -u nullglob
+
+exclude_flags=()
+
+for precious_dirs_file in "${found_precious_dirs_files[@]}"; do
+    # Make sure the precious directories (e.g. SOURCES_PATH, BASE_CACHE, SDK_PATH)
+    # are excluded from git-clean
+    echo "Found precious_dirs file: '${precious_dirs_file}'"
+
+    # Exclude the precious_dirs file itself
+    if dirs_file_exclude_fragment=$(dir_under_git_root "$(dirname "$precious_dirs_file")"); then
+        exclude_flags+=( --exclude="${dirs_file_exclude_fragment}/precious_dirs" )
+    fi
+
+    # Read each 'name=dir' pair from the precious_dirs file
+    while IFS='=' read -r name dir; do
+        # Add an exclusion flag if the precious directory is under the git root.
+        if under=$(dir_under_git_root "$dir"); then
+            echo "Avoiding ${name}: ${under}"
+            exclude_flags+=( --exclude="$under" )
+        fi
+    done < "$precious_dirs_file"
+done
+
+git clean -xdff "${exclude_flags[@]}"