guix: Add source-able bash prelude and utils

contrib/guix/libexec/prelude.bash

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+export LC_ALL=C
+set -e -o pipefail
+
+# shellcheck source=../../shell/realpath.bash
+source contrib/shell/realpath.bash
+
+# shellcheck source=../../shell/git-utils.bash
+source contrib/shell/git-utils.bash
+
+################
+# Required non-builtin commands should be invokable
+################
+
+check_tools() {
+    for cmd in "$@"; do
+        if ! command -v "$cmd" > /dev/null 2>&1; then
+            echo "ERR: This script requires that '$cmd' is installed and available in your \$PATH"
+            exit 1
+        fi
+    done
+}
+
+check_tools cat env readlink dirname basename git
+
+################
+# We should be at the top directory of the repository
+################
+
+same_dir() {
+    local resolved1 resolved2
+    resolved1="$(bash_realpath "${1}")"
+    resolved2="$(bash_realpath "${2}")"
+    [ "$resolved1" = "$resolved2" ]
+}
+
+if ! same_dir "${PWD}" "$(git_root)"; then
+cat << EOF
+ERR: This script must be invoked from the top level of the git repository
+
+Hint: This may look something like:
+    env FOO=BAR ./contrib/guix/guix-<blah>
+
+EOF
+exit 1
+fi
+
+################
+# Set common variables
+################
+
+VERSION="${VERSION:-$(git_head_version)}"
+DISTNAME="${DISTNAME:-bitcoin-${VERSION}}"
+
+version_base_prefix="${PWD}/guix-build-"
+VERSION_BASE="${version_base_prefix}${VERSION}"  # TOP
+
+DISTSRC_BASE="${DISTSRC_BASE:-${VERSION_BASE}}"
+
+OUTDIR_BASE="${OUTDIR_BASE:-${VERSION_BASE}/output}"