tools: add update-oss.sh

This script updates the version of tailscale.com in go.mod to match the version
used in the specified repository checkout.

tools/update-oss.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+#
+# Usage: update-oss.sh <directory-path>
+#
+# Update the version of the tailscale.com module to be in sync with the version
+# used by the repo checked out in the specified directory.
+#
+# Requires: git, go, jq
+#
+set -euo pipefail
+
+# The repository path to compare to.
+repo="${1:?missing repository path}"
+
+cd "$(dirname ${BASH_SOURCE[0]})/.."
+if ! git diff --quiet ; then
+    echo ">> WARNING: The working directory is not clean." 1>&2
+    echo "   Commit or stash your changes first." 1>&2
+    git diff --stat
+    exit 1
+fi
+git checkout --quiet main && git pull --rebase --quiet
+
+# The module to update.
+module=tailscale.com
+
+# The branch name to use when making an update.
+branch="$USER"/update-oss-version
+
+digest="$(
+    cd "$repo"
+    go list -json "$module" | \
+        jq -r .Module.Version | \
+        cut -d- -f3
+)"
+go get "$module"@"$digest"
+go mod tidy
+if git diff --quiet ; then
+    echo "Module $module is up-to-date at commit $digest" 1>&2
+else
+    git checkout -b "$branch"
+    git commit -m "go.mod: update $module to commit $digest" go.mod go.sum
+    git push -u origin "$branch"
+    echo "Module $module updated to commit $digest" 1>&2
+    echo "Branch $branch created" 1>&2
+fi