tools: one Gradle lock helper (gradle-lock.sh) for gradle-serial.sh and clean-builds.sh

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

tools/gradle-lock.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# The one-Gradle-build-at-a-time lock (CLAUDE.md › Build and verify). Sourced, not run, by tools/gradle-serial.sh,
+# tools/clean-builds.sh, infra/dev/build.sh and infra/build-images.sh; plain POSIX sh, so sh and bash scripts share it.
+# The lock is a directory holding the pid of its owner; a lock whose owner has died is taken over.
+# shellcheck disable=SC2034  # KVITT_GRADLE_LOCK is used by the scripts that source this file
+KVITT_GRADLE_LOCK=/tmp/kvitt-gradle.lock
+
+# Prints the pid of the live process holding the lock and succeeds; fails when nobody holds it.
+gradle_lock_holder() {
+  _pid=$(cat "$KVITT_GRADLE_LOCK/pid" 2>/dev/null) || return 1
+  [ -n "$_pid" ] && kill -0 "$_pid" 2>/dev/null || return 1
+  printf '%s\n' "$_pid"
+}
+
+# Waits until the lock is free, then takes it for this shell ($$). The caller removes it on exit:
+#   trap 'rm -rf "$KVITT_GRADLE_LOCK"' EXIT INT TERM HUP
+gradle_lock_acquire() {
+  while ! mkdir "$KVITT_GRADLE_LOCK" 2>/dev/null; do
+    _pid=$(cat "$KVITT_GRADLE_LOCK/pid" 2>/dev/null)
+    if [ -n "$_pid" ] && ! kill -0 "$_pid" 2>/dev/null; then rm -rf "$KVITT_GRADLE_LOCK"; continue; fi
+    sleep 5
+  done
+  echo $$ > "$KVITT_GRADLE_LOCK/pid"
+}