# build-lock.sh — exclusive per-build-dir lock for xce-build.sh (bug-1604).
#
# Sourced, not executed. Two xce-build.sh runs against the SAME build dir used
# to race: one run's `rm -rf $BUILD_DIR` hit files the other was writing
# ("Directory not empty") or silently deleted a binary the other had just
# built and tested. The lock serializes them for the whole run (wipe, setup,
# compile, tests); runs against DIFFERENT build dirs stay concurrent.
#
# The lock is a directory created with mkdir (atomic; needs nothing beyond
# coreutils, unlike flock(1), which the nix dev shell does not provide),
# holding the owner's PID. It lives outside the build dir -- the wipe would
# delete it -- under $XCE_BUILD_LOCK_ROOT (default $TMPDIR or /tmp), named
# after a hash of the build dir's absolute path, so separate checkouts never
# share a lock. A lock whose owner is no longer running (a build killed with
# SIGKILL) is taken over.
#
#   build_lock_acquire <build_dir>   blocks until held
#   build_lock_release               releases it (safe to call twice)
#
# Tunables: XCE_BUILD_LOCK_ROOT, XCE_BUILD_LOCK_POLL_SECS (default 2).

BUILD_LOCK_DIR=""

build_lock_path() {
    local abs sum
    abs="$(realpath -m -- "$1")"
    sum="$(printf '%s' "$abs" | cksum | cut -d' ' -f1)"
    printf '%s/xce-build-%s.lock' "${XCE_BUILD_LOCK_ROOT:-${TMPDIR:-/tmp}}" "$sum"
}

build_lock_acquire() {
    local lock owner waited=0 poll="${XCE_BUILD_LOCK_POLL_SECS:-2}"

    lock="$(build_lock_path "$1")"
    while ! mkdir "$lock" 2>/dev/null; do
        owner="$(cat "$lock/pid" 2>/dev/null || true)"
        if [ -n "$owner" ] && ! kill -0 "$owner" 2>/dev/null; then
            # The owner died holding it. If two waiters take over the same
            # stale lock at once, mkdir below still lets only one of them in.
            echo "=== Build lock for $1 was left by PID $owner, which is gone -- taking it over ===" >&2
            rm -rf -- "$lock"
            continue
        fi
        if [ "$waited" = 0 ]; then
            echo "=== Waiting for another xce-build.sh run (PID ${owner:-?}) to finish with $1 ... ===" >&2
            waited=1
        fi
        sleep "$poll"
    done
    echo "${BASHPID:-$$}" > "$lock/pid"
    BUILD_LOCK_DIR="$lock"
}

build_lock_release() {
    if [ -n "$BUILD_LOCK_DIR" ]; then
        rm -rf -- "$BUILD_LOCK_DIR"
        BUILD_LOCK_DIR=""
    fi
}