how about this?

Author: cometkim

scripts/rescript_bsb.js

@@ -6,47 +6,47 @@ var os = require("os");
 const child_process = require("child_process");
 const rescript_exe = require("./bin_path").rescript_exe;
 
-/**
- * @typedef {Object} ProjectFiles
- * @property {Array<string>} dirs
- * @property {Array<string>} generated
- */
-
-/**
- * @typedef {Object} WatcherRef
- * @property {string} dir
- * @property {fs.FSWatcher} watcher
- */
-
 const cwd = process.cwd();
 const lockFileName = path.join(cwd, ".bsb.lock");
 
-let isBuilding = false;
+/**
+ * @type {child_process.ChildProcess | null}
+ */
+let ownerProcess = null;
 function releaseBuild() {
-  if (isBuilding) {
+  if (ownerProcess) {
     try {
+      ownerProcess.kill();
       fs.unlinkSync(lockFileName);
-    } catch (err) {}
-    isBuilding = false;
+    } catch {}
+    ownerProcess = null;
   }
 }
 
-// We use [~perm:0o664] rather than our usual default perms, [0o666], because
-// lock files shouldn't rely on the umask to disallow tampering by other.
-function acquireBuild() {
-  if (isBuilding) {
-    return false;
+/**
+ * We use [~perm:0o664] rather than our usual default perms, [0o666], because
+ * lock files shouldn't rely on the umask to disallow tampering by other.
+ *
+ * @param {Array<string>} args
+ */
+function acquireBuild(args) {
+  if (ownerProcess) {
+    return null;
   } else {
     try {
-      const fid = fs.openSync(lockFileName, "wx", 0o664);
-      fs.closeSync(fid);
-      isBuilding = true;
+      ownerProcess = child_process.spawn(rescript_exe, args, {
+        stdio: "inherit",
+      });
+      fs.writeFileSync(lockFileName, ownerProcess.pid.toString(), {
+        encoding: "utf8",
+        mode: 0o664,
+      });
     } catch (err) {
       if (err.code === "EEXIST") {
         console.warn(lockFileName, "already exists, try later");
       } else console.log(err);
     }
-    return isBuilding;
+    return ownerProcess;
   }
 }
 
@@ -55,23 +55,17 @@ function acquireBuild() {
  * @param {(code: number) => void} [maybeOnClose]
  */
 function delegate(args, maybeOnClose) {
-  if (acquireBuild()) {
-    /**
-     * @type {child_process.ChildProcess}
-     */
-    const p = child_process.spawn(rescript_exe, args, {
-      stdio: "inherit",
-    });
-
-    p.on("error", e => {
+  let p;
+  if ((p = acquireBuild(args))) {
+    p.once("error", e => {
       console.error(String(e));
       releaseBuild();
       process.exit(2);
     });
 
     // The 'close' event will always emit after 'exit' was already emitted, or
     // 'error' if the child failed to spawn.
-    p.on("close", code => {
+    p.once("close", code => {
       releaseBuild();
       const exitCode = code === null ? 1 : code;
       if (maybeOnClose) {
@@ -376,7 +370,7 @@ Please pick a different one using the \`-ws [host:]port\` flag from bsb.`);
     } else {
       dlog(`Rebuilding since ${reasonsToRebuild}`);
     }
-    if (acquireBuild()) {
+    if (acquireBuild(args)) {
       logStartCompiling();
       child_process
         .spawn(rescript_exe, rescriptWatchBuildArgs, {