watch: strip underscore aliases of watch flags

lib/internal/main/watch_mode.js

@@ -7,6 +7,9 @@ const {
   ArrayPrototypePushApply,
   ArrayPrototypeSlice,
   StringPrototypeIncludes,
+  StringPrototypeIndexOf,
+  StringPrototypeReplaceAll,
+  StringPrototypeSlice,
   StringPrototypeStartsWith,
 } = primordials;
 
@@ -41,9 +44,23 @@ const kPreserveOutput = getOptionValue('--watch-preserve-output');
 const kCommand = ArrayPrototypeSlice(process.argv, 1);
 const kCommandStr = inspect(ArrayPrototypeJoin(kCommand, ' '));
 
+// The C++ option parser treats `_` and `-` as equivalent in option names
+// (e.g. --watch_path is --watch-path), so normalize before matching.
+function normalizeOptionName(arg) {
+  if (!StringPrototypeStartsWith(arg, '--')) {
+    return arg;
+  }
+  const equalsIndex = StringPrototypeIndexOf(arg, '=');
+  if (equalsIndex === -1) {
+    return StringPrototypeReplaceAll(arg, '_', '-');
+  }
+  return StringPrototypeReplaceAll(StringPrototypeSlice(arg, 0, equalsIndex), '_', '-') +
+    StringPrototypeSlice(arg, equalsIndex);
+}
+
 const argsWithoutWatchOptions = [];
 for (let i = 0; i < process.execArgv.length; i++) {
-  const arg = process.execArgv[i];
+  const arg = normalizeOptionName(process.execArgv[i]);
   if (StringPrototypeStartsWith(arg, '--watch=')) {
     continue;
   }
@@ -74,7 +91,7 @@ for (let i = 0; i < process.execArgv.length; i++) {
   if (arg === '--experimental-default-config-file') {
     continue;
   }
-  ArrayPrototypePush(argsWithoutWatchOptions, arg);
+  ArrayPrototypePush(argsWithoutWatchOptions, process.execArgv[i]);
 }
 
 ArrayPrototypePushApply(argsWithoutWatchOptions, kCommand);
@@ -88,15 +105,16 @@ if (kNodeOptions != null) {
   const parts = parseNodeOptionsEnvVar(kNodeOptions);
   for (let i = 0; i < parts.length; i++) {
     const part = parts[i];
-    if (part === '--watch' ||
-        part === '--watch-preserve-output' ||
-        StringPrototypeStartsWith(part, '--watch=') ||
-        StringPrototypeStartsWith(part, '--watch-preserve-output=') ||
-        StringPrototypeStartsWith(part, '--watch-path=') ||
-        StringPrototypeStartsWith(part, '--watch-kill-signal=')) {
+    const name = normalizeOptionName(part);
+    if (name === '--watch' ||
+        name === '--watch-preserve-output' ||
+        StringPrototypeStartsWith(name, '--watch=') ||
+        StringPrototypeStartsWith(name, '--watch-preserve-output=') ||
+        StringPrototypeStartsWith(name, '--watch-path=') ||
+        StringPrototypeStartsWith(name, '--watch-kill-signal=')) {
       continue;
     }
-    if (part === '--watch-path' || part === '--watch-kill-signal') {
+    if (name === '--watch-path' || name === '--watch-kill-signal') {
       // Skip the flag and its separate value argument
       i++;
       continue;

test/sequential/test-watch-mode.mjs

@@ -1033,6 +1033,60 @@ process.on('message', (message) => {
     }
   });
 
+  it('should strip underscore aliases of watch flags from NODE_OPTIONS', async () => {
+    // Avoid recursively watching the repository's source and test trees.
+    const cwd = tmpdir.resolve('node-options-underscore');
+    mkdirSync(path.join(cwd, 'src'), { recursive: true });
+    mkdirSync(path.join(cwd, 'test'));
+    const file = createTmpFile('console.log(process.env.NODE_OPTIONS);');
+    const nodeOptions = [
+      '--watch_path=./src',
+      '--watch_path', './test',
+      '--watch_preserve_output',
+      '--watch_preserve_output=true',
+      '--watch_kill_signal=SIGKILL',
+      '--watch_kill_signal', 'SIGINT',
+      '--max_old_space_size=4096',
+      '--no-warnings',
+    ].join(' ');
+    const { done, restart } = runInBackground({
+      args: ['--watch', file],
+      options: {
+        cwd,
+        env: { ...process.env, NODE_OPTIONS: nodeOptions },
+      },
+    });
+
+    try {
+      const { stdout, stderr } = await restart();
+
+      assert.strictEqual(stderr, '');
+      const nodeOptionsLine = stdout.find((line) => line.includes('--max_old_space_size'));
+      assert.ok(nodeOptionsLine);
+      assert.strictEqual(nodeOptionsLine, '--max_old_space_size=4096 --no-warnings');
+    } finally {
+      await done();
+    }
+  });
+
+  it('should strip --watch_path from execArgv in child process', async () => {
+    const dir = tmpdir.resolve('exec-argv-underscore');
+    mkdirSync(dir, { recursive: true });
+    const file = createTmpFile('console.log(JSON.stringify(process.execArgv));');
+    const { done, restart } = runInBackground({
+      args: [`--watch_path=${dir}`, file],
+    });
+
+    try {
+      const { stdout, stderr } = await restart();
+
+      assert.strictEqual(stderr, '');
+      assert.ok(stdout.includes('["--no-warnings"]'));
+    } finally {
+      await done();
+    }
+  });
+
   it('should not strip --watch when it appears inside a quoted NODE_OPTIONS value', {
     // Honoring --require from NODE_OPTIONS is required for this test.
     skip: !!process.config.variables.node_without_node_options,