fix: make sure node_modules dir is exclusively created

Refs: #2462

Author: simschla

lib/src/main/java/com/diffplug/spotless/npm/ExclusiveFolderAccess.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2025 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.spotless.npm;
+
+import java.io.File;
+import java.util.Objects;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
+import javax.annotation.Nonnull;
+
+import com.diffplug.spotless.ThrowingEx;
+
+interface ExclusiveFolderAccess {
+
+	static ExclusiveFolderAccess forFolder(@Nonnull File folder) {
+		return forFolder(folder.getAbsolutePath());
+	}
+
+	static ExclusiveFolderAccess forFolder(@Nonnull String path) {
+		return new ExclusiveFolderAccessSharedMutex(Objects.requireNonNull(path));
+	}
+
+	void runExclusively(ThrowingEx.Runnable runnable);
+
+	class ExclusiveFolderAccessSharedMutex implements ExclusiveFolderAccess {
+
+		private static final ConcurrentHashMap<String, Lock> mutexes = new ConcurrentHashMap<>();
+
+		private final String path;
+
+		private ExclusiveFolderAccessSharedMutex(@Nonnull String path) {
+			this.path = Objects.requireNonNull(path);
+		}
+
+		private Lock getMutex() {
+			return mutexes.computeIfAbsent(path, k -> new ReentrantLock());
+		}
+
+		@Override
+		public void runExclusively(ThrowingEx.Runnable runnable) {
+			final Lock lock = getMutex();
+			try {
+				lock.lock();
+				runnable.run();
+			} catch (Exception e) {
+				throw ThrowingEx.asRuntime(e);
+			} finally {
+				lock.unlock();
+			}
+		}
+	}
+}

lib/src/main/java/com/diffplug/spotless/npm/NpmFormatterStepStateBase.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2024 DiffPlug
+ * Copyright 2016-2025 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -87,14 +87,17 @@ protected void prepareNodeServer() throws IOException {
 		}
 
 		protected void assertNodeServerDirReady() throws IOException {
-			if (needsPrepareNodeServerLayout()) {
-				// reinstall if missing
-				prepareNodeServerLayout();
-			}
-			if (needsPrepareNodeServer()) {
-				// run npm install if node_modules is missing
-				prepareNodeServer();
-			}
+			ExclusiveFolderAccess.forFolder(nodeServerLayout.nodeModulesDir())
+					.runExclusively(() -> {
+						if (needsPrepareNodeServerLayout()) {
+							// reinstall if missing
+							prepareNodeServerLayout();
+						}
+						if (needsPrepareNodeServer()) {
+							// run npm install if node_modules is missing
+							prepareNodeServer();
+						}
+					});
 		}
 
 		protected boolean needsPrepareNodeServer() {