Semaphore class to keep track of concurrent access to Git repos

Author: MarkRobbo

src/main/java/org/commonwl/view/git/GitSemaphore.java

@@ -0,0 +1,66 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.commonwl.view.git;
+
+import org.springframework.stereotype.Component;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Object to manage concurrent access to Git repositories
+ */
+@Component
+public class GitSemaphore {
+
+    private static Map<String, Integer> currentRepos = new HashMap<>();
+
+    /**
+     * Note that a thread will be accessing the repository
+     * @param repoUrl The url of the repository
+     * @return Whether the resource can be accessed safely
+     * (no other threads are using it)
+     */
+    synchronized boolean acquire(String repoUrl) {
+        if (currentRepos.containsKey(repoUrl)) {
+            currentRepos.put(repoUrl, currentRepos.get(repoUrl) + 1);
+            return false;
+        } else {
+            currentRepos.put(repoUrl, 1);
+            return true;
+        }
+    }
+
+    /**
+     * Release use of the shared resource
+     * @param repoUrl The url of the repository
+     */
+    synchronized void release(String repoUrl) {
+        if (currentRepos.containsKey(repoUrl)) {
+            int threadCountUsing = currentRepos.get(repoUrl);
+            if (threadCountUsing > 1) {
+                currentRepos.put(repoUrl, threadCountUsing - 1);
+            } else {
+                currentRepos.remove(repoUrl);
+            }
+        }
+    }
+
+}

src/test/java/org/commonwl/view/git/GitSemaphoreTest.java

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.commonwl.view.git;
+
+import org.junit.Test;
+
+import static junit.framework.TestCase.assertFalse;
+import static junit.framework.TestCase.assertTrue;
+
+public class GitSemaphoreTest {
+
+    @Test
+    public void basicOperation() throws Exception {
+
+        GitSemaphore sem = new GitSemaphore();
+        boolean first = sem.acquire("1");
+        boolean second = sem.acquire("1");
+        sem.acquire("2");
+        sem.release("1");
+        boolean third = sem.acquire("1");
+        sem.release("1");
+        sem.release("1");
+        boolean fourth = sem.acquire("1");
+
+        assertTrue(first);
+        assertFalse(second);
+        assertFalse(third);
+        assertTrue(fourth);
+
+    }
+
+}