f-fix-deadlock -> dev

Summary

fix #122 (closed)

Focus

Problem was a non-atomic operation (integer decrement). Concurrent reading thread would fail to decrement the acquired counter. See code annotations.

Test Environment

Please use b2faf630 to see failing tests. Please use 9b85d817 for fixed DatabaseAccessManager

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• All automated tests pass
• Reference related Issues
• Up-to-date CHANGELOG.md
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
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Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
• The test environment setup works and the intended behavior is reproducible in the test environment
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src/test/java/org/caosdb/server/database/DatabaseAccessManagerTest.java

@@ -22,10 +22,91 @@
  */
 package org.caosdb.server.database;
 
+import static org.junit.Assert.assertFalse;
+
+import java.util.LinkedList;
+import java.util.List;
 import org.junit.Assert;
 import org.junit.Test;
 
 public class DatabaseAccessManagerTest {
+  Thread createReadThread(long wait, String name, ReadAccessSemaphore readAccess) {
+    return new Thread(
+        new Runnable() {
+
+          @Override
+          public void run() {
+            try {
+              readAccess.acquire();
+              Thread.sleep(wait);
+            } catch (InterruptedException e) {
+              e.printStackTrace();
+            } finally {
+              readAccess.release();
+            }
+          }
+        },
+        name);
+  }
+
+  Thread createWriteThread(long wait, String name, WriteAccessLock writeAccess) {
+    return new Thread(
+        new Runnable() {
+
+          @Override
+          public void run() {
+            try {
+              writeAccess.reserve();
+              Thread.sleep(wait);
+              writeAccess.lockInterruptibly();
+              Thread.sleep(wait);
+            } catch (InterruptedException e) {
+              e.printStackTrace();
+            } finally {
+              writeAccess.release();
+            }
+          }
+        },
+        name);
+  }
+
+  @Test
+  public void testDeadLock() throws InterruptedException {
+    final ReadAccessSemaphore readAccess = new ReadAccessSemaphore();
+    final WriteAccessLock writeAccess = new WriteAccessLock(readAccess);
+    List<Thread> ts = new LinkedList<>();
+    for (int i = 0; i < 1000; i++) {
+      Thread t1 = createReadThread(1, "Ra" + i, readAccess);
+      Thread t2 = createReadThread(2, "Rb" + i, readAccess);
+      Thread t3 = createReadThread(3, "Rc" + i, readAccess);
+      Thread t5 = createReadThread(5, "Rd" + i, readAccess);
+      Thread t7 = createReadThread(7, "Re" + i, readAccess);
+      Thread t11 = createReadThread(11, "Rf" + i, readAccess);
+      Thread w5 = createWriteThread(2, "W" + i, writeAccess);
+
+      t1.start();
+      t2.start();
+      w5.start();
+      t3.start();
+      t5.start();
+      t7.start();
+      t11.start();
+
+      ts.add(t1);
+      ts.add(t2);
+      ts.add(t3);
+      ts.add(t5);
+      ts.add(t7);
+      ts.add(t11);
+      ts.add(w5);
+    }
+
+    for (Thread t : ts) {
+      t.join(10000);
+      assertFalse(t.isAlive());
+    }
+  }
+
   public static final ReadAccessSemaphore readAccess = new ReadAccessSemaphore();
   public static final WriteAccessLock writeAccess = new WriteAccessLock(readAccess);