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
• In-code documentation and comments are up-to-date.
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src/main/java/org/caosdb/server/database/DatabaseAccessManager.java

@@ -26,6 +26,7 @@
 package org.caosdb.server.database;
 
 import java.util.concurrent.Semaphore;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.ReentrantLock;
 import org.caosdb.server.database.access.Access;
 import org.caosdb.server.database.access.AccessControlAccess;
@@ -56,7 +57,7 @@ import org.caosdb.server.utils.Releasable;
 class ReadAccessSemaphore extends Semaphore implements Releasable {
 
   private static final long serialVersionUID = 4384921156838881337L;
-  private int acquired = 0; // how many threads have read access
+  private AtomicInteger acquired = new AtomicInteger(0); // how many threads have read access
   Semaphore writersBlock =
       new Semaphore(1, true); // This semaphore is blocked as long as there are any
   // unreleased read permits.
@@ -73,10 +74,9 @@ class ReadAccessSemaphore extends Semaphore implements Releasable {
   @Override
   public void acquire() throws InterruptedException {
     super.acquire(); // Protect the next few lines
-    if (this.acquired == 0) {
+    if (this.acquired.getAndIncrement() == 0) {
       this.writersBlock.acquire();
     }
-    this.acquired++;
     super.release();
   }
 
@@ -87,9 +87,7 @@ class ReadAccessSemaphore extends Semaphore implements Releasable {
    */
   @Override
   public void release() {
-    this.acquired--;
-    if (this.acquired <= 0) { // Last permit: release
-      this.acquired = 0;
+    if (this.acquired.decrementAndGet() == 0) { // Last permit: release
       if (this.writersBlock.availablePermits() <= 0) {
         this.writersBlock.release();
       }