[GitHub] [maven-resolver] Tibor17 commented on a change in pull request #65: [MRESOLVER-123] Provide a global locking sync context by default

[GitBox <gi...@apache.org>]

Tibor17 commented on a change in pull request #65:
URL: https://github.com/apache/maven-resolver/pull/65#discussion_r456789040



##########
File path: maven-resolver-impl/src/main/java/org/eclipse/aether/internal/impl/DefaultSyncContextFactory.java
##########
@@ -28,31 +30,59 @@
 import org.eclipse.aether.artifact.Artifact;
 import org.eclipse.aether.impl.SyncContextFactory;
 import org.eclipse.aether.metadata.Metadata;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.google.inject.Singleton;
 
 /**
- * A factory to create synchronization contexts. This default implementation actually does not provide any real
- * synchronization but merely completes the repository system.
+ * A factory to create synchronization contexts. This default implementation uses fair global locking
+ * based on {@link ReentrantReadWriteLock}. Explicit artifacts and metadata passed are ignored.
  */
 @Named
+@Singleton
 public class DefaultSyncContextFactory
     implements SyncContextFactory
 {
+    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock( true );
 
     public SyncContext newInstance( RepositorySystemSession session, boolean shared )
     {
-        return new DefaultSyncContext();
+        return new DefaultSyncContext( shared ? lock.readLock() : lock.writeLock(), shared );
     }
 
     static class DefaultSyncContext
         implements SyncContext
     {
+        private static final Logger LOGGER = LoggerFactory.getLogger( DefaultSyncContext.class );
+
+        private final Lock lock;
+        private final boolean shared;
+        private int lockHoldCount;
+
+        DefaultSyncContext( Lock lock, boolean shared )
+        {
+            this.lock = lock;
+            this.shared = shared;
+        }
 
         public void acquire( Collection<? extends Artifact> artifact, Collection<? extends Metadata> metadata )
         {
+            LOGGER.trace( "Acquiring global {} lock (currently held: {})",
+                          shared ? "read" : "write", lockHoldCount );
+            lock.lock();

Review comment:
       This is really dangerous and antipattern because you here can call lock/unlock in different threads since you have two methods. See the Javadoc. It says to use try-finally. And it is because the method contaxt is always called in one thread and that's the point of locking that you lock and unlock your thread. But unparking another thread is dangerous.
   Maybe change the API or the abstraction in this class but do not use these principles.




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