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From GitBox <...@apache.org>
Subject [GitHub] [maven-resolver] Tibor17 commented on a change in pull request #65: [MRESOLVER-123] Provide a global locking sync context by default
Date Sun, 19 Jul 2020 09:56:42 GMT

Tibor17 commented on a change in pull request #65:
URL: https://github.com/apache/maven-resolver/pull/65#discussion_r456887720



##########
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:
       I don't like this style of code passing thread-unsafe Collections from T1 to T2, etc.
Again dangerous but not because of modifications. It is because of memory visibility between
the CPU cache and the main memory. The `ArrayList` does not make it!




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