michael-o commented on a change in pull request #65:
URL: https://github.com/apache/maven-resolver/pull/65#discussion_r456887921



##########
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:
       The collections passed has there for years, I did not invent this and it 
is also ignored on the default implementation. How can this be a problem? Let's 
focus solely on the PR and not the outer code because that would be a different 
discussion.




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