allow limited concurrent indexing.

Intention is to allow remote index file downloads during indexing.

java/maven.indexer/src/org/netbeans/modules/maven/indexer/NexusRepositoryIndexerImpl.java

@@ -35,6 +35,7 @@
 import java.nio.file.StandardCopyOption;
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Semaphore;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import java.util.regex.Matcher;
@@ -73,6 +74,7 @@
 import org.apache.maven.wagon.authentication.AuthenticationException;
 import org.apache.maven.wagon.authentication.AuthenticationInfo;
 import org.apache.maven.wagon.authorization.AuthorizationException;
+import org.apache.maven.wagon.events.TransferEvent;
 import org.apache.maven.wagon.events.TransferListener;
 import org.apache.maven.wagon.providers.http.HttpWagon;
 import org.apache.maven.wagon.proxy.ProxyInfo;
@@ -149,7 +151,19 @@ public class NexusRepositoryIndexerImpl implements RepositoryIndexerImplementati
     private static final HashMap<String,Mutex> repoMutexMap = new HashMap<>(4);
 
     private static final Set<Mutex> indexingMutexes = new HashSet<>();
-    private static final RequestProcessor RP = new RequestProcessor("indexing", 1);
+
+    /**
+     * Fixed to two Threads to allow concurrent remote index downloads during local indexing
+     */
+    private static final RequestProcessor RP = new RequestProcessor("indexing", 2);
+
+    /**
+     * Best effort attempt to prevent two _local_ repos to be indexed at the same time to avoid unnecessary IO contention.
+     * Concurrent local+remote indexing are allowed.
+     * This isn't perfect since a download can finish before local repos finished indexing, tests showed
+     * performance dosn't degrade significantly on solid state drives.
+     */
+    private static final Semaphore localIndexingPermit = new Semaphore(1);
 
     @Override
     public boolean handlesRepository(RepositoryInfo repo) {
@@ -642,9 +656,18 @@ private void spawnIndexLoadedRepo(final RepositoryInfo repo) {
         RP.post(() -> {
             getRepoMutex(repo).writeAccess((Mutex.Action<Void>) () -> {
                 try {
+                    if (repo.isLocal()) {
+                        localIndexingPermit.acquire();
+                    }
                     indexLoadedRepo(repo, true);
                 } catch (IOException ex) {
                     LOGGER.log(Level.INFO, "could not (re-)index " + repo.getId(), ex);
+                } catch (InterruptedException ex) {
+                    LOGGER.log(Level.INFO, "got interrupting while waiting for indexing permit of " + repo.getId(), ex);
+                } finally {
+                    if (repo.isLocal()) {
+                        localIndexingPermit.release();
+                    }
                 }
                 return null;
             });