HDFS-16535: SlotReleaser should reuse the domain socket based on socket paths

Author: stiga-huang

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/shortcircuit/ShortCircuitCache.java

@@ -189,16 +189,18 @@ public void run() {
       final DfsClientShm shm = (DfsClientShm)slot.getShm();
       final DomainSocket shmSock = shm.getPeer().getDomainSocket();
       final String path = shmSock.getPath();
+      DomainSocket domainSocket = pathToDomainSocket.get(path);
       DataOutputStream out = null;
       boolean success = false;
       int retries = 2;
       try {
         while (retries > 0) {
           try {
             if (domainSocket == null || !domainSocket.isOpen()) {
-              // we are running in single thread mode, no protection needed for
-              // domainSocket
               domainSocket = DomainSocket.connect(path);
+              // we are running in single thread mode, no protection needed for
+              // pathToDomainSocket
+              pathToDomainSocket.put(path, domainSocket);
             }
 
             out = new DataOutputStream(
@@ -221,8 +223,11 @@ public void run() {
           } catch (SocketException se) {
             // the domain socket on datanode may be timed out, we retry once
             retries--;
-            domainSocket.close();
-            domainSocket = null;
+            if (domainSocket != null) {
+              domainSocket.close();
+              domainSocket = null;
+              pathToDomainSocket.remove(path);
+            }
             if (retries == 0) {
               throw new SocketException("Create domain socket failed");
             }
@@ -240,7 +245,7 @@ public void run() {
         } else {
           shm.getEndpointShmManager().shutdown(shm);
           IOUtilsClient.cleanupWithLogger(LOG, domainSocket, out);
-          domainSocket = null;
+          pathToDomainSocket.remove(path);
         }
       }
     }
@@ -354,7 +359,11 @@ public interface ShortCircuitReplicaCreator {
    */
   private final DfsClientShmManager shmManager;
 
-  private DomainSocket domainSocket = null;
+  /**
+   * A map contains all DomainSockets used in SlotReleaser. Keys are the domain socket
+   * paths of short-circuit shared memory segments.
+   */
+  private Map<String, DomainSocket> pathToDomainSocket = new HashMap<>();
 
   public static ShortCircuitCache fromConf(ShortCircuitConf conf) {
     return new ShortCircuitCache(