HADOOP-13599. s3a close() to be non-synchronized, so avoid risk of deadlock on shutdown. Contributed by Steve Loughran.

(cherry picked from commit 47f80922dc7cb2fa6d084e6fb1f354c4ec1d4c69)
(cherry picked from commit 1f1e47e41198760b0ddddceb489f2de2b569f38a)

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java

@@ -127,6 +127,7 @@ public class S3AFileSystem extends FileSystem {
   private S3AInputPolicy inputPolicy;
   private static final AtomicBoolean warnedOfCoreThreadDeprecation =
       new AtomicBoolean(false);
+  private final AtomicBoolean closed = new AtomicBoolean(false);
 
   // The maximum number of entries that can be deleted in any call to s3
   private static final int MAX_ENTRIES_TO_DELETE = 1000;
@@ -1514,7 +1515,11 @@ public class S3AFileSystem extends FileSystem {
    * @throws IOException IO problem
    */
   @Override
-  public synchronized void close() throws IOException {
+  public void close() throws IOException {
+    if (closed.getAndSet(true)) {
+      // already closed
+      return;
+    }
     try {
       super.close();
     } finally {

hadoop-tools/hadoop-aws/src/test/java/org/apache/hadoop/fs/s3a/TestS3AConfiguration.java

@@ -401,6 +401,14 @@ public class TestS3AConfiguration {
         awsConf.getUserAgent());
   }
 
+  @Test
+  public void testCloseIdempotent() throws Throwable {
+    conf = new Configuration();
+    fs = S3ATestUtils.createTestFileSystem(conf);
+    fs.close();
+    fs.close();
+  }
+
   /**
    * Reads and returns a field from an object using reflection.  If the field
    * cannot be found, is null, or is not the expected type, then this method