HBASE-21292 IdLock.getLockEntry() may hang if interrupted

hbase-common/src/main/java/org/apache/hadoop/hbase/util/IdLock.java

@@ -81,6 +81,17 @@ public class IdLock {
               existing.wait();
             } catch (InterruptedException e) {
               --existing.numWaiters;  // Remove ourselves from waiters.
+              // HBASE-21292
+              // There is a rare case that interrupting and the lock owner thread call
+              // releaseLockEntry at the same time. Since the owner thread found there
+              // still one waiting, it won't remove the entry from the map. If the interrupted
+              // thread is the last one waiting on the lock, and since an exception is thrown,
+              // the 'existing' entry will stay in the map forever. Later threads which try to
+              // get this lock will stuck in a infinite loop because
+              // existing = map.putIfAbsent(entry.id, entry)) != null and existing.locked=false.
+              if (!existing.locked && existing.numWaiters == 0) {
+                map.remove(existing.id);
+              }
               throw new InterruptedIOException(
                   "Interrupted waiting to acquire sparse lock");
             }
@@ -135,6 +146,12 @@ public class IdLock {
 
             }
           } catch (InterruptedException e) {
+            // HBASE-21292
+            // Please refer to the comments in getLockEntry()
+            // the difference here is that we decrease numWaiters in finally block
+            if (!existing.locked && existing.numWaiters == 1) {
+              map.remove(existing.id);
+            }
             throw new InterruptedIOException(
                 "Interrupted waiting to acquire sparse lock");
           } finally {