HBASE-18906 Provide Region#waitForFlushes API.

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegion.java

@@ -1768,30 +1768,39 @@ public class HRegion implements HeapSize, PropagatingConfigurationObserver, Regi
     }
   }
 
-  /** Wait for all current flushes of the region to complete
+  /**
+   * Wait for all current flushes of the region to complete
    */
-  // TODO HBASE-18906. Check the usage (if any) in Phoenix and expose this or give alternate way for
-  // Phoenix needs.
   public void waitForFlushes() {
+    waitForFlushes(0);// Unbound wait
+  }
+
+  @Override
+  public boolean waitForFlushes(long timeout) {
     synchronized (writestate) {
       if (this.writestate.readOnly) {
         // we should not wait for replayed flushed if we are read only (for example in case the
         // region is a secondary replica).
-        return;
+        return true;
       }
-      if (!writestate.flushing) return;
+      if (!writestate.flushing) return true;
       long start = System.currentTimeMillis();
+      long duration = 0;
       boolean interrupted = false;
-      try {
-        while (writestate.flushing) {
       LOG.debug("waiting for cache flush to complete for region " + this);
       try {
-            writestate.wait();
+        while (writestate.flushing) {
+          if (timeout > 0 && duration >= timeout) break;
+          try {
+            long toWait = timeout == 0 ? 0 : (timeout - duration);
+            writestate.wait(toWait);
           } catch (InterruptedException iex) {
             // essentially ignore and propagate the interrupt back up
             LOG.warn("Interrupted while waiting");
             interrupted = true;
             break;
+          } finally {
+            duration = System.currentTimeMillis() - start;
           }
         }
       } finally {
@@ -1799,10 +1808,11 @@ public class HRegion implements HeapSize, PropagatingConfigurationObserver, Regi
           Thread.currentThread().interrupt();
         }
       }
-      long duration = System.currentTimeMillis() - start;
       LOG.debug("Waited " + duration + " ms for flush to complete");
+      return !(writestate.flushing);
     }
   }
+
   protected ThreadPoolExecutor getStoreOpenAndCloseThreadPool(
       final String threadNamePrefix) {
     int numStores = Math.max(1, this.htableDescriptor.getColumnFamilyCount());

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/Region.java

@@ -473,4 +473,13 @@ public interface Region extends ConfigurationObserver {
    * Request flush on this region.
    */
   void requestFlush(FlushLifeCycleTracker tracker) throws IOException;
+
+  /**
+   * Wait for all current flushes of the region to complete
+   *
+   * @param timeout The maximum time to wait in milliseconds.
+   * @return False when timeout elapsed but flushes are not over. True when flushes are over within
+   * max wait time period.
+   */
+  boolean waitForFlushes(long timeout);
 }