HBASE-22941 merge operation returns parent regions in random order

hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterRpcServices.java

@@ -812,10 +812,6 @@ public MergeTableRegionsResponse mergeTableRegions(
 
     RegionStates regionStates = master.getAssignmentManager().getRegionStates();
 
-    if (request.getRegionCount() != 2) {
-      throw new ServiceException(new DoNotRetryIOException(
-        "Only support merging 2 regions but " + request.getRegionCount() + " region passed"));
-    }
     RegionInfo[] regionsToMerge = new RegionInfo[request.getRegionCount()];
     for (int i = 0; i < request.getRegionCount(); i++) {
       final byte[] encodedNameOfRegion = request.getRegion(i).getValue().toByteArray();

hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/RegionStateStore.java

@@ -19,9 +19,9 @@
 
 import java.io.IOException;
 import java.util.Collections;
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
+import java.util.SortedMap;
+import java.util.TreeMap;
 
 import org.apache.hadoop.hbase.Cell;
 import org.apache.hadoop.hbase.CellBuilderFactory;
@@ -263,7 +263,7 @@ public void mergeRegions(RegionInfo child, RegionInfo [] parents, ServerName ser
       throws IOException {
     TableDescriptor htd = getDescriptor(child.getTable());
     boolean globalScope = htd.hasGlobalReplicationScope();
-    Map<RegionInfo, Long> parentSeqNums = new HashMap<>(parents.length);
+    SortedMap<RegionInfo, Long> parentSeqNums = new TreeMap<>();
     for (RegionInfo ri: parents) {
       parentSeqNums.put(ri, globalScope? getOpenSeqNumForParentRegion(ri): -1);
     }

hbase-server/src/test/java/org/apache/hadoop/hbase/TestSplitMerge.java

@@ -19,7 +19,9 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
 
+import java.util.List;
 import java.util.concurrent.TimeUnit;
 import org.apache.hadoop.hbase.Waiter.ExplainingPredicate;
 import org.apache.hadoop.hbase.client.AsyncConnection;
@@ -104,4 +106,50 @@ public String explainFailure() throws Exception {
         .getRegionLocation(Bytes.toBytes(1), true).get().getServerName());
     }
   }
+
+  @Test
+  public void testMergeRegionOrder() throws Exception {
+
+    int regionCount= 20;
+
+    TableName tableName = TableName.valueOf("MergeRegionOrder");
+    byte[] family = Bytes.toBytes("CF");
+    TableDescriptor td = TableDescriptorBuilder.newBuilder(tableName)
+        .setColumnFamily(ColumnFamilyDescriptorBuilder.of(family)).build();
+
+    byte[][] splitKeys = new byte[regionCount-1][];
+
+    for (int c = 0; c < regionCount-1; c++) {
+      splitKeys[c] = Bytes.toBytes(c+1 * 1000);
+    }
+
+    UTIL.getAdmin().createTable(td, splitKeys);
+    UTIL.waitTableAvailable(tableName);
+
+    List<RegionInfo> regions = UTIL.getAdmin().getRegions(tableName);
+
+    byte[][] regionNames = new byte[20][];
+    for (int c = 0; c < regionCount; c++) {
+      regionNames[c] = regions.get(c).getRegionName();
+    }
+
+    UTIL.getAdmin().mergeRegionsAsync(regionNames, false).get(60, TimeUnit.SECONDS);
+
+    List<RegionInfo> mergedRegions =
+        MetaTableAccessor.getTableRegions(UTIL.getConnection(), tableName);
+
+    assertEquals(1, mergedRegions.size());
+
+    RegionInfo mergedRegion = mergedRegions.get(0);
+
+    List<RegionInfo> mergeParentRegions = MetaTableAccessor.getMergeRegions(UTIL.getConnection(),
+      mergedRegion.getEncodedNameAsBytes());
+
+    assertEquals(mergeParentRegions.size(), 20);
+
+    for (int c = 0; c < 19; c++) {
+      assertTrue(Bytes.compareTo(mergeParentRegions.get(c).getStartKey(), mergeParentRegions.get(c+1).getStartKey())<0);
+    }
+
+  }
 }