HBASE-25835 Ignore duplicate split requests from regionserver reports

A SplitTableRegionProcedure may already be running when a regionserver
report is received that includes a split request. The outcome is
multiple SplitTableRegionProcedure procedures scheduled for the split
request, only one of which can succeed. The others error out.

Do not create a split procedure in response to a region state change
report if the region is not open or already splitting.

Author: apurtell

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

@@ -1126,7 +1126,13 @@ private void updateRegionSplitTransition(final ServerName serverName, final Tran
       LOG.debug("Split request from " + serverName +
           ", parent=" + parent + " splitKey=" + Bytes.toStringBinary(splitKey));
     }
-    master.getMasterProcedureExecutor().submitProcedure(createSplitProcedure(parent, splitKey));
+    if (regionStates.getRegionState(parent).isOpened() &&
+          !regionStates.getRegionState(parent).isSplitting()) {
+      master.getMasterProcedureExecutor().submitProcedure(createSplitProcedure(parent, splitKey));
+    } else {
+      LOG.info("Ignoring split request from " + serverName +
+        ", parent=" + parent + " because parent is already splitting or not online");
+    }
 
     // If the RS is < 2.0 throw an exception to abort the operation, we are handling the split
     if (master.getServerManager().getVersionNumber(serverName) < 0x0200000) {