HBASE-23596 HBCKServerCrashProcedure can double assign

Change its behavior so it will use Regions found in
Master context or, if none found, only then will it
search hbase:meta for references. Normal operation
where we read Master context is usual and sufficient.
The scan of hbase:meta is only for case where Master
state has been corrupted and we need to clear out
'Unknown Servers'.

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

@@ -65,31 +65,41 @@ public HBCKServerCrashProcedure(final MasterProcedureEnv env, final ServerName s
   public HBCKServerCrashProcedure() {}
 
   /**
-   * Adds Regions found by super method any found scanning hbase:meta.
+   * If no Regions found in Master context, then we will search hbase:meta for references
+   * to the passed server. Operator may have passed ServerName because they have found
+   * references to 'Unknown Servers'. They are using HBCKSCP to clear them out.
    */
   @Override
   @edu.umd.cs.findbugs.annotations.SuppressWarnings(value="NP_NULL_ON_SOME_PATH_EXCEPTION",
     justification="FindBugs seems confused on ps in below.")
   List<RegionInfo> getRegionsOnCrashedServer(MasterProcedureEnv env) {
-    // Super can return immutable emptyList.
+    // Super will return an immutable list (empty if nothing on this server).
     List<RegionInfo> ris = super.getRegionsOnCrashedServer(env);
+    if (!ris.isEmpty()) {
+      return ris;
+    }
+    // If NOT empty, then presume operator schedule an SCP because they are trying to
+    // purge 'Unknown Servers' -- servers that are neither online nor in dead servers
+    // list but that ARE in hbase:meta and so showing as unknown in places like the
+    // 'HBCK Report'.
     List<Pair<RegionInfo, ServerName>> ps = null;
     try {
       ps = MetaTableAccessor.getTableRegionsAndLocations(env.getMasterServices().getConnection(),
               null, false);
     } catch (IOException ioe) {
-      LOG.warn("Failed get of all regions; continuing", ioe);
+      LOG.warn("Failed scan of hbase:meta for 'Unknown Servers'", ioe);
+      return ris;
     }
     if (ps == null || ps.isEmpty()) {
-      LOG.warn("No regions found in hbase:meta");
+      LOG.warn("No regions found scanning hbase:meta");
       return ris;
     }
-    List<RegionInfo> aggregate = ris == null || ris.isEmpty()?
-        new ArrayList<>(): new ArrayList<>(ris);
-    int before = aggregate.size();
+    // Use a Set here in case same Region in master memory and in hbase:meta (the usual case!).
+    // Otherwise, we will double-assign.
+    List<RegionInfo> aggregate = new ArrayList<>();
     ps.stream().filter(p -> p.getSecond() != null && p.getSecond().equals(getServerName())).
         forEach(p -> aggregate.add(p.getFirst()));
-    LOG.info("Found {} mentions of {} in hbase:meta", aggregate.size() - before, getServerName());
-    return aggregate;
+    LOG.info("Found {} mentions of {} in hbase:meta", aggregate.size(), getServerName());
+    return new ArrayList<RegionInfo>(aggregate);
   }
 }