fix region aware placement policy use disk weight not work (apache#2981)

### Motivation
When we meet the following conditions:
1. configured region aware placement policy
2. enable disk weight based placement
3. fallback random selection when selecting ensemble bookies, such as:
     - not enough regions
     - rack number less than 2 in one region

It will throw the following exception, and create ledger failed.
```
12:15:36.459 [bookkeeper-ml-scheduler-OrderedScheduler-1-0] ERROR org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl - [public/default/persistent/test_v2] Encountered unexpected error when creating ledger
java.lang.NullPointerException: null
        at org.apache.bookkeeper.client.WeightedRandomSelectionImpl.getNextRandom(WeightedRandomSelectionImpl.java:150) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.RackawareEnsemblePlacementPolicyImpl.selectRandomInternal(RackawareEnsemblePlacementPolicyImpl.java:748) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.RackawareEnsemblePlacementPolicyImpl.selectRandom(RackawareEnsemblePlacementPolicyImpl.java:698) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.RackawareEnsemblePlacementPolicyImpl.newEnsembleInternal(RackawareEnsemblePlacementPolicyImpl.java:409) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.RackawareEnsemblePlacementPolicyImpl.newEnsemble(RackawareEnsemblePlacementPolicyImpl.java:372) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.RackawareEnsemblePlacementPolicy.newEnsemble(RackawareEnsemblePlacementPolicy.java:159) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.RegionAwareEnsemblePlacementPolicy.newEnsemble(RegionAwareEnsemblePlacementPolicy.java:303) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.BookieWatcherImpl.newEnsemble(BookieWatcherImpl.java:270) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.LedgerCreateOp.initiate(LedgerCreateOp.java:161) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.client.BookKeeper.asyncCreateLedger(BookKeeper.java:860) ~[io.streamnative-bookkeeper-server-4.14.3.1.jar:4.14.3.1]
        at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.asyncCreateLedger(ManagedLedgerImpl.java:3657) ~[io.streamnative-managed-ledger-2.8.1.30.jar:2.8.1.30]
        at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.initializeBookKeeper(ManagedLedgerImpl.java:460) ~[io.streamnative-managed-ledger-2.8.1.30.jar:2.8.1.30]
        at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.access$400(ManagedLedgerImpl.java:141) ~[io.streamnative-managed-ledger-2.8.1.30.jar:2.8.1.30]
        at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl$1.operationComplete(ManagedLedgerImpl.java:396) ~[io.streamnative-managed-ledger-2.8.1.30.jar:2.8.1.30]
        at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl$1.operationComplete(ManagedLedgerImpl.java:328) ~[io.streamnative-managed-ledger-2.8.1.30.jar:2.8.1.30]
        at org.apache.bookkeeper.mledger.impl.MetaStoreImpl.lambda$getManagedLedgerInfo$2(MetaStoreImpl.java:97) ~[io.streamnative-managed-ledger-2.8.1.30.jar:2.8.1.30]
        at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:714) [?:?]
        at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:478) [?:?]
        at org.apache.bookkeeper.common.util.OrderedExecutor$TimedRunnable.run(OrderedExecutor.java:203) [io.streamnative-bookkeeper-common-4.14.3.1.jar:4.14.3.1]
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) [?:?]
        at java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) [?:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [io.netty-netty-common-4.1.72.Final.jar:4.1.72.Final]
        at java.lang.Thread.run(Thread.java:834) [?:?]
```

The root cause of this case it that in `selectRandomInternal`, the `wRselection` haven't ever update any bookie map and the filed `randomMax` and `cummulativeMap` doesn't initialized.

### Modification
1. update the `wRSelection`'s map on `selectRandomInternal` method whenever the `wRSelection` have ever set or not.

(cherry picked from commit 002725e)

Author: hangc0276

bookkeeper-server/src/main/java/org/apache/bookkeeper/client/RackawareEnsemblePlacementPolicyImpl.java

@@ -715,20 +715,21 @@ protected List<BookieNode> selectRandomInternal(List<BookieNode> bookiesToSelect
                 throw new BKNotEnoughBookiesException();
             }
             if (wRSelection == null) {
-                Map<BookieNode, WeightedObject> rackMap = new HashMap<BookieNode, WeightedObject>();
-                for (BookieNode n : bookiesToSelectFrom) {
-                    if (excludeBookies.contains(n)) {
-                        continue;
-                    }
-                    if (this.bookieInfoMap.containsKey(n)) {
-                        rackMap.put(n, this.bookieInfoMap.get(n));
-                    } else {
-                        rackMap.put(n, new BookieInfo());
-                    }
-                }
                 wRSelection = new WeightedRandomSelectionImpl<BookieNode>(this.maxWeightMultiple);
-                wRSelection.updateMap(rackMap);
             }
+
+            Map<BookieNode, WeightedObject> rackMap = new HashMap<BookieNode, WeightedObject>();
+            for (BookieNode n : bookiesToSelectFrom) {
+                if (excludeBookies.contains(n)) {
+                    continue;
+                }
+                if (this.bookieInfoMap.containsKey(n)) {
+                    rackMap.put(n, this.bookieInfoMap.get(n));
+                } else {
+                    rackMap.put(n, new BookieInfo());
+                }
+            }
+            wRSelection.updateMap(rackMap);
         } else {
             Collections.shuffle(bookiesToSelectFrom);
         }

bookkeeper-server/src/main/java/org/apache/bookkeeper/client/WeightedRandomSelectionImpl.java

@@ -150,8 +150,6 @@ public T getNextRandom() {
             Double randomNum = randomMax * Math.random();
             // find the nearest key in the map corresponding to the randomNum
             Double key = cummulativeMap.floorKey(randomNum);
-            //LOG.info("Random max: {} CummulativeMap size: {} selected key: {}", randomMax, cummulativeMap.size(),
-            //    key);
             return cummulativeMap.get(key);
         } finally {
             rwLock.readLock().unlock();

bookkeeper-server/src/test/java/org/apache/bookkeeper/client/TestRegionAwareEnsemblePlacementPolicy.java

@@ -1572,4 +1572,39 @@ public void testNewEnsembleSetWithFiveRegions() throws Exception {
             fail("Should not get not enough bookies exception even there is only one rack.");
         }
     }
+
+    public void testRegionsWithDiskWeight() throws Exception {
+        repp.uninitalize();
+        repp = new RegionAwareEnsemblePlacementPolicy();
+        conf.setProperty(REPP_ENABLE_VALIDATION, false);
+        conf.setDiskWeightBasedPlacementEnabled(true);
+        repp.initialize(conf, Optional.empty(), timer, DISABLE_ALL,
+            NullStatsLogger.INSTANCE, BookieSocketAddress.LEGACY_BOOKIEID_RESOLVER);
+        BookieSocketAddress addr1 = new BookieSocketAddress("127.0.0.2", 3181);
+        BookieSocketAddress addr2 = new BookieSocketAddress("127.0.0.3", 3181);
+        BookieSocketAddress addr3 = new BookieSocketAddress("127.0.0.4", 3181);
+        BookieSocketAddress addr4 = new BookieSocketAddress("127.0.0.5", 3181);
+        BookieSocketAddress addr5 = new BookieSocketAddress("127.0.0.6", 3181);
+
+        // update dns mapping
+        StaticDNSResolver.addNodeToRack(addr1.getHostName(), "/region1/r1");
+        StaticDNSResolver.addNodeToRack(addr2.getHostName(), "/region2/r3");
+        StaticDNSResolver.addNodeToRack(addr3.getHostName(), "/region3/r11");
+        StaticDNSResolver.addNodeToRack(addr4.getHostName(), "/region4/r13");
+        StaticDNSResolver.addNodeToRack(addr5.getHostName(), "/region5/r23");
+        // Update cluster
+        Set<BookieId> addrs = new HashSet<BookieId>();
+        addrs.add(addr1.toBookieId());
+        addrs.add(addr2.toBookieId());
+        addrs.add(addr3.toBookieId());
+        addrs.add(addr4.toBookieId());
+        addrs.add(addr5.toBookieId());
+
+        repp.onClusterChanged(addrs, new HashSet<BookieId>());
+
+        List<BookieId> ensemble = repp.newEnsemble(3, 3, 2, null,
+            new HashSet<>()).getResult();
+
+        assertEquals(3, ensemble.size());
+    }
 }