RackAware placement policy can not try-my-best to ensure replicas located into different racks.

[hangc0276]

Describe

Add the following test into TestRackawareEnsemblePlacementPolicy.java can reproduce the issue.

@Test
    public void testNewEnsemblePolicyWithMultipleRacks() throws Exception {
        BookieSocketAddress addr1 = new BookieSocketAddress("127.0.0.1", 3181);
        BookieSocketAddress addr2 = new BookieSocketAddress("127.0.0.2", 3181);
        BookieSocketAddress addr3 = new BookieSocketAddress("127.0.0.3", 3181);
        BookieSocketAddress addr4 = new BookieSocketAddress("127.0.0.4", 3181);
        BookieSocketAddress addr5 = new BookieSocketAddress("127.0.0.5", 3181);
        // update dns mapping
        StaticDNSResolver.addNodeToRack(addr1.getHostName(), "/default-region/r1");
        StaticDNSResolver.addNodeToRack(addr2.getHostName(), "/default-region/r1");
        StaticDNSResolver.addNodeToRack(addr3.getHostName(), "/default-region/r2");
        StaticDNSResolver.addNodeToRack(addr4.getHostName(), "/default-region/r3");
        //StaticDNSResolver.addNodeToRack(addr5.getHostName(), "/default-region/r1");
        // Update cluster
        Set<BookieId> addrs = new HashSet<BookieId>();
        addrs.add(addr1.toBookieId());
        addrs.add(addr2.toBookieId());
        addrs.add(addr3.toBookieId());
        //addrs.add(addr5.toBookieId());
        addrs.add(addr4.toBookieId());
        repp.onClusterChanged(addrs, new HashSet<BookieId>());

        try {
            int ensembleSize = 3;
            int writeQuorumSize = 3;
            int ackQuorumSize = 2;

            Set<BookieId> excludeBookies = new HashSet<>();
            //excludeBookies.add(addr4.toBookieId());
            //excludeBookies.add(addr3.toBookieId());

            for (int i = 0; i < 50; ++i) {
                EnsemblePlacementPolicy.PlacementResult<List<BookieId>> ensembleResponse =
                    repp.newEnsemble(ensembleSize, writeQuorumSize,
                        ackQuorumSize, null, excludeBookies);
                List<BookieId> ensemble = ensembleResponse.getResult();

                if (ensemble.contains(addr1.toBookieId()) && ensemble.contains(addr2.toBookieId())) {
                    LOG.error("The same ensemble.");
                    ensemble.forEach(t -> {
                        LOG.info("[hangc] {}", t);
                    });
                }
                LOG.info("==========");
            }
        } catch (Exception e ){
            LOG.error("failed ", e);
        }
    }

[horizonzy]

I will solve it.