create test case which shows quorum calculation is wrong

atlasdb-cassandra/src/main/java/com/palantir/atlasdb/keyvalue/cassandra/CassandraTopologyValidator.java

@@ -264,7 +264,7 @@ private ClusterTopologyResult maybeGetConsistentClusterTopology(
 
         // Only consider hosts that have the endpoint for quorum calculations.
         // Otherwise, we will never add hosts when we're in a mixed state
-        int quorum = (hostIdsByServerWithoutSoftFailures.size() + 1) / 2;
+        int quorum = (hostIdsByServerWithoutSoftFailures.size() / 2) + 1;
 
         // If too many hosts are unreachable, then we cannot come to a consensus
         if (hostIdsWithoutFailures.size() < quorum) {

atlasdb-cassandra/src/test/java/com/palantir/atlasdb/keyvalue/cassandra/CassandraTopologyValidatorTest.java

@@ -280,7 +280,11 @@ public void validateNewlyAddedHostsNewHostsNotAddedIfTheyDisagreeWithOldHostsOnP
 
     @Test
     public void validateNewlyAddedHostsNoNewHostsAddedIfNewHostsDoNotHaveQuorumAndNoCurrentServers() {
-        Map<CassandraServer, CassandraClientPoolingContainer> allHosts = setupHosts(NEW_HOSTS);
+        Set<String> hosts = ImmutableSet.<String>builder()
+                .addAll(NEW_HOSTS)
+                .add(OLD_HOST_ONE)
+                .build();
+        Map<CassandraServer, CassandraClientPoolingContainer> allHosts = setupHosts(hosts);
         Set<String> hostsOffline = ImmutableSet.of(NEW_HOST_ONE, NEW_HOST_TWO);
         setHostIds(filterContainers(allHosts, hostsOffline::contains), HostIdResult.hardFailure());
         setHostIds(filterContainers(allHosts, server -> !hostsOffline.contains(server)), HostIdResult.success(UUIDS));

changelog/@unreleased/pr-6427.v2.yml

@@ -0,0 +1,5 @@
+type: fix
+fix:
+  description: 'Fixes bug where quorum calculation was incorrect for topology validation. '
+  links:
+  - https://github.com/palantir/atlasdb/pull/6427