andreachild commented on code in PR #3139:
URL: https://github.com/apache/tinkerpop/pull/3139#discussion_r2159580880


##########
gremlin-driver/src/main/java/org/apache/tinkerpop/gremlin/driver/Client.java:
##########
@@ -543,9 +543,20 @@ protected void initializeImplementation() {
                 this.initializationFailure = ex;
             }
 
-            // throw an error if there is no host available after initializing 
connection pool.
-            if (cluster.availableHosts().isEmpty())
+            // throw an error if there is no host available after initializing 
connection pool. we used
+            // to test cluster.availableHosts().isEmpty() but checking if we 
actually have hosts in
+            // the connection pool seems a bit more fireproof. if we look at 
initializeConnectionSetupForHost
+            // we can see that a successful initialization of the 
host/connection pool pair is followed by
+            // marking the host available and notifying the load balancing 
strategy. by relying directly on
+            // the state of hostConnectionPools we ensure that there is 
actually a concrete
+            // host/connection pool pair. even if the connection pool has 
immediate problems, it can fallback
+            // to its normal reconnection operation and won't put 
chooseConnection in a state where it can
+            // get a NPE if hostConnectionPools ends up being empty. it seems 
as if the safest minimum
+            // requirement for leaving this method is to ensure that at least 
one ConnectionPool constructor
+            // completed for at least one Host.
+            if (hostConnectionPools.isEmpty()) {

Review Comment:
   This change seems OK but I am wondering if there is a deeper issue in that 
`hostConnectionPools.isEmpty()` has somehow become inconsistent with 
`cluster.availableHosts().isEmpty()`?



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