From e52f281118e31ca6ae3752b2e694750f245bb5ae Mon Sep 17 00:00:00 2001
From: Dan Wang <wangdan@apache.org>
Date: Wed, 22 Jan 2025 16:16:58 +0800
Subject: [PATCH] add comments and fix tests

---
 .../org/apache/pegasus/rpc/async/MetaSession.java   | 13 +++++++++----
 .../apache/pegasus/rpc/async/MetaSessionTest.java   |  2 +-
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/java-client/src/main/java/org/apache/pegasus/rpc/async/MetaSession.java b/java-client/src/main/java/org/apache/pegasus/rpc/async/MetaSession.java
index 8b002db050..8931afc993 100644
--- a/java-client/src/main/java/org/apache/pegasus/rpc/async/MetaSession.java
+++ b/java-client/src/main/java/org/apache/pegasus/rpc/async/MetaSession.java
@@ -264,26 +264,30 @@ void resolveHost(String hostPort) throws IllegalArgumentException {
       return;
     }
 
-    Set<rpc_address> newSet = new TreeSet<>(Arrays.asList(addrs));
     Set<rpc_address> oldSet =
         metaList.stream()
             .map(ReplicaSession::getAddress)
             .collect(Collectors.toCollection(TreeSet::new));
+    Set<rpc_address> newSet = new TreeSet<>(Arrays.asList(addrs));
 
-    // fast path: do nothing if meta list is unchanged.
+    // Do nothing if meta list is unchanged.
     if (newSet.equals(oldSet)) {
       return;
     }
 
-    // removed metas
+    // Find the meta servers that should be removed.
     Set<rpc_address> removedSet = new HashSet<>(oldSet);
     removedSet.removeAll(newSet);
 
+    // Iterate over the current meta list: once a meta server is found in the removed set,
+    // it would be removed from the meta list after its session is closed.
     Iterator<ReplicaSession> iterator = metaList.iterator();
     while (iterator.hasNext()) {
       ReplicaSession session = iterator.next();
       rpc_address addr = session.getAddress();
       if (!removedSet.contains(addr)) {
+        // This meta server is not found in the removed set, which means it should just be
+        // retained.
         continue;
       }
 
@@ -292,10 +296,11 @@ void resolveHost(String hostPort) throws IllegalArgumentException {
       logger.info("meta server {} was removed", addr);
     }
 
-    // newly added metas
+    // Find the meta servers that should be added.
     Set<rpc_address> addedSet = new HashSet<>(newSet);
     addedSet.removeAll(oldSet);
 
+    // Add each new meta servers to the meta list.
     for (rpc_address addr : addedSet) {
       metaList.add(clusterManager.getReplicaSession(addr));
       logger.info("meta server {} was added", addr);
diff --git a/java-client/src/test/java/org/apache/pegasus/rpc/async/MetaSessionTest.java b/java-client/src/test/java/org/apache/pegasus/rpc/async/MetaSessionTest.java
index a9d7a4fe0b..01261757a5 100644
--- a/java-client/src/test/java/org/apache/pegasus/rpc/async/MetaSessionTest.java
+++ b/java-client/src/test/java/org/apache/pegasus/rpc/async/MetaSessionTest.java
@@ -256,7 +256,7 @@ public void testMetaForwardUnknownPrimary() throws Exception {
     FieldUtils.writeField(op, "response", new query_cfg_response(), true);
     op.get_response().err = new error_code();
     op.get_response().err.errno = error_code.error_types.ERR_FORWARD_TO_OTHERS;
-    op.get_response().partitions = Collections.singletonList(new partition_configuration[1]);
+    op.get_response().partitions = Collections.singletonList(new partition_configuration());
     op.get_response().partitions.set(0, new partition_configuration());
     op.get_response().partitions.get(0).primary = rpc_address.fromIpPort("172.0.0.3:34601");
     MetaSession.MetaRequestRound round =