fix(spanner): lazily initialize cloudpath fallback pool

Author: rahul2393

java-spanner/google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpc.java

@@ -705,18 +705,19 @@ public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(
           ManagedChannelBuilder<?> fallbackBuilder = cloudPathBuilder;
           if (options.isGrpcGcpExtensionEnabled()) {
             String jsonApiConfig = parseGrpcGcpApiConfig();
-            GcpManagedChannelOptions gcpOptions = grpcGcpOptionsWithMetricsAndDcp(options);
-            if (gcpOptions == null) {
-              gcpOptions = GcpManagedChannelOptions.newBuilder().build();
-            }
+            GcpManagedChannelOptions primaryGcpOptions = grpcGcpOptionsWithMetricsAndDcp(options);
+            GcpManagedChannelOptions fallbackGcpOptions =
+                GcpManagedChannelOptions.newBuilder(primaryGcpOptions)
+                    .withChannelPoolOptions(getGrpcGcpLazyFallbackChannelPoolOptions(options))
+                    .build();
             primaryBuilder =
                 GcpManagedChannelBuilder.forDelegateBuilder(builder)
                     .withApiConfigJsonString(jsonApiConfig)
-                    .withOptions(gcpOptions);
+                    .withOptions(primaryGcpOptions);
             fallbackBuilder =
                 GcpManagedChannelBuilder.forDelegateBuilder(cloudPathBuilder)
                     .withApiConfigJsonString(jsonApiConfig)
-                    .withOptions(gcpOptions);
+                    .withOptions(fallbackGcpOptions);
           }
 
           GcpFallbackOpenTelemetry fallbackTelemetry =
@@ -846,6 +847,14 @@ static GcpChannelPoolOptions getGrpcGcpChannelPoolOptions(SpannerOptions options
         .build();
   }
 
+  @VisibleForTesting
+  static GcpChannelPoolOptions getGrpcGcpLazyFallbackChannelPoolOptions(SpannerOptions options) {
+    return GcpChannelPoolOptions.newBuilder(getGrpcGcpChannelPoolOptions(options))
+        .setMinSize(0)
+        .setInitSize(0)
+        .build();
+  }
+
   @VisibleForTesting
   static GcpChannelPoolOptions getGrpcGcpEndpointChannelPoolOptions(SpannerOptions options) {
     GcpChannelPoolOptions channelPoolOptions = options.getGcpChannelPoolOptions();

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpcTest.java

@@ -1264,6 +1264,27 @@ public void testGrpcGcpOptionsIncludeStaticChannelPoolSettingsWithoutDcp() throw
     assertEquals(Duration.ZERO, grpcGcpOptions.getChannelPoolOptions().getScaleDownInterval());
   }
 
+  @Test
+  public void testGrpcGcpLazyFallbackChannelPoolOptionsStartWithoutChannels() {
+    SpannerOptions options =
+        SpannerOptions.newBuilder()
+            .setProjectId("[PROJECT]")
+            .enableGrpcGcpExtension()
+            .disableDynamicChannelPool()
+            .setNumChannels(8)
+            .build();
+
+    GcpChannelPoolOptions poolOptions =
+        GapicSpannerRpc.getGrpcGcpLazyFallbackChannelPoolOptions(options);
+
+    assertEquals(8, poolOptions.getMaxSize());
+    assertEquals(0, poolOptions.getMinSize());
+    assertEquals(0, poolOptions.getInitSize());
+    assertEquals(0, poolOptions.getMinRpcPerChannel());
+    assertEquals(0, poolOptions.getMaxRpcPerChannel());
+    assertEquals(Duration.ZERO, poolOptions.getScaleDownInterval());
+  }
+
   @Test
   public void testGrpcGcpOptionsRetainDynamicChannelPoolSettingsWithDcp() throws Exception {
     Duration affinityKeyLifetime = Duration.ofMinutes(10);
@@ -1434,7 +1455,7 @@ private SpannerOptions createSpannerOptions() {
   }
 
   @Test
-  public void testDirectPathFallbackCreatesOneGrpcGcpLayerPerPath() {
+  public void testDirectPathFallbackCreatesLazyCloudPathGrpcGcpPool() {
     SpannerOptions.useEnvironment(new SpannerOptions.SpannerEnvironment() {});
     GapicSpannerRpc rpc = null;
     try {
@@ -1445,8 +1466,8 @@ public void testDirectPathFallbackCreatesOneGrpcGcpLayerPerPath() {
       GrpcGcpObjectCounts before = countGrpcGcpObjectsFromChannelz();
       rpc = new GapicSpannerRpc(options);
       GrpcGcpObjectCounts counts = countGrpcGcpObjectsFromChannelz().minus(before);
-      assertEquals(counts.debugString(), 6, counts.gcpManagedChannels);
-      assertEquals(counts.debugString(), 48, counts.channelRefs);
+      assertEquals(counts.debugString(), 3, counts.gcpManagedChannels);
+      assertEquals(counts.debugString(), 24, counts.channelRefs);
     } finally {
       if (rpc != null) {
         rpc.shutdown();