xds: Avoid default bootstrap when global override in XdsNameResolver

This fixes a regression with an internal API from 27d1508 where
overridding the global bootstrap didn't impact parsing the default
bootstrap. So if no global bootstrap was available XdsNameResolver would
fail to start even though an override was in place in
SharedXdsClientPoolProvider. Instead of dealing with the override in
SharedXdsClientPoolProvider, do it in GrpcBootstrapperImpl so
XdsNameResolver is ignorant of the source of the default bootstrap.

We want all of this to go away in favor of XDS_CLIENT_SUPPLIER
injection, but there needs to be some overlap for migration.

cl/826085025

Author: ejona86

xds/src/main/java/io/grpc/xds/GrpcBootstrapperImpl.java

@@ -100,12 +100,22 @@ protected Object getImplSpecificConfig(Map<String, ?> serverConfig, String serve
     return getChannelCredentials(serverConfig, serverUri);
   }
 
+  @GuardedBy("GrpcBootstrapperImpl.class")
+  private static Map<String, ?> defaultBootstrapOverride;
   @GuardedBy("GrpcBootstrapperImpl.class")
   private static BootstrapInfo defaultBootstrap;
 
+  static synchronized void setDefaultBootstrapOverride(Map<String, ?> rawBootstrap) {
+    defaultBootstrapOverride = rawBootstrap;
+  }
+
   static synchronized BootstrapInfo defaultBootstrap() throws XdsInitializationException {
     if (defaultBootstrap == null) {
-      defaultBootstrap = new GrpcBootstrapperImpl().bootstrap();
+      if (defaultBootstrapOverride == null) {
+        defaultBootstrap = new GrpcBootstrapperImpl().bootstrap();
+      } else {
+        defaultBootstrap = new GrpcBootstrapperImpl().bootstrap(defaultBootstrapOverride);
+      }
     }
     return defaultBootstrap;
   }

xds/src/main/java/io/grpc/xds/InternalSharedXdsClientPoolProvider.java

@@ -41,7 +41,7 @@ private InternalSharedXdsClientPoolProvider() {}
    */
   @Deprecated
   public static void setDefaultProviderBootstrapOverride(Map<String, ?> bootstrap) {
-    SharedXdsClientPoolProvider.getDefaultProvider().setBootstrapOverride(bootstrap);
+    GrpcBootstrapperImpl.setDefaultBootstrapOverride(bootstrap);
   }
 
   /**

xds/src/main/java/io/grpc/xds/SharedXdsClientPoolProvider.java

@@ -37,7 +37,6 @@
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ScheduledExecutorService;
-import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import javax.annotation.Nullable;
@@ -55,29 +54,24 @@ final class SharedXdsClientPoolProvider implements XdsClientPoolFactory {
   private static final ExponentialBackoffPolicy.Provider BACKOFF_POLICY_PROVIDER =
       new ExponentialBackoffPolicy.Provider();
 
+  @Nullable
   private final Bootstrapper bootstrapper;
   private final Object lock = new Object();
-  private final AtomicReference<Map<String, ?>> bootstrapOverride = new AtomicReference<>();
   private final Map<String, ObjectPool<XdsClient>> targetToXdsClientMap = new ConcurrentHashMap<>();
 
   SharedXdsClientPoolProvider() {
-    this(new GrpcBootstrapperImpl());
+    this(null);
   }
 
   @VisibleForTesting
-  SharedXdsClientPoolProvider(Bootstrapper bootstrapper) {
-    this.bootstrapper = checkNotNull(bootstrapper, "bootstrapper");
+  SharedXdsClientPoolProvider(@Nullable Bootstrapper bootstrapper) {
+    this.bootstrapper = bootstrapper;
   }
 
   static SharedXdsClientPoolProvider getDefaultProvider() {
     return SharedXdsClientPoolProviderHolder.instance;
   }
 
-  @Deprecated
-  public void setBootstrapOverride(Map<String, ?> bootstrap) {
-    bootstrapOverride.set(bootstrap);
-  }
-
   @Override
   @Nullable
   public ObjectPool<XdsClient> get(String target) {
@@ -89,11 +83,10 @@ public ObjectPool<XdsClient> getOrCreate(
       String target, MetricRecorder metricRecorder, CallCredentials transportCallCredentials)
       throws XdsInitializationException {
     BootstrapInfo bootstrapInfo;
-    Map<String, ?> rawBootstrap = bootstrapOverride.get();
-    if (rawBootstrap != null) {
-      bootstrapInfo = bootstrapper.bootstrap(rawBootstrap);
-    } else {
+    if (bootstrapper != null) {
       bootstrapInfo = bootstrapper.bootstrap();
+    } else {
+      bootstrapInfo = GrpcBootstrapperImpl.defaultBootstrap();
     }
     return getOrCreate(target, bootstrapInfo, metricRecorder, transportCallCredentials);
   }