Revert "Remove overrideChannelBuilder" (#756)

Author: schmidt-sebastian

firebase-firestore/src/main/java/com/google/firebase/firestore/remote/GrpcCallProvider.java

@@ -15,6 +15,7 @@
 package com.google.firebase.firestore.remote;
 
 import android.content.Context;
+import androidx.annotation.VisibleForTesting;
 import com.google.android.gms.common.GooglePlayServicesNotAvailableException;
 import com.google.android.gms.common.GooglePlayServicesRepairableException;
 import com.google.android.gms.security.ProviderInstaller;
@@ -24,6 +25,7 @@
 import com.google.firebase.firestore.util.AsyncQueue;
 import com.google.firebase.firestore.util.Executors;
 import com.google.firebase.firestore.util.Logger;
+import com.google.firebase.firestore.util.Supplier;
 import com.google.firestore.v1.FirestoreGrpc;
 import io.grpc.CallCredentials;
 import io.grpc.CallOptions;
@@ -41,11 +43,26 @@ public class GrpcCallProvider {
 
   private static final String LOG_TAG = "GrpcCallProvider";
 
+  private static Supplier<ManagedChannelBuilder<?>> overrideChannelBuilderSupplier;
+
   private final Task<ManagedChannel> channelTask;
   private final AsyncQueue asyncQueue;
 
   private CallOptions callOptions;
 
+  /**
+   * Helper function to globally override the channel that RPCs use. Useful for testing when you
+   * want to bypass SSL certificate checking.
+   *
+   * @param channelBuilderSupplier The supplier for a channel builder that is used to create gRPC
+   *     channels.
+   */
+  @VisibleForTesting
+  public static void overrideChannelBuilder(
+      Supplier<ManagedChannelBuilder<?>> channelBuilderSupplier) {
+    overrideChannelBuilderSupplier = channelBuilderSupplier;
+  }
+
   GrpcCallProvider(
       AsyncQueue asyncQueue,
       Context context,
@@ -88,12 +105,16 @@ private ManagedChannel initChannel(Context context, DatabaseInfo databaseInfo) {
       Logger.warn(LOG_TAG, "Failed to update ssl context: %s", e);
     }
 
-    ManagedChannelBuilder<?> channelBuilder =
-        ManagedChannelBuilder.forTarget(databaseInfo.getHost());
-    if (!databaseInfo.isSslEnabled()) {
-      // Note that the boolean flag does *NOT* switch the wire format from Protobuf to Plaintext.
-      // It merely turns off SSL encryption.
-      channelBuilder.usePlaintext();
+    ManagedChannelBuilder<?> channelBuilder;
+    if (overrideChannelBuilderSupplier != null) {
+      channelBuilder = overrideChannelBuilderSupplier.get();
+    } else {
+      channelBuilder = ManagedChannelBuilder.forTarget(databaseInfo.getHost());
+      if (!databaseInfo.isSslEnabled()) {
+        // Note that the boolean flag does *NOT* switch the wire format from Protobuf to Plaintext.
+        // It merely turns off SSL encryption.
+        channelBuilder.usePlaintext();
+      }
     }
 
     // Ensure gRPC recovers from a dead connection. (Not typically necessary, as the OS will