core: PoC - MirroringInterceptor for traffic shadowing (#12448)

core/src/main/java/io/grpc/util/MirroringInterceptor.java

@@ -0,0 +1,116 @@
+/*
+ * Copyright 2025 The gRPC Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.grpc.util;
+
+import com.google.common.base.Preconditions;
+import io.grpc.CallOptions;
+import io.grpc.Channel;
+import io.grpc.ClientCall;
+import io.grpc.ClientInterceptor;
+import io.grpc.ForwardingClientCall;
+import io.grpc.Metadata;
+import io.grpc.MethodDescriptor;
+import java.util.concurrent.Executor;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+/**
+ * A ClientInterceptor that mirrors calls to a shadow channel.
+ * Designed to support Unary, Client-Streaming, Server-Streaming, and Bidi calls.
+ */
+public final class MirroringInterceptor implements ClientInterceptor {
+  private static final Logger logger = Logger.getLogger(MirroringInterceptor.class.getName());
+
+  private final Channel mirrorChannel;
+  private final Executor executor;
+
+  public MirroringInterceptor(Channel mirrorChannel, Executor executor) {
+    this.mirrorChannel = Preconditions.checkNotNull(mirrorChannel, "mirrorChannel");
+    this.executor = Preconditions.checkNotNull(executor, "executor");
+  }
+
+  @Override
+  public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(
+      MethodDescriptor<ReqT, RespT> method, CallOptions callOptions, Channel next) {
+
+    return new ForwardingClientCall.SimpleForwardingClientCall<ReqT, RespT>(
+        next.newCall(method, callOptions)) {
+
+      private ClientCall<ReqT, RespT> mirrorCall;
+
+      @Override
+      public void start(Listener<RespT> responseListener, Metadata headers) {
+        // 1. Capture and copy headers immediately (thread-safe for the executor)
+        final Metadata mirrorHeaders = new Metadata();
+        mirrorHeaders.merge(headers);
+
+        executor.execute(() -> {
+          try {
+            // 2. Initialize the shadow call once per stream
+            mirrorCall = mirrorChannel.newCall(method, callOptions);
+            mirrorCall.start(new ClientCall.Listener<RespT>() {}, mirrorHeaders);
+          } catch (Exception e) {
+            logger.log(Level.WARNING, "Failed to start mirror call", e);
+          }
+        });
+        super.start(responseListener, headers);
+      }
+
+      @Override
+      public void sendMessage(ReqT message) {
+        executor.execute(() -> {
+          if (mirrorCall != null) {
+            try {
+              mirrorCall.sendMessage(message);
+            } catch (Exception e) {
+              logger.log(Level.WARNING, "Mirroring message failed", e);
+            }
+          }
+        });
+        super.sendMessage(message);
+      }
+
+      @Override
+      public void halfClose() {
+        executor.execute(() -> {
+          if (mirrorCall != null) {
+            try {
+              mirrorCall.halfClose();
+            } catch (Exception e) {
+              logger.log(Level.WARNING, "Mirroring halfClose failed", e);
+            }
+          }
+        });
+        super.halfClose();
+      }
+
+      @Override
+      public void cancel(String message, Throwable cause) {
+        executor.execute(() -> {
+          if (mirrorCall != null) {
+            try {
+              mirrorCall.cancel(message, cause);
+            } catch (Exception e) {
+              logger.log(Level.WARNING, "Mirroring cancel failed", e);
+            }
+          }
+        });
+        super.cancel(message, cause);
+      }
+    };
+  }
+}