remote: Add interceptor for logging gRPC calls during remote execution/caching

This provides a io.grpc.ClientInterceptor implementation that can be used to log gRPC call information. The interceptor can select a logging handler to use based on the gRPC method being called (Watch, Execute, Write, etc) to build a LogEntry, which can then be logged after the call has finished. Unit tests for the interceptor are included.

In this change, the interceptor is never invoked, nor are there any handlers implemented for any gRPC methods. The interceptor also never tries to log any entries.

To avoid circular dependency issues (Remote library will depend on logger which depends on remote library for utils), I've factored out the utility classes from the remote library into their own directory/package as part of this change.

PiperOrigin-RevId: 187926516

20 files changed, 249 insertions, 12 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/BUILD b/src/main/java/com/google/devtools/build/lib/BUILD
index 8b5135122e..dafeecec87 100644
--- a/src/main/java/com/google/devtools/build/lib/BUILD
+++ b/src/main/java/com/google/devtools/build/lib/BUILD
@@ -40,6 +40,8 @@ filegroup(
         "//src/main/java/com/google/devtools/build/lib/remote:srcs",
         "//src/main/java/com/google/devtools/build/lib/remote/blobstore:srcs",
         "//src/main/java/com/google/devtools/build/lib/remote/blobstore/http:srcs",
+        "//src/main/java/com/google/devtools/build/lib/remote/logging:srcs",
+        "//src/main/java/com/google/devtools/build/lib/remote/util:srcs",
         "//src/main/java/com/google/devtools/build/lib/rules/apple/cpp:srcs",
         "//src/main/java/com/google/devtools/build/lib/rules/apple:srcs",
         "//src/main/java/com/google/devtools/build/lib/rules/apple/swift:srcs",
diff --git a/src/main/java/com/google/devtools/build/lib/remote/AbstractRemoteActionCache.java b/src/main/java/com/google/devtools/build/lib/remote/AbstractRemoteActionCache.java
index 6b2c4560f1..d0a05fa473 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/AbstractRemoteActionCache.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/AbstractRemoteActionCache.java
@@ -17,6 +17,7 @@ import com.google.devtools.build.lib.actions.EnvironmentalExecException;
 import com.google.devtools.build.lib.actions.ExecException;
 import com.google.devtools.build.lib.concurrent.ThreadSafety;
 import com.google.devtools.build.lib.remote.TreeNodeRepository.TreeNode;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
 import com.google.devtools.build.lib.util.io.FileOutErr;
 import com.google.devtools.build.lib.vfs.Dirent;
 import com.google.devtools.build.lib.vfs.FileSystemUtils;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/BUILD b/src/main/java/com/google/devtools/build/lib/remote/BUILD
index 050a555698..42bf402027 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/BUILD
+++ b/src/main/java/com/google/devtools/build/lib/remote/BUILD
@@ -31,6 +31,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/exec/local:options",
         "//src/main/java/com/google/devtools/build/lib/remote/blobstore",
         "//src/main/java/com/google/devtools/build/lib/remote/blobstore/http",
+        "//src/main/java/com/google/devtools/build/lib/remote/util",
         "//src/main/java/com/google/devtools/build/lib/vfs",
         "//src/main/java/com/google/devtools/common/options",
         "//third_party:auth",
diff --git a/src/main/java/com/google/devtools/build/lib/remote/ByteStreamUploader.java b/src/main/java/com/google/devtools/build/lib/remote/ByteStreamUploader.java
index 609ee3c669..000bfc38ee 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/ByteStreamUploader.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/ByteStreamUploader.java
@@ -33,6 +33,7 @@ import com.google.common.util.concurrent.ListeningScheduledExecutorService;
 import com.google.common.util.concurrent.MoreExecutors;
 import com.google.common.util.concurrent.SettableFuture;
 import com.google.devtools.build.lib.remote.Retrier.RetryException;
+import com.google.devtools.build.lib.remote.util.TracingMetadataUtils;
 import com.google.devtools.remoteexecution.v1test.Digest;
 import io.grpc.CallCredentials;
 import io.grpc.CallOptions;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/CacheNotFoundException.java b/src/main/java/com/google/devtools/build/lib/remote/CacheNotFoundException.java
index 2f9e0c3ed0..717d5f538f 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/CacheNotFoundException.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/CacheNotFoundException.java
@@ -14,6 +14,7 @@
 
 package com.google.devtools.build.lib.remote;
 
+import com.google.devtools.build.lib.remote.util.DigestUtil;
 import com.google.devtools.remoteexecution.v1test.Digest;
 import java.io.IOException;
 
diff --git a/src/main/java/com/google/devtools/build/lib/remote/Chunker.java b/src/main/java/com/google/devtools/build/lib/remote/Chunker.java
index 9f14fa3f0b..fb3202c214 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/Chunker.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/Chunker.java
@@ -22,6 +22,7 @@ import com.google.common.base.Throwables;
 import com.google.common.io.ByteStreams;
 import com.google.devtools.build.lib.actions.ActionInput;
 import com.google.devtools.build.lib.actions.MetadataProvider;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.remoteexecution.v1test.Digest;
 import com.google.protobuf.ByteString;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/GrpcRemoteCache.java b/src/main/java/com/google/devtools/build/lib/remote/GrpcRemoteCache.java
index 377c50d091..3f5a48176b 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/GrpcRemoteCache.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/GrpcRemoteCache.java
@@ -27,9 +27,11 @@ import com.google.common.util.concurrent.MoreExecutors;
 import com.google.devtools.build.lib.actions.ActionInput;
 import com.google.devtools.build.lib.actions.MetadataProvider;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
-import com.google.devtools.build.lib.remote.DigestUtil.ActionKey;
 import com.google.devtools.build.lib.remote.Retrier.RetryException;
 import com.google.devtools.build.lib.remote.TreeNodeRepository.TreeNode;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
+import com.google.devtools.build.lib.remote.util.DigestUtil.ActionKey;
+import com.google.devtools.build.lib.remote.util.TracingMetadataUtils;
 import com.google.devtools.build.lib.util.io.FileOutErr;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.remoteexecution.v1test.ActionCacheGrpc;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/GrpcRemoteExecutor.java b/src/main/java/com/google/devtools/build/lib/remote/GrpcRemoteExecutor.java
index dbff8e7e88..3b91174a06 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/GrpcRemoteExecutor.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/GrpcRemoteExecutor.java
@@ -16,6 +16,7 @@ package com.google.devtools.build.lib.remote;
 
 import com.google.common.base.Preconditions;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
+import com.google.devtools.build.lib.remote.util.TracingMetadataUtils;
 import com.google.devtools.remoteexecution.v1test.ExecuteRequest;
 import com.google.devtools.remoteexecution.v1test.ExecuteResponse;
 import com.google.devtools.remoteexecution.v1test.ExecutionGrpc;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/RemoteActionContextProvider.java b/src/main/java/com/google/devtools/build/lib/remote/RemoteActionContextProvider.java
index c761c10f3c..67acd0f93e 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/RemoteActionContextProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/RemoteActionContextProvider.java
@@ -27,6 +27,7 @@ import com.google.devtools.build.lib.exec.local.LocalExecutionOptions;
 import com.google.devtools.build.lib.exec.local.LocalSpawnRunner;
 import com.google.devtools.build.lib.exec.local.PosixLocalEnvProvider;
 import com.google.devtools.build.lib.exec.local.WindowsLocalEnvProvider;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
 import com.google.devtools.build.lib.runtime.CommandEnvironment;
 import com.google.devtools.build.lib.util.OS;
 import javax.annotation.Nullable;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/RemoteModule.java b/src/main/java/com/google/devtools/build/lib/remote/RemoteModule.java
index 235f97137c..72c58d312b 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/RemoteModule.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/RemoteModule.java
@@ -22,6 +22,7 @@ import com.google.devtools.build.lib.buildeventstream.PathConverter;
 import com.google.devtools.build.lib.buildtool.BuildRequest;
 import com.google.devtools.build.lib.events.Event;
 import com.google.devtools.build.lib.exec.ExecutorBuilder;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
 import com.google.devtools.build.lib.runtime.BlazeModule;
 import com.google.devtools.build.lib.runtime.Command;
 import com.google.devtools.build.lib.runtime.CommandEnvironment;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnCache.java b/src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnCache.java
index 8886f37de4..6d766923ab 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnCache.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnCache.java
@@ -26,8 +26,10 @@ import com.google.devtools.build.lib.events.Event;
 import com.google.devtools.build.lib.events.Reporter;
 import com.google.devtools.build.lib.exec.SpawnCache;
 import com.google.devtools.build.lib.exec.SpawnRunner.SpawnExecutionPolicy;
-import com.google.devtools.build.lib.remote.DigestUtil.ActionKey;
 import com.google.devtools.build.lib.remote.TreeNodeRepository.TreeNode;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
+import com.google.devtools.build.lib.remote.util.DigestUtil.ActionKey;
+import com.google.devtools.build.lib.remote.util.TracingMetadataUtils;
 import com.google.devtools.build.lib.skyframe.FileArtifactValue;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnRunner.java b/src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnRunner.java
index a061bedcf0..0d113830b9 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnRunner.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnRunner.java
@@ -33,9 +33,11 @@ import com.google.devtools.build.lib.events.Event;
 import com.google.devtools.build.lib.events.Reporter;
 import com.google.devtools.build.lib.exec.SpawnExecException;
 import com.google.devtools.build.lib.exec.SpawnRunner;
-import com.google.devtools.build.lib.remote.DigestUtil.ActionKey;
 import com.google.devtools.build.lib.remote.Retrier.RetryException;
 import com.google.devtools.build.lib.remote.TreeNodeRepository.TreeNode;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
+import com.google.devtools.build.lib.remote.util.DigestUtil.ActionKey;
+import com.google.devtools.build.lib.remote.util.TracingMetadataUtils;
 import com.google.devtools.build.lib.util.ExitCode;
 import com.google.devtools.build.lib.util.io.FileOutErr;
 import com.google.devtools.build.lib.vfs.Path;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/SimpleBlobStoreActionCache.java b/src/main/java/com/google/devtools/build/lib/remote/SimpleBlobStoreActionCache.java
index 21ad4489bb..656288b7c0 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/SimpleBlobStoreActionCache.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/SimpleBlobStoreActionCache.java
@@ -18,9 +18,10 @@ import com.google.devtools.build.lib.actions.ActionInput;
 import com.google.devtools.build.lib.actions.MetadataProvider;
 import com.google.devtools.build.lib.actions.cache.VirtualActionInput;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
-import com.google.devtools.build.lib.remote.DigestUtil.ActionKey;
 import com.google.devtools.build.lib.remote.TreeNodeRepository.TreeNode;
 import com.google.devtools.build.lib.remote.blobstore.SimpleBlobStore;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
+import com.google.devtools.build.lib.remote.util.DigestUtil.ActionKey;
 import com.google.devtools.build.lib.util.io.FileOutErr;
 import com.google.devtools.build.lib.vfs.FileSystemUtils;
 import com.google.devtools.build.lib.vfs.Path;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/TreeNodeRepository.java b/src/main/java/com/google/devtools/build/lib/remote/TreeNodeRepository.java
index 8a1dfe9fa4..2d2de7e92a 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/TreeNodeRepository.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/TreeNodeRepository.java
@@ -30,6 +30,7 @@ import com.google.devtools.build.lib.actions.cache.VirtualActionInput;
 import com.google.devtools.build.lib.concurrent.BlazeInterners;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
+import com.google.devtools.build.lib.remote.util.DigestUtil;
 import com.google.devtools.build.lib.vfs.Dirent;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
diff --git a/src/main/java/com/google/devtools/build/lib/remote/logging/BUILD b/src/main/java/com/google/devtools/build/lib/remote/logging/BUILD
new file mode 100644
index 0000000000..9d8af5875d
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/remote/logging/BUILD
@@ -0,0 +1,23 @@
+package(default_visibility = ["//src:__subpackages__"])
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+    visibility = ["//src/main/java/com/google/devtools/build/lib:__pkg__"],
+)
+
+java_library(
+    name = "logging",
+    srcs = glob(["*.java"]),
+    tags = ["bazel"],
+    deps = [
+        "//src/main/java/com/google/devtools/build/lib/remote/util",
+        "//src/main/protobuf:remote_execution_log_java_proto",
+        "//third_party:guava",
+        "//third_party/grpc:grpc-jar",
+        "@googleapis//:google_devtools_remoteexecution_v1test_remote_execution_java_proto",
+        "@googleapis//:google_longrunning_operations_java_proto",
+        "@googleapis//:google_rpc_status_java_proto",
+        "@googleapis//:google_watch_v1_java_proto",
+    ],
+)
diff --git a/src/main/java/com/google/devtools/build/lib/remote/logging/LoggingHandler.java b/src/main/java/com/google/devtools/build/lib/remote/logging/LoggingHandler.java
new file mode 100644
index 0000000000..fb23d6ca4b
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/remote/logging/LoggingHandler.java
@@ -0,0 +1,44 @@
+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// 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 com.google.devtools.build.lib.remote.logging;
+
+import com.google.devtools.build.lib.remote.logging.RemoteExecutionLog.LogEntry;
+
+/**
+ * An interface for building {@link LogEntry}s specialized for a specific gRPC call with specific
+ * request and response types.
+ *
+ * @param <ReqT> request type of the gRPC call
+ * @param <RespT> response type of the gRPC call
+ */
+public interface LoggingHandler<ReqT, RespT> {
+
+  /**
+   * Handle logging for an issued message.
+   *
+   * @param message the issued request message
+   */
+  void handleReq(ReqT message);
+
+  /**
+   * Handle logging for a received response.
+   *
+   * @param message the received response message
+   */
+  void handleResp(RespT message);
+
+  /** Returns a {@link LogEntry} based on the requests and responses handled by this handler * */
+  LogEntry getEntry();
+}
diff --git a/src/main/java/com/google/devtools/build/lib/remote/logging/LoggingInterceptor.java b/src/main/java/com/google/devtools/build/lib/remote/logging/LoggingInterceptor.java
new file mode 100644
index 0000000000..37629d7ae1
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/remote/logging/LoggingInterceptor.java
@@ -0,0 +1,121 @@
+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// 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 com.google.devtools.build.lib.remote.logging;
+
+import com.google.devtools.build.lib.remote.logging.RemoteExecutionLog.LogEntry;
+import com.google.devtools.build.lib.remote.util.TracingMetadataUtils;
+import com.google.devtools.remoteexecution.v1test.RequestMetadata;
+import io.grpc.CallOptions;
+import io.grpc.Channel;
+import io.grpc.ClientCall;
+import io.grpc.ClientInterceptor;
+import io.grpc.ForwardingClientCall;
+import io.grpc.ForwardingClientCallListener;
+import io.grpc.Metadata;
+import io.grpc.MethodDescriptor;
+import io.grpc.Status;
+import javax.annotation.Nullable;
+
+/** Client interceptor for logging details of certain gRPC calls. */
+public class LoggingInterceptor implements ClientInterceptor {
+
+  /**
+   * Returns a {@link LoggingHandler} to handle logging details for the specified method. If there
+   * is no handler for the given method, returns {@code null}.
+   *
+   * @param method Method to return handler for.
+   */
+  protected <ReqT, RespT> @Nullable LoggingHandler<ReqT, RespT> selectHandler(
+      MethodDescriptor<ReqT, RespT> method) {
+    // TODO(cdlee): add handlers for methods
+    return null;
+  }
+
+  @Override
+  public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(
+      MethodDescriptor<ReqT, RespT> method, CallOptions callOptions, Channel next) {
+    ClientCall<ReqT, RespT> call = next.newCall(method, callOptions);
+    LoggingHandler<ReqT, RespT> handler = selectHandler(method);
+    if (handler != null) {
+      return new LoggingForwardingCall<>(call, handler, method);
+    } else {
+      return call;
+    }
+  }
+
+  /**
+   * Wraps client call to log call details by building a {@link LogEntry} and writing it to a log.
+   */
+  private static class LoggingForwardingCall<ReqT, RespT>
+      extends ForwardingClientCall.SimpleForwardingClientCall<ReqT, RespT> {
+    private final LoggingHandler<ReqT, RespT> handler;
+    private final LogEntry.Builder entryBuilder;
+
+    protected LoggingForwardingCall(
+        ClientCall<ReqT, RespT> delegate,
+        LoggingHandler<ReqT, RespT> handler,
+        MethodDescriptor<ReqT, RespT> method) {
+      super(delegate);
+      this.handler = handler;
+      this.entryBuilder = LogEntry.newBuilder().setMethodName(method.getFullMethodName());
+    }
+
+    @Override
+    public void start(Listener<RespT> responseListener, Metadata headers) {
+      RequestMetadata metadata = TracingMetadataUtils.requestMetadataFromHeaders(headers);
+      if (metadata != null) {
+        entryBuilder.setMetadata(metadata);
+      }
+      super.start(
+          new ForwardingClientCallListener.SimpleForwardingClientCallListener<RespT>(
+              responseListener) {
+            @Override
+            public void onMessage(RespT message) {
+              handler.handleResp(message);
+              super.onMessage(message);
+            }
+
+            @Override
+            public void onClose(Status status, Metadata trailers) {
+              entryBuilder.setStatus(makeStatusProto(status));
+              // TODO(cdlee): Actually store this and log the entry.
+              entryBuilder.mergeFrom(handler.getEntry()).build();
+              super.onClose(status, trailers);
+            }
+          },
+          headers);
+    }
+
+    @Override
+    public void sendMessage(ReqT message) {
+      handler.handleReq(message);
+      super.sendMessage(message);
+    }
+  }
+
+  /** Converts io.grpc.Status to com.google.rpc.Status proto for logging. */
+  private static com.google.rpc.Status makeStatusProto(Status status) {
+    String message = "";
+    if (status.getCause() != null) {
+      message = status.getCause().toString();
+    } else if (status.getDescription() != null) {
+      message = status.getDescription();
+    }
+    return com.google.rpc.Status.newBuilder()
+        .setCode(status.getCode().value())
+        .setMessage(message)
+        .build();
+  }
+}
diff --git a/src/main/java/com/google/devtools/build/lib/remote/util/BUILD b/src/main/java/com/google/devtools/build/lib/remote/util/BUILD
new file mode 100644
index 0000000000..41834376ff
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/remote/util/BUILD
@@ -0,0 +1,22 @@
+package(default_visibility = ["//src:__subpackages__"])
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+    visibility = ["//src/main/java/com/google/devtools/build/lib:__pkg__"],
+)
+
+java_library(
+    name = "util",
+    srcs = glob(["*.java"]),
+    tags = ["bazel"],
+    deps = [
+        "//src/main/java/com/google/devtools/build/lib:build-base",
+        "//src/main/java/com/google/devtools/build/lib/actions",
+        "//src/main/java/com/google/devtools/build/lib/vfs",
+        "//third_party:guava",
+        "//third_party/grpc:grpc-jar",
+        "//third_party/protobuf:protobuf_java",
+        "@googleapis//:google_devtools_remoteexecution_v1test_remote_execution_java_proto",
+    ],
+)
diff --git a/src/main/java/com/google/devtools/build/lib/remote/DigestUtil.java b/src/main/java/com/google/devtools/build/lib/remote/util/DigestUtil.java
index d3c93bbde2..afd31ccdeb 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/DigestUtil.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/util/DigestUtil.java
@@ -11,7 +11,7 @@
 // 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 com.google.devtools.build.lib.remote;
+package com.google.devtools.build.lib.remote.util;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
 
diff --git a/src/main/java/com/google/devtools/build/lib/remote/TracingMetadataUtils.java b/src/main/java/com/google/devtools/build/lib/remote/util/TracingMetadataUtils.java
index a7958617af..eac9e5aac8 100644
--- a/src/main/java/com/google/devtools/build/lib/remote/TracingMetadataUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/remote/util/TracingMetadataUtils.java
@@ -11,11 +11,11 @@
 // 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 com.google.devtools.build.lib.remote;
+package com.google.devtools.build.lib.remote.util;
 
 import com.google.common.annotations.VisibleForTesting;
 import com.google.devtools.build.lib.analysis.BlazeVersionInfo;
-import com.google.devtools.build.lib.remote.DigestUtil.ActionKey;
+import com.google.devtools.build.lib.remote.util.DigestUtil.ActionKey;
 import com.google.devtools.remoteexecution.v1test.RequestMetadata;
 import com.google.devtools.remoteexecution.v1test.ToolDetails;
 import io.grpc.ClientInterceptor;
@@ -28,6 +28,7 @@ import io.grpc.ServerCallHandler;
 import io.grpc.ServerInterceptor;
 import io.grpc.protobuf.ProtoUtils;
 import io.grpc.stub.MetadataUtils;
+import javax.annotation.Nullable;
 
 /** Utility functions to handle Metadata for remote Grpc calls. */
 public class TracingMetadataUtils {
@@ -42,11 +43,11 @@ public class TracingMetadataUtils {
       ProtoUtils.keyForProto(RequestMetadata.getDefaultInstance());
 
   /**
-   * Returns a new gRPC context derived from the current context, with
-   * {@link RequestMetadata} accessible by the {@link fromCurrentContext()} method.
+   * Returns a new gRPC context derived from the current context, with {@link RequestMetadata}
+   * accessible by the {@link fromCurrentContext()} method.
    *
-   * <p>The {@link RequestMetadata} is constructed using the provided arguments
-   * and the current tool version.
+   * <p>The {@link RequestMetadata} is constructed using the provided arguments and the current tool
+   * version.
    */
   public static Context contextWithMetadata(
       String buildRequestId, String commandId, ActionKey actionKey) {
@@ -88,6 +89,14 @@ public class TracingMetadataUtils {
     return headers;
   }
 
+  /**
+   * Extracts a {@link RequestMetadata} from a {@link Metadata} and returns it if it exists. If it
+   * does not exist, returns {@code null}.
+   */
+  public static @Nullable RequestMetadata requestMetadataFromHeaders(Metadata headers) {
+    return headers.get(METADATA_KEY);
+  }
+
   public static ClientInterceptor attachMetadataFromContextInterceptor() {
     return MetadataUtils.newAttachHeadersInterceptor(headersFromCurrentContext());
   }
@@ -97,7 +106,7 @@ public class TracingMetadataUtils {
     @Override
     public <ReqT, RespT> Listener<ReqT> interceptCall(
         ServerCall<ReqT, RespT> call, Metadata headers, ServerCallHandler<ReqT, RespT> next) {
-      RequestMetadata meta = headers.get(METADATA_KEY);
+      RequestMetadata meta = requestMetadataFromHeaders(headers);
       if (meta == null) {
         throw new IllegalStateException("RequestMetadata not received from the client.");
       }