Set the digest_function field as part of all relevant gRPC requests

In the following PR we extended most of the REv2 *Request messages to
take an explicit digest function:

bazelbuild/remote-apis#235

This eliminates the ambiguity between digest functions that have the
same hash length (e.g., MD5 and MURMUR3, SHA256 and SHA256TREE). This
change extends the REv2 client in Bazel to set the digest_function field
explicitly, so that the server does not need to use any heuristics to
pick a digest function when processing requests.

As we are now going to call DigestUtil.getDigestFunction() far more
frequently, alter DigestUtil to precompute the value upon construction.

src/main/java/com/google/devtools/build/lib/remote/GrpcCacheClient.java

@@ -117,6 +117,7 @@ private int computeMaxMissingBlobsDigestsPerMessage() {
     final int overhead =
         FindMissingBlobsRequest.newBuilder()
             .setInstanceName(options.remoteInstanceName)
+            .setDigestFunction(digestUtil.getDigestFunction())
             .build()
             .getSerializedSize();
     final int tagSize =
@@ -185,7 +186,9 @@ public ListenableFuture<ImmutableSet<Digest>> findMissingDigests(
     }
     // Need to potentially split the digests into multiple requests.
     FindMissingBlobsRequest.Builder requestBuilder =
-        FindMissingBlobsRequest.newBuilder().setInstanceName(options.remoteInstanceName);
+        FindMissingBlobsRequest.newBuilder()
+            .setInstanceName(options.remoteInstanceName)
+            .setDigestFunction(digestUtil.getDigestFunction());
     List<ListenableFuture<FindMissingBlobsResponse>> getMissingDigestCalls = new ArrayList<>();
     for (Digest digest : digests) {
       requestBuilder.addBlobDigests(digest);
@@ -260,6 +263,7 @@ public ListenableFuture<CachedActionResult> downloadActionResult(
     GetActionResultRequest request =
         GetActionResultRequest.newBuilder()
             .setInstanceName(options.remoteInstanceName)
+            .setDigestFunction(digestUtil.getDigestFunction())
             .setActionDigest(actionKey.getDigest())
             .setInlineStderr(inlineOutErr)
             .setInlineStdout(inlineOutErr)
@@ -289,6 +293,7 @@ public ListenableFuture<Void> uploadActionResult(
                                         .updateActionResult(
                                             UpdateActionResultRequest.newBuilder()
                                                 .setInstanceName(options.remoteInstanceName)
+                                                .setDigestFunction(digestUtil.getDigestFunction())
                                                 .setActionDigest(actionKey.getDigest())
                                                 .setActionResult(actionResult)
                                                 .build())),

src/main/java/com/google/devtools/build/lib/remote/RemoteExecutionService.java

@@ -1486,6 +1486,7 @@ public RemoteActionResult executeRemotely(
     ExecuteRequest.Builder requestBuilder =
         ExecuteRequest.newBuilder()
             .setInstanceName(remoteOptions.remoteInstanceName)
+            .setDigestFunction(digestUtil.getDigestFunction())
             .setActionDigest(action.getActionKey().getDigest())
             .setSkipCacheLookup(!acceptCachedResult);
     if (remoteOptions.remoteResultCachePriority != 0) {

src/main/java/com/google/devtools/build/lib/remote/RemoteRepositoryRemoteExecutor.java

@@ -167,6 +167,7 @@ public ExecutionResult execute(
             ExecuteRequest.newBuilder()
                 .setActionDigest(actionDigest)
                 .setInstanceName(remoteInstanceName)
+                .setDigestFunction(digestUtil.getDigestFunction())
                 .setSkipCacheLookup(!acceptCached)
                 .build();
 

src/main/java/com/google/devtools/build/lib/remote/util/DigestUtil.java

@@ -35,14 +35,15 @@
 public class DigestUtil {
   private final XattrProvider xattrProvider;
   private final DigestHashFunction hashFn;
+  private final DigestFunction.Value digestFunction;
 
   public DigestUtil(XattrProvider xattrProvider, DigestHashFunction hashFn) {
     this.xattrProvider = xattrProvider;
     this.hashFn = hashFn;
+    this.digestFunction = getDigestFunctionFromHashFunction(hashFn);
   }
 
-  /** Returns the currently used digest function. */
-  public DigestFunction.Value getDigestFunction() {
+  private static DigestFunction.Value getDigestFunctionFromHashFunction(DigestHashFunction hashFn) {
     for (String name : hashFn.getNames()) {
       try {
         return DigestFunction.Value.valueOf(name);
@@ -53,6 +54,12 @@ public DigestFunction.Value getDigestFunction() {
     return DigestFunction.Value.UNKNOWN;
   }
 
+
+  /** Returns the currently used digest function. */
+  public DigestFunction.Value getDigestFunction() {
+    return digestFunction;
+  }
+
   public Digest compute(byte[] blob) {
     return buildDigest(hashFn.getHashFunction().hashBytes(blob).toString(), blob.length);
   }

src/test/java/com/google/devtools/build/lib/remote/GrpcCacheClientTest.java

@@ -30,6 +30,7 @@
 import build.bazel.remote.execution.v2.Command;
 import build.bazel.remote.execution.v2.ContentAddressableStorageGrpc.ContentAddressableStorageImplBase;
 import build.bazel.remote.execution.v2.Digest;
+import build.bazel.remote.execution.v2.DigestFunction;
 import build.bazel.remote.execution.v2.Directory;
 import build.bazel.remote.execution.v2.DirectoryNode;
 import build.bazel.remote.execution.v2.FileNode;
@@ -798,6 +799,7 @@ public void updateActionResult(
             assertThat(request)
                 .isEqualTo(
                     UpdateActionResultRequest.newBuilder()
+                        .setDigestFunction(DigestFunction.Value.SHA256)
                         .setActionDigest(fooDigest)
                         .setActionResult(result)
                         .build());