Handle null digest returned by batchStat

When RemoteOutputService.BatchStat returns a null digest, the remote
output service knows the file exists, but it is incapable of returning
the digest. This may be caused by the kernel holding on to dirty pages
in its write-back cache.

By returning null, ActionMetadataHandler's
fileArtifactValueFromArtifact() will compute the file's digest
separately.

Make sure to run a recent build of bb-clientd to detect these cases.

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

@@ -398,7 +398,19 @@ public List<FileStatusWithDigest> batchStat(Iterable<PathFragment> paths)
                 }
                 FileStatus fileStatus = response.getFileStatus();
                 if (fileStatus.hasFile()) {
-                  Digest digest = fileStatus.getFile().getDigest();
+                  FileStatus.File regularFileStatus = fileStatus.getFile();
+                  if (!regularFileStatus.hasDigest()) {
+                    // The remote output service knows the file exists, but it
+                    // is incapable of returning the digest. This may be caused
+                    // by the kernel holding on to dirty pages in its write-back
+                    // cache.
+                    //
+                    // By returning null, ActionMetadataHandler's
+                    // fileArtifactValueFromArtifact() will compute the file's
+                    // digest separately.
+                    return null;
+                  }
+                  Digest digest = regularFileStatus.getDigest();
                   return new RegularFileStatus(digest.getSizeBytes(), DigestUtil.toBinaryDigest(digest));
                 }
                 if (fileStatus.hasDirectory()) {
@@ -558,6 +570,13 @@ protected byte[] getFastDigest(PathFragment path) throws IOException {
       FileStatus fileStatus = response.getFileStatus();
       if (fileStatus.hasFile()) {
         FileStatus.File regularFileStatus = fileStatus.getFile();
+        if (!regularFileStatus.hasDigest()) {
+          // The remote output service knows the file exists, but it is
+          // incapable of returning the digest. This may be caused by
+          // the kernel holding on to dirty pages in its write-back
+          // cache. We must compute the digest ourselves.
+          return super.getFastDigest(path);
+        }
         return DigestUtil.toBinaryDigest(regularFileStatus.getDigest());
       }
       if (fileStatus.hasExternal()) {