Take no action to prefetch empty artifacts.

Previously, Bazel would attempt to overwrite /dev/null.

Closes bazelbuild#12514.

PiperOrigin-RevId: 343824719

Author: benjaminp

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

@@ -28,6 +28,7 @@
 import com.google.devtools.build.lib.actions.FileArtifactValue;
 import com.google.devtools.build.lib.actions.MetadataProvider;
 import com.google.devtools.build.lib.actions.cache.VirtualActionInput;
+import com.google.devtools.build.lib.actions.cache.VirtualActionInput.EmptyActionInput;
 import com.google.devtools.build.lib.profiler.Profiler;
 import com.google.devtools.build.lib.profiler.ProfilerTask;
 import com.google.devtools.build.lib.profiler.SilentCloseable;
@@ -94,9 +95,11 @@ public void prefetchFiles(
       Map<Path, ListenableFuture<Void>> downloadsToWaitFor = new HashMap<>();
       for (ActionInput input : inputs) {
         if (input instanceof VirtualActionInput) {
-          VirtualActionInput virtualActionInput = (VirtualActionInput) input;
-          Path outputPath = execRoot.getRelative(virtualActionInput.getExecPath());
-          SandboxHelpers.atomicallyWriteVirtualInput(virtualActionInput, outputPath, ".remote");
+          if (!(input instanceof EmptyActionInput)) {
+            VirtualActionInput virtualActionInput = (VirtualActionInput) input;
+            Path outputPath = execRoot.getRelative(virtualActionInput.getExecPath());
+            SandboxHelpers.atomicallyWriteVirtualInput(virtualActionInput, outputPath, ".remote");
+          }
         } else {
           FileArtifactValue metadata = metadataProvider.getMetadata(input);
           if (metadata == null || !metadata.isRemote()) {

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

@@ -36,6 +36,7 @@
 import com.google.devtools.build.lib.actions.cache.VirtualActionInput;
 import com.google.devtools.build.lib.actions.util.ActionsTestUtil;
 import com.google.devtools.build.lib.clock.JavaClock;
+import com.google.devtools.build.lib.exec.SpawnInputExpander;
 import com.google.devtools.build.lib.remote.options.RemoteOptions;
 import com.google.devtools.build.lib.remote.util.DigestUtil;
 import com.google.devtools.build.lib.remote.util.InMemoryCacheClient;
@@ -74,6 +75,9 @@ public void setUp() throws IOException {
     FileSystem fs = new InMemoryFileSystem(new JavaClock(), HASH_FUNCTION);
     execRoot = fs.getPath("/exec");
     execRoot.createDirectoryAndParents();
+    Path dev = fs.getPath("/dev");
+    dev.createDirectory();
+    dev.setWritable(false);
     artifactRoot = ArtifactRoot.asDerivedRoot(execRoot, "root");
     artifactRoot.getRoot().asPath().createDirectoryAndParents();
     options = Options.getDefaults(RemoteOptions.class);
@@ -127,6 +131,23 @@ public void testStagingVirtualActionInput() throws Exception {
     assertThat(actionInputFetcher.downloadsInProgress).isEmpty();
   }
 
+  @Test
+  public void testStagingEmptyVirtualActionInput() throws Exception {
+    // arrange
+    MetadataProvider metadataProvider = new StaticMetadataProvider(new HashMap<>());
+    RemoteCache remoteCache = newCache(options, digestUtil, new HashMap<>());
+    RemoteActionInputFetcher actionInputFetcher =
+        new RemoteActionInputFetcher(remoteCache, execRoot, RequestMetadata.getDefaultInstance());
+
+    // act
+    actionInputFetcher.prefetchFiles(
+        ImmutableList.of(SpawnInputExpander.EMPTY_FILE), metadataProvider);
+
+    // assert that nothing happened
+    assertThat(actionInputFetcher.downloadedFiles()).isEmpty();
+    assertThat(actionInputFetcher.downloadsInProgress).isEmpty();
+  }
+
   @Test
   public void testFileNotFound() throws Exception {
     // Test that we get an exception if an input file is missing