[6.2.0]Skip empty directories instead of throwing in prefetcher.

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

@@ -197,10 +197,16 @@ protected ListenableFuture<Void> prefetchFiles(
     Map<SpecialArtifact, List<TreeFileArtifact>> trees = new HashMap<>();
     List<ActionInput> files = new ArrayList<>();
     for (ActionInput input : inputs) {
+      // Source artifacts don't need to be fetched.
       if (input instanceof Artifact && ((Artifact) input).isSourceArtifact()) {
         continue;
       }
 
+      // Skip empty tree artifacts (non-empty tree artifacts should have already been expanded).
+      if (input.isDirectory()) {
+        continue;
+      }
+
       if (input instanceof TreeFileArtifact) {
         TreeFileArtifact treeFile = (TreeFileArtifact) input;
         SpecialArtifact treeArtifact = treeFile.getParent();

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

@@ -520,6 +520,58 @@ public void treeOutputsFromLocalFileSystem_works() throws Exception {
     assertValidOutputFile("out/foobar.txt", "file-1\nbar\n");
   }
 
+  @Test
+  public void emptyTreeConsumedByLocalAction() throws Exception {
+    // Disable remote execution so that the empty tree artifact is prefetched.
+    addOptions("--modify_execution_info=Genrule=+no-remote-exec");
+    setDownloadToplevel();
+    writeOutputDirRule();
+    write(
+        "BUILD",
+        "load(':output_dir.bzl', 'output_dir')",
+        "output_dir(",
+        "  name = 'foo',",
+        "  manifest = ':manifest',",
+        ")",
+        "genrule(",
+        "  name = 'foobar',",
+        "  srcs = [':foo'],",
+        "  outs = ['foobar.txt'],",
+        "  cmd = 'touch $@',",
+        ")");
+    write("manifest"); // no files
+
+    buildTarget("//:foobar");
+    waitDownloads();
+  }
+
+  @Test
+  public void multiplePackagePaths_buildsSuccessfully() throws Exception {
+    write(
+        "../a/src/BUILD",
+        "genrule(",
+        "  name = 'foo',",
+        "  srcs = [],",
+        "  outs = ['out/foo.txt'],",
+        "  cmd = 'echo foo > $@',",
+        ")");
+    write(
+        "BUILD",
+        "genrule(",
+        "  name = 'foobar',",
+        "  srcs = ['//src:foo'],",
+        "  outs = ['out/foobar.txt'],",
+        "  cmd = 'cat $(location //src:foo) > $@ && echo bar >> $@',",
+        ")");
+    addOptions("--package_path=%workspace%:%workspace%/../a");
+    setDownloadToplevel();
+
+    buildTarget("//:foobar");
+    waitDownloads();
+
+    assertValidOutputFile("out/foobar.txt", "foo\nbar\n");
+  }
+
   @Test
   public void incrementalBuild_deleteOutputsInUnwritableParentDirectory() throws Exception {
     write(