Fix artifact/symlink mismatch detection

src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java

@@ -600,6 +600,12 @@ private static FileArtifactValue fileArtifactValueFromArtifact(
     checkState(!artifact.isTreeArtifact() && !artifact.isMiddlemanArtifact(), artifact);
 
     Path pathNoFollow = artifactPathResolver.toPath(artifact);
+    // If we expect a symlink, we can readlink it directly and handle errors appropriately - there
+    // is no need for the stat below.
+    if (artifact.isSymlink()) {
+      return FileArtifactValue.createForUnresolvedSymlink(pathNoFollow);
+    }
+
     RootedPath rootedPathNoFollow =
         RootedPath.toRootedPath(
             artifactPathResolver.transformRoot(artifact.getRoot().getRoot()),
@@ -618,10 +624,6 @@ private static FileArtifactValue fileArtifactValueFromArtifact(
           rootedPathNoFollow, statNoFollow, digestWillBeInjected, xattrProvider, tsgm);
     }
 
-    if (artifact.isSymlink()) {
-      return FileArtifactValue.createForUnresolvedSymlink(pathNoFollow);
-    }
-
     // We use FileStatus#isSymbolicLink over Path#isSymbolicLink to avoid the unnecessary stat
     // done by the latter.  We need to protect against symlink cycles since
     // ArtifactFileMetadata#value assumes it's dealing with a file that's not in a symlink cycle.

src/main/java/com/google/devtools/build/lib/skyframe/SkyframeActionExecutor.java

@@ -98,6 +98,7 @@
 import com.google.devtools.build.lib.util.DetailedExitCode;
 import com.google.devtools.build.lib.util.io.FileOutErr;
 import com.google.devtools.build.lib.vfs.FileSystem;
+import com.google.devtools.build.lib.vfs.FileSystem.NotASymlinkException;
 import com.google.devtools.build.lib.vfs.OutputService;
 import com.google.devtools.build.lib.vfs.OutputService.ActionFileSystemType;
 import com.google.devtools.build.lib.vfs.Path;
@@ -1541,8 +1542,11 @@ private boolean checkOutputs(
           success = false;
           if (output.isTreeArtifact()) {
             reportOutputTreeArtifactErrors(action, output, reporter, e);
+          } else if (output.isSymlink() && e instanceof NotASymlinkException) {
+            reporter.handle(Event.error(action.getOwner().getLocation(),
+                String.format("declared output '%s' is not a symlink", output.prettyPrint())));
           } else {
-            // Are all exceptions caught due to missing files?
+            // Are all other exceptions caught due to missing files?
             reportMissingOutputFile(action, output, reporter, output.getPath().isSymbolicLink(), e);
           }
         }

src/main/java/com/google/devtools/build/lib/vfs/FileSystem.java

@@ -52,7 +52,7 @@ public DigestHashFunction getDigestFunction() {
   /**
    * An exception thrown when attempting to resolve an ordinary file as a symlink.
    */
-  protected static final class NotASymlinkException extends IOException {
+  public static final class NotASymlinkException extends IOException {
     public NotASymlinkException(PathFragment path) {
       super(path.getPathString() + " is not a symlink");
     }

src/main/java/com/google/devtools/build/lib/vfs/Path.java

@@ -492,8 +492,8 @@ public void createSymbolicLink(PathFragment target) throws IOException {
    * an {@link UnsupportedOperationException} if the link points to a non-existent file.
    *
    * @return the content (i.e. target) of the symbolic link
-   * @throws IOException if the current path is not a symbolic link, or the contents of the link
-   *     could not be read for any reason
+   * @throws FileSystem.NotASymlinkException if the current path is not a symbolic link.
+   * @throws IOException if the contents of the link could not be read for any reason
    */
   public PathFragment readSymbolicLink() throws IOException {
     return fileSystem.readSymbolicLink(asFragment());
@@ -504,8 +504,8 @@ public PathFragment readSymbolicLink() throws IOException {
    * are intentionally left underspecified otherwise to permit efficient implementations.
    *
    * @return the content (i.e. target) of the symbolic link
-   * @throws IOException if the current path is not a symbolic link, or the contents of the link
-   *     could not be read for any reason
+   * @throws FileSystem.NotASymlinkException if the current path is not a symbolic link.
+   * @throws IOException if the contents of the link could not be read for any reason
    */
   public PathFragment readSymbolicLinkUnchecked() throws IOException {
     return fileSystem.readSymbolicLinkUnchecked(asFragment());