Include unresolved symlink target into the source directory hash

This makes it so that changes to the targets of unresolved symlinks invalidate actions that depend on the source directory containing them.

Author: fmeum

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

@@ -187,9 +187,7 @@ public RecursiveFilesystemTraversalValue compute(SkyKey skyKey, Environment env)
         if (rootInfo.type.isSymlink()) {
           return RecursiveFilesystemTraversalValue.of(
               ResolvedFileFactory.danglingSymlink(
-                  traversal.root().asRootedPath(),
-                  rootInfo.unresolvedSymlinkTarget,
-                  rootInfo.metadata));
+                  traversal.root().asRootedPath(), rootInfo.unresolvedSymlinkTarget));
         } else {
           return RecursiveFilesystemTraversalValue.EMPTY;
         }

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

@@ -14,12 +14,14 @@
 package com.google.devtools.build.lib.skyframe;
 
 import static com.google.common.base.Preconditions.checkNotNull;
+import static java.nio.charset.StandardCharsets.ISO_8859_1;
 
 import com.google.common.base.Objects;
 import com.google.devtools.build.lib.actions.HasDigest;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.collect.nestedset.Order;
+import com.google.devtools.build.lib.vfs.DigestHashFunction;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import com.google.devtools.build.lib.vfs.RootedPath;
 import com.google.devtools.build.skyframe.SkyValue;
@@ -574,9 +576,15 @@ public static ResolvedFile symlinkToDirectory(
       return new SymlinkToDirectory(targetPath, linkNamePath, linkValue, metadata);
     }
 
-    public static ResolvedFile danglingSymlink(
-        RootedPath linkNamePath, PathFragment linkValue, HasDigest metadata) {
-      return new DanglingSymlink(linkNamePath, linkValue, metadata);
+    public static ResolvedFile danglingSymlink(RootedPath linkNamePath, PathFragment linkValue) {
+      byte[] digest =
+          DigestHashFunction.SHA256
+              .getHashFunction()
+              .hashString(linkValue.getPathString(), ISO_8859_1)
+              .asBytes();
+      // Ensure that the digest does not collide with that of a regular file.
+      digest[0] ^= 1;
+      return new DanglingSymlink(linkNamePath, linkValue, new HasDigest.ByteStringDigest(digest));
     }
   }
 

src/test/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunctionTest.java

@@ -830,8 +830,7 @@ public void testTraversalOfDanglingSymlink() throws Exception {
     PathFragment linkTarget = PathFragment.create("non_existent");
     parentOf(link).asPath().createDirectory();
     link.asPath().createSymbolicLink(linkTarget);
-    traverseAndAssertFiles(
-        fileLikeRoot(linkArtifact), danglingSymlink(link, linkTarget, EMPTY_METADATA));
+    traverseAndAssertFiles(fileLikeRoot(linkArtifact), danglingSymlink(link, linkTarget));
   }
 
   @Test
@@ -845,7 +844,7 @@ public void testTraversalOfDanglingSymlinkInADirectory() throws Exception {
     traverseAndAssertFiles(
         fileLikeRoot(dirArtifact),
         regularFile(file, EMPTY_METADATA),
-        danglingSymlink(link, linkTarget, EMPTY_METADATA));
+        danglingSymlink(link, linkTarget));
   }
 
   @Test

src/test/java/com/google/devtools/build/lib/skyframe/SourceDirectoryIntegrationTest.java

@@ -13,6 +13,7 @@
 // limitations under the License.
 package com.google.devtools.build.lib.skyframe;
 
+import static java.nio.charset.StandardCharsets.ISO_8859_1;
 import static org.junit.Assert.assertThrows;
 
 import com.google.common.collect.ImmutableSet;
@@ -195,13 +196,21 @@ public void emptyDirReplacedWithDifferentSymlink_invalidatesAction() throws Exce
   }
 
   @Test
-  @Ignore("TODO(#25834)")
   public void danglingSymlinkModified_invalidatesAction() throws Exception {
     FileSystemUtils.ensureSymbolicLink(
         sourceDir.getRelative("dangling_symlink"), PathFragment.create("still_does_not_exist"));
     assertInvalidatedByBuild();
   }
 
+  @Test
+  public void danglingSymlinkReplacedWithFile_invalidatesAction() throws Exception {
+    Path danglingSymlink = sourceDir.getRelative("dangling_symlink");
+    String target = danglingSymlink.readSymbolicLink().getPathString();
+    danglingSymlink.delete();
+    FileSystemUtils.writeContent(danglingSymlink, ISO_8859_1, target);
+    assertInvalidatedByBuild();
+  }
+
   @Test
   public void crossingPackageBoundary_fails() throws Exception {
     FileSystemUtils.touchFile(sourceDir.getRelative("subdir/BUILD"));