Rename FileStateValue.DEFAULT_MIDDLEMAN to RUNFILES_TREE_MARKER.

Also add a Javadoc explaining why it exists; it really shouldn't, but at least then we should have a record as to why it does.

RELNOTES: None.
PiperOrigin-RevId: 695692425
Change-Id: I16ba3d739a45e6a031e987a36ffec0c7c77fcbd6

Author: lberki

src/main/java/com/google/devtools/build/lib/actions/FileArtifactValue.java

@@ -189,8 +189,20 @@ public Optional<PathFragment> getMaterializationExecPath() {
    */
   interface Singleton {}
 
+  /**
+   * Metadata for runfiles trees.
+   *
+   * <p>This should really be more nuanced so that runfiles trees don't need to be special-cased in
+   * the local action cache, but it works well enough. The only downsides are that we don't detect
+   * when someone changed a runfiles tree like we do for other output artifacts and a number of
+   * extra branches.
+   *
+   * <p>In Skyframe, we check whether a runfiles tree changed based on {@link
+   * RunfilesArtifactValue}, which does contain data about its contents.
+   */
   @SerializationConstant
-  public static final FileArtifactValue DEFAULT_MIDDLEMAN = new SingletonMarkerValue();
+  public static final FileArtifactValue RUNFILES_TREE_MARKER = new SingletonMarkerValue();
+
   /** Data that marks that a file is not present on the filesystem. */
   @SerializationConstant
   public static final FileArtifactValue MISSING_FILE_MARKER = new SingletonMarkerValue();

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

@@ -197,8 +197,8 @@ public FileArtifactValue getOutputMetadata(ActionInput actionInput)
       if (value != null) {
         return checkExists(value, artifact);
       }
-      putArtifactData(artifact, FileArtifactValue.DEFAULT_MIDDLEMAN);
-      return FileArtifactValue.DEFAULT_MIDDLEMAN;
+      putArtifactData(artifact, FileArtifactValue.RUNFILES_TREE_MARKER);
+      return FileArtifactValue.RUNFILES_TREE_MARKER;
     }
 
     if (artifact.isTreeArtifact()) {

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

@@ -417,7 +417,7 @@ public SkyValue compute(SkyKey skyKey, Environment env) throws InterruptedExcept
                   .build();
           treeArtifactData.put(output, tree);
         } else if (output.isRunfilesTree()) {
-          artifactData.put(output, FileArtifactValue.DEFAULT_MIDDLEMAN);
+          artifactData.put(output, FileArtifactValue.RUNFILES_TREE_MARKER);
         } else {
           Path path = output.getPath();
           FileArtifactValue noDigest =

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

@@ -92,7 +92,7 @@ public void testEqualsAndHashCode() {
             RemoteFileArtifactValue.create(toBytes("00112233445566778899AABBCCDDEEFF"), 1, 1, 1),
             RemoteFileArtifactValue.create(toBytes("00112233445566778899AABBCCDDEEFF"), 1, 1, 2))
         .addEqualityGroup(FileArtifactValue.MISSING_FILE_MARKER)
-        .addEqualityGroup(FileArtifactValue.DEFAULT_MIDDLEMAN)
+        .addEqualityGroup(FileArtifactValue.RUNFILES_TREE_MARKER)
         .addEqualityGroup("a string")
         .testEquals();
   }