Clarify comments concerning output artifact data, remove index check

Note that 0 is a legal backend index to use when representing local,
empty, or omitted outputs.

Also, remove backend index check in injectRemoteFile to accommodate
storing omitted output information.

RELNOTES: None.
PiperOrigin-RevId: 230345657

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

@@ -118,7 +118,7 @@ public BigInteger getValueFingerprint() {
    * Index used to resolve remote files.
    *
    * <p>0 indicates that no such information is available which can mean that it's either a local
-   * file or empty.
+   * file, empty, or an omitted output.
    */
   public int getLocationIndex() {
     return 0;

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

@@ -476,13 +476,6 @@ public void injectDigest(ActionInput output, FileStatus statNoFollow, byte[] dig
   public void injectRemoteFile(Artifact output, byte[] digest, long size, int locationIndex) {
     Preconditions.checkState(
         executionMode.get(), "Tried to inject %s outside of execution.", output);
-    Preconditions.checkArgument(
-        locationIndex != 0 || size == 0,
-        "output = %s, size = %s, locationIndex =%s",
-        output,
-        size,
-        locationIndex);
-
     store.injectRemoteFile(output, digest, size, locationIndex);
   }
 

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

@@ -39,7 +39,4 @@ void putArtifactData(Artifact artifact, ArtifactFileMetadata value) {}
 
   @Override
   void addTreeArtifactContents(Artifact artifact, TreeFileArtifact contents) {}
-
-  @Override
-  void injectRemoteFile(Artifact output, byte[] digest, long size, int locationIndex) {}
 }

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

@@ -166,7 +166,7 @@ final void injectOutputData(Artifact output, FileArtifactValue artifactValue) {
     injectedFiles.add(output);
 
     // While `artifactValue` carries the important information, consumers also require an entry in
-    // `outputArtifactData` so a `PLACEHOLDER` is added to `outputArtifactData`.
+    // `artifactData` so a `PLACEHOLDER` is added to `artifactData`.
     artifactData.put(output, ArtifactFileMetadata.PLACEHOLDER);
     additionalOutputData.put(output, artifactValue);
   }

src/main/java/com/google/devtools/build/lib/util/RegexFilter.java

@@ -130,11 +130,13 @@ public boolean isIncluded(String value) {
     return inclusionPattern.matcher(value).find();
   }
 
-  String getInclusionRegex() {
+  @Nullable
+  public String getInclusionRegex() {
     return inclusionPattern == null ? null : inclusionPattern.pattern();
   }
 
-  String getExclusionRegex() {
+  @Nullable
+  public String getExclusionRegex() {
     return exclusionPattern == null ? null : exclusionPattern.pattern();
   }