Introduces a SourceArtifact type.

SourceArtifact.getPath() is considered safe and eventually we may restrict
getPath() to only SourceArtifact.

PiperOrigin-RevId: 191768519

3 files changed, 40 insertions, 13 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
index bb8cd9875c..58c72204a3 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
@@ -94,20 +94,18 @@ import javax.annotation.Nullable;
  *   <li>A 'Fileset' special Artifact. This is a legacy type of Artifact and should not be used by
  *       new rule implementations.
  * </ul>
- *
- * <p>This class is "theoretically" final; it should not be subclassed except by {@link
- * SpecialArtifact}.
  */
 @Immutable
 @SkylarkModule(
-    name = "File",
-    category = SkylarkModuleCategory.BUILTIN,
-    doc = "This object is created during the analysis phase to represent a file or directory that "
-        + "will be read or written during the execution phase. It is not an open file handle, and "
-        + "cannot be used to directly read or write file contents. Rather, you use it to construct "
-        + "the action graph in a rule implementation function by passing it to action-creating "
-        + "functions. See the <a href='../rules.$DOC_EXT#files'>Rules page</a> for more "
-        + "information."
+  name = "File",
+  category = SkylarkModuleCategory.BUILTIN,
+  doc =
+      "This object is created during the analysis phase to represent a file or directory that "
+          + "will be read or written during the execution phase. It is not an open file handle, "
+          + "and cannot be used to directly read or write file contents. Rather, you use it to "
+          + "construct the action graph in a rule implementation function by passing it to "
+          + "action-creating functions. See the "
+          + "<a href='../rules.$DOC_EXT#files'>Rules page</a> for more information."
 )
 @AutoCodec
 public class Artifact
@@ -422,6 +420,31 @@ public class Artifact
     return false;
   }
 
+  /** Only callable if isSourceArtifact() is true. */
+  public SourceArtifact asSourceArtifact() {
+    throw new IllegalStateException("Not a source artifact!");
+  }
+
+  /** {@link Artifact#isSourceArtifact() is true.
+   *
+   * <p>Source artifacts have the property that unlike for output artifacts, direct file system
+   * access for their contents should be safe, even in a distributed context.
+   *
+   * TODO(shahan): move {@link Artifact#getPath} to this subclass.
+   * */
+  @AutoCodec
+  public static final class SourceArtifact extends Artifact {
+    @AutoCodec.VisibleForSerialization
+    SourceArtifact(ArtifactRoot root, PathFragment execPath, ArtifactOwner owner) {
+      super(root, execPath, owner);
+    }
+
+    @Override
+    public SourceArtifact asSourceArtifact() {
+      return this;
+    }
+  }
+
   /**
    * Special artifact types.
    *
diff --git a/src/main/java/com/google/devtools/build/lib/actions/ArtifactFactory.java b/src/main/java/com/google/devtools/build/lib/actions/ArtifactFactory.java
index d23637a0fb..3011dd87b4 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/ArtifactFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/ArtifactFactory.java
@@ -258,7 +258,9 @@ public class ArtifactFactory implements ArtifactResolver {
       @Nullable SpecialArtifactType type) {
     Preconditions.checkNotNull(owner);
     if (type == null) {
-      return new Artifact(root, execPath, owner);
+      return root.isSourceRoot()
+          ? new Artifact.SourceArtifact(root, execPath, owner)
+          : new Artifact(root, execPath, owner);
     } else {
       return new Artifact.SpecialArtifact(root, execPath, owner, type);
     }
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
index b4f3e69aa8..6fee875c33 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
@@ -78,7 +78,9 @@ public final class EvalUtils {
               && ((SkylarkList) o1).isTuple() == ((SkylarkList) o2).isTuple()) {
             return compareLists((SkylarkList) o1, (SkylarkList) o2);
           }
-          if (!o1.getClass().equals(o2.getClass())) {
+
+          if (!(o1.getClass().isAssignableFrom(o2.getClass())
+              || o2.getClass().isAssignableFrom(o1.getClass()))) {
             throw new ComparisonException(
                 "Cannot compare " + getDataTypeName(o1) + " with " + getDataTypeName(o2));
           }