Prevent nested set flattening in ActionSpawn if there are no input fi…

…lesets.

This is always the case in Bazel. The nested set structure can be exploited to perform optimizations, notably by --experimental_remote_merkle_tree_cache.

PiperOrigin-RevId: 585657594
Change-Id: I83042b5c40b3b9e6ce511a230a0cbd0677dda5b3

src/main/java/com/google/devtools/build/lib/analysis/actions/SpawnAction.java

@@ -520,14 +520,7 @@ private ActionSpawn(
           parent.getRunfilesSupplier(),
           parent,
           parent.resourceSetOrBuilder);
-      NestedSetBuilder<ActionInput> inputsBuilder = NestedSetBuilder.stableOrder();
-      for (Artifact input : inputs.toList()) {
-        if (!input.isFileset()) {
-          inputsBuilder.add(input);
-        }
-      }
-      inputsBuilder.addAll(additionalInputs);
-      this.inputs = inputsBuilder.build();
+      this.inputs = getNonFilesetInputs(inputs).addAll(additionalInputs).build();
       this.filesetMappings = filesetMappings;
       this.pathMapper = pathMapper;
 
@@ -542,6 +535,27 @@ private ActionSpawn(
       this.reportOutputs = reportOutputs;
     }
 
+    /** Returns a {@link NestedSetBuilder} containing only the non-fileset inputs. */
+    private static NestedSetBuilder<ActionInput> getNonFilesetInputs(NestedSet<Artifact> inputs) {
+      NestedSetBuilder<ActionInput> builder = NestedSetBuilder.stableOrder();
+      // TODO(tjgq): Investigate whether we can avoid flattening when filesetMappings is empty.
+      // This requires auditing getSpawnForExtraAction(), which doesn't appear to propagate the
+      // filesetMappings from the shadowed action.
+      boolean hasFilesets = false;
+      for (Artifact input : inputs.toList()) {
+        if (!input.isFileset()) {
+          builder.add(input);
+        } else {
+          hasFilesets = true;
+        }
+      }
+      // If possible, keep the original nested set. This aids callers that exploit the nested set
+      // structure to perform optimizations (see SpawnInputExpander#walkInputs and its callers).
+      return hasFilesets
+          ? builder
+          : NestedSetBuilder.<ActionInput>stableOrder().addTransitive(inputs);
+    }
+
     @Override
     public PathMapper getPathMapper() {
       return pathMapper;