prune out empty bazel depsets

Instead of using a sentinel file that was created under bazel_workspace
prune them out. The sentinel file was being created under bazel_workspace,
which would be deleted and thus rendered useless from the perspective of
not retriggering a build.

Test: `touch Android.bp` then `NINJA_ARGS="-d explain" m --bazel-mode-staging adbd_test`
Bug: 265155778
Change-Id: If5d8f0ea7f3b8828fda0646faafd1a621f0cb27c
diff --git a/bazel/aquery.go b/bazel/aquery.go
index bc823b3..80cf70a 100644
--- a/bazel/aquery.go
+++ b/bazel/aquery.go
@@ -118,12 +118,11 @@
 // A helper type for aquery processing which facilitates retrieval of path IDs from their
 // less readable Bazel structures (depset and path fragment).
 type aqueryArtifactHandler struct {
-	// Switches to true if any depset contains only `bazelToolsDependencySentinel`
-	bazelToolsDependencySentinelNeeded bool
 	// Maps depset id to AqueryDepset, a representation of depset which is
 	// post-processed for middleman artifact handling, unhandled artifact
 	// dropping, content hashing, etc.
 	depsetIdToAqueryDepset map[depsetId]AqueryDepset
+	emptyDepsetIds         map[depsetId]struct{}
 	// Maps content hash to AqueryDepset.
 	depsetHashToAqueryDepset map[string]AqueryDepset
 
@@ -145,9 +144,6 @@
 // The file name of py3wrapper.sh, which is used by py_binary targets.
 const py3wrapperFileName = "/py3wrapper.sh"
 
-// A file to be put into depsets that are otherwise empty
-const bazelToolsDependencySentinel = "BAZEL_TOOLS_DEPENDENCY_SENTINEL"
-
 func indexBy[K comparable, V any](values []V, keyFn func(v V) K) map[K]V {
 	m := map[K]V{}
 	for _, v := range values {
@@ -192,6 +188,7 @@
 		depsetIdToAqueryDepset:         map[depsetId]AqueryDepset{},
 		depsetHashToAqueryDepset:       map[string]AqueryDepset{},
 		depsetHashToArtifactPathsCache: map[string][]string{},
+		emptyDepsetIds:                 make(map[depsetId]struct{}, 0),
 		artifactIdToPath:               artifactIdToPath,
 	}
 
@@ -208,16 +205,16 @@
 
 // Ensures that the handler's depsetIdToAqueryDepset map contains an entry for the given
 // depset.
-func (a *aqueryArtifactHandler) populateDepsetMaps(depset depSetOfFiles, middlemanIdToDepsetIds map[artifactId][]depsetId, depsetIdToDepset map[depsetId]depSetOfFiles) (AqueryDepset, error) {
+func (a *aqueryArtifactHandler) populateDepsetMaps(depset depSetOfFiles, middlemanIdToDepsetIds map[artifactId][]depsetId, depsetIdToDepset map[depsetId]depSetOfFiles) (*AqueryDepset, error) {
 	if aqueryDepset, containsDepset := a.depsetIdToAqueryDepset[depset.Id]; containsDepset {
-		return aqueryDepset, nil
+		return &aqueryDepset, nil
 	}
 	transitiveDepsetIds := depset.TransitiveDepSetIds
 	var directArtifactPaths []string
 	for _, artifactId := range depset.DirectArtifactIds {
 		path, pathExists := a.artifactIdToPath[artifactId]
 		if !pathExists {
-			return AqueryDepset{}, fmt.Errorf("undefined input artifactId %d", artifactId)
+			return nil, fmt.Errorf("undefined input artifactId %d", artifactId)
 		}
 		// Filter out any inputs which are universally dropped, and swap middleman
 		// artifacts with their corresponding depsets.
@@ -226,6 +223,7 @@
 			transitiveDepsetIds = append(transitiveDepsetIds, depsetsToUse...)
 		} else if strings.HasSuffix(path, py3wrapperFileName) ||
 			strings.HasPrefix(path, "../bazel_tools") {
+			continue
 			// Drop these artifacts.
 			// See go/python-binary-host-mixed-build for more details.
 			// 1) Drop py3wrapper.sh, just use python binary, the launcher script generated by the
@@ -241,20 +239,23 @@
 	for _, childDepsetId := range transitiveDepsetIds {
 		childDepset, exists := depsetIdToDepset[childDepsetId]
 		if !exists {
-			return AqueryDepset{}, fmt.Errorf("undefined input depsetId %d (referenced by depsetId %d)", childDepsetId, depset.Id)
+			if _, empty := a.emptyDepsetIds[childDepsetId]; empty {
+				continue
+			} else {
+				return nil, fmt.Errorf("undefined input depsetId %d (referenced by depsetId %d)", childDepsetId, depset.Id)
+			}
 		}
-		childAqueryDepset, err := a.populateDepsetMaps(childDepset, middlemanIdToDepsetIds, depsetIdToDepset)
-		if err != nil {
-			return AqueryDepset{}, err
+		if childAqueryDepset, err := a.populateDepsetMaps(childDepset, middlemanIdToDepsetIds, depsetIdToDepset); err != nil {
+			return nil, err
+		} else if childAqueryDepset == nil {
+			continue
+		} else {
+			childDepsetHashes = append(childDepsetHashes, childAqueryDepset.ContentHash)
 		}
-		childDepsetHashes = append(childDepsetHashes, childAqueryDepset.ContentHash)
 	}
 	if len(directArtifactPaths) == 0 && len(childDepsetHashes) == 0 {
-		// We could omit this depset altogether but that requires cleanup on
-		// transitive dependents.
-		// As a simpler alternative, we use this sentinel file as a dependency.
-		directArtifactPaths = append(directArtifactPaths, bazelToolsDependencySentinel)
-		a.bazelToolsDependencySentinelNeeded = true
+		a.emptyDepsetIds[depset.Id] = struct{}{}
+		return nil, nil
 	}
 	aqueryDepset := AqueryDepset{
 		ContentHash:            depsetContentHash(directArtifactPaths, childDepsetHashes),
@@ -263,7 +264,7 @@
 	}
 	a.depsetIdToAqueryDepset[depset.Id] = aqueryDepset
 	a.depsetHashToAqueryDepset[aqueryDepset.ContentHash] = aqueryDepset
-	return aqueryDepset, nil
+	return &aqueryDepset, nil
 }
 
 // getInputPaths flattens the depsets of the given IDs and returns all transitive
@@ -392,14 +393,6 @@
 	}
 
 	var buildStatements []BuildStatement
-	if aqueryHandler.bazelToolsDependencySentinelNeeded {
-		buildStatements = append(buildStatements, BuildStatement{
-			Command:     fmt.Sprintf("touch '%s'", bazelToolsDependencySentinel),
-			OutputPaths: []string{bazelToolsDependencySentinel},
-			Mnemonic:    bazelToolsDependencySentinel,
-		})
-	}
-
 	for _, actionEntry := range aqueryResult.Actions {
 		if shouldSkipAction(actionEntry) {
 			continue
@@ -484,7 +477,9 @@
 	var hashes []string
 	for _, depsetId := range inputDepsetIds {
 		if aqueryDepset, exists := a.depsetIdToAqueryDepset[depsetId]; !exists {
-			return nil, fmt.Errorf("undefined input depsetId %d", depsetId)
+			if _, empty := a.emptyDepsetIds[depsetId]; !empty {
+				return nil, fmt.Errorf("undefined (not even empty) input depsetId %d", depsetId)
+			}
 		} else {
 			hashes = append(hashes, aqueryDepset.ContentHash)
 		}