feat: support only updating existing BUILD files

Close #1832

walk/config.go

@@ -33,14 +33,27 @@ import (
 	gzflag "github.com/bazelbuild/bazel-gazelle/flag"
 )
 
+// GenerationModeType represents one of the generation modes.
+type GenerationModeType string
+
+// Generation modes
+const (
+	// Update: update and maintain existing BUILD files
+	GenerationModeUpdate GenerationModeType = "update"
+
+	// Create: create new and update existing BUILD files
+	GenerationModeCreate GenerationModeType = "create"
+)
+
 // TODO(#472): store location information to validate each exclude. They
 // may be set in one directory and used in another. Excludes work on
 // declared generated files, so we can't just stat.
 
 type walkConfig struct {
-	excludes []string
-	ignore   bool
-	follow   []string
+	updateOnly bool
+	excludes   []string
+	ignore     bool
+	follow     []string
 }
 
 const walkName = "_walk"
@@ -73,7 +86,7 @@ func (*Configurer) RegisterFlags(fs *flag.FlagSet, cmd string, c *config.Config)
 func (*Configurer) CheckFlags(fs *flag.FlagSet, c *config.Config) error { return nil }
 
 func (*Configurer) KnownDirectives() []string {
-	return []string{"exclude", "follow", "ignore"}
+	return []string{"generation_mode", "exclude", "follow", "ignore"}
 }
 
 func (cr *Configurer) Configure(c *config.Config, rel string, f *rule.File) {
@@ -85,6 +98,8 @@ func (cr *Configurer) Configure(c *config.Config, rel string, f *rule.File) {
 	if f != nil {
 		for _, d := range f.Directives {
 			switch d.Key {
+			case "generation_mode":
+				wcCopy.updateOnly = GenerationModeType(strings.TrimSpace(d.Value)) == GenerationModeUpdate
 			case "exclude":
 				if err := checkPathMatchPattern(path.Join(rel, d.Value)); err != nil {
 					log.Printf("the exclusion pattern is not valid %q: %s", path.Join(rel, d.Value), err)

walk/walk.go

@@ -125,9 +125,14 @@ func Walk(c *config.Config, cexts []config.Configurer, dirs []string, mode Mode,
 	visit(c, cexts, isBazelIgnored, knownDirectives, updateRels, wf, c.RepoRoot, "", false)
 }
 
-func visit(c *config.Config, cexts []config.Configurer, isBazelIgnored isIgnoredFunc, knownDirectives map[string]bool, updateRels *UpdateFilter, wf WalkFunc, dir, rel string, updateParent bool) {
+// Read and traverse the 'rel' directory, calling the WalkFunc for each directory
+// while respecting the `isBazelIgnored` and exclusion directives.
+//
+// If the given directory is not configured to produce a BUILD the collected files
+// will be returned and the WalkFunc will not be called.
+func visit(c *config.Config, cexts []config.Configurer, isBazelIgnored isIgnoredFunc, knownDirectives map[string]bool, updateRels *UpdateFilter, wf WalkFunc, dir, rel string, updateParent bool) ([]string, bool) {
 	if isBazelIgnored(rel) {
-		return
+		return nil, false
 	}
 
 	haveError := false
@@ -138,7 +143,7 @@ func visit(c *config.Config, cexts []config.Configurer, isBazelIgnored isIgnored
 	ents, err := os.ReadDir(dir)
 	if err != nil {
 		log.Print(err)
-		return
+		return nil, false
 	}
 
 	f, err := loadBuildFile(c, rel, dir, ents)
@@ -152,14 +157,22 @@ func visit(c *config.Config, cexts []config.Configurer, isBazelIgnored isIgnored
 		haveError = true
 	}
 
-	c = configure(cexts, knownDirectives, c, rel, f)
 	wc := getWalkConfig(c)
+	collectionOnly := f == nil && wc.updateOnly
+
+	// Configure the current directory if not only collecting files
+	if !collectionOnly {
+		c = configure(cexts, knownDirectives, c, rel, f)
+		wc = getWalkConfig(c)
 
-	if wc.isExcluded(rel, ".") {
-		return
+		// If the config of this directory has excluded itself
+		if wc.isExcluded(rel, ".") {
+			return nil, false
+		}
 	}
 
-	var subdirs, regularFiles []string
+	// Divide the directory entries while filtering out ignored/excluded.
+	var dirs, regularFiles []string
 	for _, ent := range ents {
 		base := ent.Name()
 		if isBazelIgnored(path.Join(rel, base)) || wc.isExcluded(rel, base) {
@@ -170,24 +183,39 @@ func visit(c *config.Config, cexts []config.Configurer, isBazelIgnored isIgnored
 		case ent == nil:
 			continue
 		case ent.IsDir():
-			subdirs = append(subdirs, base)
+			dirs = append(dirs, base)
 		default:
 			regularFiles = append(regularFiles, base)
 		}
 	}
 
+	var subdirs []string
+
+	// Visit subdirectories dividing them into `regularFiles` and `subdirs``
 	shouldUpdate := updateRels.shouldUpdate(rel, updateParent)
-	for _, sub := range subdirs {
+	for _, sub := range dirs {
 		if subRel := path.Join(rel, sub); updateRels.shouldVisit(subRel, shouldUpdate) {
-			visit(c, cexts, isBazelIgnored, knownDirectives, updateRels, wf, filepath.Join(dir, sub), subRel, shouldUpdate)
+			subFiles, shouldMerge := visit(c, cexts, isBazelIgnored, knownDirectives, updateRels, wf, filepath.Join(dir, sub), subRel, shouldUpdate)
+			if shouldMerge {
+				for _, f := range subFiles {
+					regularFiles = append(regularFiles, path.Join(sub, f))
+				}
+			} else {
+				subdirs = append(subdirs, sub)
+			}
 		}
 	}
 
+	if collectionOnly {
+		return regularFiles, true
+	}
+
 	update := !haveError && !wc.ignore && shouldUpdate
 	if updateRels.shouldCall(rel, updateParent) {
 		genFiles := findGenFiles(wc, f)
 		wf(dir, rel, c, update, f, subdirs, regularFiles, genFiles)
 	}
+	return nil, false
 }
 
 // An UpdateFilter tracks which directories need to be updated