feat: support only updating existing BUILD files

Close bazel-contrib#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"
@@ -70,7 +83,7 @@ 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) {
@@ -82,6 +95,8 @@ 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

@@ -140,7 +140,7 @@ func Walk(c *config.Config, cexts []config.Configurer, dirs []string, mode Mode,
 //
 // Traversal may skip subtrees or files based on the config.Config exclude/ignore/follow options
 // as well as the UpdateFilter callbacks.
-func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[string]bool, updateRels *UpdateFilter, trie *pathTrie, wf WalkFunc, rel string, updateParent bool) {
+func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[string]bool, updateRels *UpdateFilter, trie *pathTrie, wf WalkFunc, rel string, updateParent bool) ([]string, bool) {
 	haveError := false
 
 	// Absolute path to the directory being visited
@@ -157,11 +157,16 @@ func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[stri
 		haveError = true
 	}
 
-	configure(cexts, knownDirectives, c, rel, f)
-	wc := getWalkConfig(c)
+	collectionOnly := f == nil && getWalkConfig(c).updateOnly
+
+	// Configure the current directory if not only collecting files
+	if !collectionOnly {
+		configure(cexts, knownDirectives, c, rel, f)
+	}
 
+	wc := getWalkConfig(c)
 	if wc.isExcluded(rel) {
-		return
+		return nil, false
 	}
 
 	// Filter and collect files
@@ -191,16 +196,28 @@ func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[stri
 			subdirs = append(subdirs, base)
 
 			if updateRels.shouldVisit(entRel, shouldUpdate) {
-				visit(c.Clone(), cexts, knownDirectives, updateRels, t, wf, entRel, shouldUpdate)
+				subFiles, shouldMerge := visit(c.Clone(), cexts, knownDirectives, updateRels, t, wf, entRel, shouldUpdate)
+				if shouldMerge {
+					for _, f := range subFiles {
+						regularFiles = append(regularFiles, path.Join(base, f))
+					}
+				} else {
+					subdirs = append(subdirs, base)
+				}
 			}
 		}
 	}
 
+	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