feat: support only updating existing BUILD files

Close bazelbuild#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

@@ -132,7 +132,12 @@ func Walk(c *config.Config, cexts []config.Configurer, dirs []string, mode Mode,
  visit(c, cexts, knownDirectives, updateRels, trie, wf, "", false)
 }
 
-func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[string]bool, updateRels *UpdateFilter, trie *pathTrie, wf WalkFunc, 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, knownDirectives map[string]bool, updateRels *UpdateFilter, trie *pathTrie, wf WalkFunc, rel string, updateParent bool) ([]string, bool) {
  haveError := false
 
  ents := make([]fs.DirEntry, 0, len(trie.children))
@@ -158,14 +163,22 @@ func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[stri
  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()
  entRel := path.Join(rel, base)
@@ -177,24 +190,39 @@ func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[stri
  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, knownDirectives, updateRels, trie.children[sub], wf, subRel, shouldUpdate)
+ subFiles, shouldMerge := visit(c, cexts, knownDirectives, updateRels, trie.children[sub], wf, 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