refactor: reduce path.join while walking repo (#1913)

**What type of PR is this?**

refactor / perf

**What package or component does this PR mostly affect?**

> all

**What does this PR do? Why is it needed?**

Reduce duplicate `path.Join` invocations. Remove duplicate
`isBazelIgnored` invocations (it is not needed on the root dir, it is
invoked before recursing).

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

walk/config.go

@@ -49,12 +49,12 @@ func getWalkConfig(c *config.Config) *walkConfig {
 	return c.Exts[walkName].(*walkConfig)
 }
 
-func (wc *walkConfig) isExcluded(rel, base string) bool {
-	return matchAnyGlob(wc.excludes, path.Join(rel, base))
+func (wc *walkConfig) isExcluded(p string) bool {
+	return matchAnyGlob(wc.excludes, p)
 }
 
-func (wc *walkConfig) shouldFollow(rel, base string) bool {
-	return matchAnyGlob(wc.follow, path.Join(rel, base))
+func (wc *walkConfig) shouldFollow(p string) bool {
+	return matchAnyGlob(wc.follow, p)
 }
 
 var _ config.Configurer = (*Configurer)(nil)

walk/walk.go

@@ -129,10 +129,10 @@ func Walk(c *config.Config, cexts []config.Configurer, dirs []string, mode Mode,
 		log.Fatalf("error walking the file system: %v\n", err)
 	}
 
-	visit(c, cexts, knownDirectives, updateRels, trie, wf, c.RepoRoot, "", false)
+	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, dir, 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) {
 	haveError := false
 
 	ents := make([]fs.DirEntry, 0, len(trie.children))
@@ -144,6 +144,9 @@ func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[stri
 		return ents[i].Name() < ents[j].Name()
 	})
 
+	// Absolute path to the directory being visited
+	dir := filepath.Join(c.RepoRoot, rel)
+
 	f, err := loadBuildFile(c, rel, dir, ents)
 	if err != nil {
 		log.Print(err)
@@ -158,17 +161,18 @@ func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[stri
 	c = configure(cexts, knownDirectives, c, rel, f)
 	wc := getWalkConfig(c)
 
-	if wc.isExcluded(rel, ".") {
+	if wc.isExcluded(rel) {
 		return
 	}
 
 	var subdirs, regularFiles []string
 	for _, ent := range ents {
 		base := ent.Name()
-		if wc.isExcluded(rel, base) {
+		entRel := path.Join(rel, base)
+		if wc.isExcluded(entRel) {
 			continue
 		}
-		ent := resolveFileInfo(wc, dir, rel, ent)
+		ent := resolveFileInfo(wc, dir, entRel, ent)
 		switch {
 		case ent == nil:
 			continue
@@ -182,7 +186,7 @@ func visit(c *config.Config, cexts []config.Configurer, knownDirectives map[stri
 	shouldUpdate := updateRels.shouldUpdate(rel, updateParent)
 	for _, sub := range subdirs {
 		if subRel := path.Join(rel, sub); updateRels.shouldVisit(subRel, shouldUpdate) {
-			visit(c, cexts, knownDirectives, updateRels, trie.children[sub], wf, path.Join(dir, sub), subRel, shouldUpdate)
+			visit(c, cexts, knownDirectives, updateRels, trie.children[sub], wf, subRel, shouldUpdate)
 		}
 	}
 
@@ -332,27 +336,23 @@ func findGenFiles(wc *walkConfig, f *rule.File) []string {
 
 	var genFiles []string
 	for _, s := range strs {
-		if !wc.isExcluded(f.Pkg, s) {
+		if !wc.isExcluded(path.Join(f.Pkg, s)) {
 			genFiles = append(genFiles, s)
 		}
 	}
 	return genFiles
 }
 
 func resolveFileInfo(wc *walkConfig, dir, rel string, ent fs.DirEntry) fs.DirEntry {
-	base := ent.Name()
-	if base == "" {
-		return nil
-	}
 	if ent.Type()&os.ModeSymlink == 0 {
 		// Not a symlink, use the original FileInfo.
 		return ent
 	}
-	if !wc.shouldFollow(rel, base) {
+	if !wc.shouldFollow(rel) {
 		// A symlink, but not one we should follow.
 		return nil
 	}
-	fi, err := os.Stat(path.Join(dir, base))
+	fi, err := os.Stat(path.Join(dir, ent.Name()))
 	if err != nil {
 		// A symlink, but not one we could resolve.
 		return nil