Reduce allocations in Walkdir (#17036)

cmd/object-api-utils.go

@@ -209,6 +209,9 @@ func checkObjectNameForLengthAndSlash(bucket, object string) error {
 // SlashSeparator - slash separator.
 const SlashSeparator = "/"
 
+// SlashSeparatorChar - slash separator.
+const SlashSeparatorChar = '/'
+
 // retainSlash - retains slash from a path.
 func retainSlash(s string) string {
 	if s == "" {
@@ -231,13 +234,102 @@ func pathsJoinPrefix(prefix string, elem ...string) (paths []string) {
 func pathJoin(elem ...string) string {
 	trailingSlash := ""
 	if len(elem) > 0 {
-		if HasSuffix(elem[len(elem)-1], SlashSeparator) {
+		if hasSuffixByte(elem[len(elem)-1], SlashSeparatorChar) {
 			trailingSlash = SlashSeparator
 		}
 	}
 	return path.Join(elem...) + trailingSlash
 }
 
+// pathJoinBuf - like path.Join() but retains trailing SlashSeparator of the last element.
+// Provide a string builder to reduce allocation.
+func pathJoinBuf(dst *bytes.Buffer, elem ...string) string {
+	trailingSlash := len(elem) > 0 && hasSuffixByte(elem[len(elem)-1], SlashSeparatorChar)
+	dst.Reset()
+	added := 0
+	for _, e := range elem {
+		if added > 0 || e != "" {
+			if added > 0 {
+				dst.WriteRune(SlashSeparatorChar)
+			}
+			dst.WriteString(e)
+			added += len(e)
+		}
+	}
+
+	if pathNeedsClean(dst.Bytes()) {
+		s := path.Clean(dst.String())
+		if trailingSlash {
+			return s + SlashSeparator
+		}
+		return s
+	}
+	if trailingSlash {
+		dst.WriteRune(SlashSeparatorChar)
+	}
+	return dst.String()
+}
+
+// hasSuffixByte returns true if the last byte of s is 'suffix'
+func hasSuffixByte(s string, suffix byte) bool {
+	return len(s) > 0 && s[len(s)-1] == suffix
+}
+
+// pathNeedsClean returns whether path.Clean may change the path.
+// Will detect all cases that will be cleaned,
+// but may produce false positives on non-trivial paths.
+func pathNeedsClean(path []byte) bool {
+	if len(path) == 0 {
+		return true
+	}
+
+	rooted := path[0] == '/'
+	n := len(path)
+
+	r, w := 0, 0
+	if rooted {
+		r, w = 1, 1
+	}
+
+	for r < n {
+		switch {
+		case path[r] > 127:
+			// Non ascii.
+			return true
+		case path[r] == '/':
+			// multiple / elements
+			return true
+		case path[r] == '.' && (r+1 == n || path[r+1] == '/'):
+			// . element - assume it has to be cleaned.
+			return true
+		case path[r] == '.' && path[r+1] == '.' && (r+2 == n || path[r+2] == '/'):
+			// .. element: remove to last / - assume it has to be cleaned.
+			return true
+		default:
+			// real path element.
+			// add slash if needed
+			if rooted && w != 1 || !rooted && w != 0 {
+				w++
+			}
+			// copy element
+			for ; r < n && path[r] != '/'; r++ {
+				w++
+			}
+			// allow one slash, not at end
+			if r < n-1 && path[r] == '/' {
+				r++
+			}
+		}
+	}
+
+	// Turn empty string into "."
+	if w == 0 {
+		return true
+	}
+
+	return false
+}
+
 // mustGetUUID - get a random UUID.
 func mustGetUUID() string {
 	u, err := uuid.NewRandom()