use buffers for pathJoin, to re-use buffers. (#17960)

```
benchmark                        old ns/op     new ns/op     delta
BenchmarkPathJoin/PathJoin-8     79.6          55.3          -30.53%

benchmark                        old allocs     new allocs     delta
BenchmarkPathJoin/PathJoin-8     2              1              -50.00%

benchmark                        old bytes     new bytes     delta
BenchmarkPathJoin/PathJoin-8     48            24            -50.00%
```

cmd/bucket-handlers.go

@@ -1290,7 +1290,10 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 		// instead of "copying" from source, we need the stream to be seekable
 		// to ensure that we can make fan-out calls concurrently.
 		buf := bytebufferpool.Get()
-		defer bytebufferpool.Put(buf)
+		defer func() {
+			buf.Reset()
+			bytebufferpool.Put(buf)
+		}()
 
 		md5w := md5.New()
 

cmd/metacache-walk.go

@@ -18,7 +18,6 @@
 package cmd
 
 import (
-	"bytes"
 	"context"
 	"fmt"
 	"io"
@@ -32,6 +31,7 @@ import (
 	xhttp "github.com/minio/minio/internal/http"
 	xioutil "github.com/minio/minio/internal/ioutil"
 	"github.com/minio/minio/internal/logger"
+	"github.com/valyala/bytebufferpool"
 )
 
 // WalkDirOptions provides options for WalkDir operations.
@@ -144,7 +144,12 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 
 	scanDir = func(current string) error {
 		// Skip forward, if requested...
-		var sb bytes.Buffer
+		sb := bytebufferpool.Get()
+		defer func() {
+			sb.Reset()
+			bytebufferpool.Put(sb)
+		}()
+
 		forward := ""
 		if len(opts.ForwardTo) > 0 && strings.HasPrefix(opts.ForwardTo, current) {
 			forward = strings.TrimPrefix(opts.ForwardTo, current)
@@ -228,7 +233,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 				if s.walkReadMu != nil {
 					s.walkReadMu.Lock()
 				}
-				meta.metadata, err = s.readMetadata(ctx, pathJoinBuf(&sb, volumeDir, current, entry))
+				meta.metadata, err = s.readMetadata(ctx, pathJoinBuf(sb, volumeDir, current, entry))
 				if s.walkReadMu != nil {
 					s.walkReadMu.Unlock()
 				}
@@ -244,7 +249,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 				}
 				meta.name = strings.TrimSuffix(entry, xlStorageFormatFile)
 				meta.name = strings.TrimSuffix(meta.name, SlashSeparator)
-				meta.name = pathJoinBuf(&sb, current, meta.name)
+				meta.name = pathJoinBuf(sb, current, meta.name)
 				meta.name = decodeDirObject(meta.name)
 				if err := send(meta); err != nil {
 					return err
@@ -254,7 +259,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 			// Check legacy.
 			if HasSuffix(entry, xlStorageFormatFileV1) && legacy {
 				var meta metaCacheEntry
-				meta.metadata, err = xioutil.ReadFile(pathJoinBuf(&sb, volumeDir, current, entry))
+				meta.metadata, err = xioutil.ReadFile(pathJoinBuf(sb, volumeDir, current, entry))
 				diskHealthCheckOK(ctx, err)
 				if err != nil {
 					if !IsErrIgnored(err, io.EOF, io.ErrUnexpectedEOF) {
@@ -264,7 +269,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 				}
 				meta.name = strings.TrimSuffix(entry, xlStorageFormatFileV1)
 				meta.name = strings.TrimSuffix(meta.name, SlashSeparator)
-				meta.name = pathJoinBuf(&sb, current, meta.name)
+				meta.name = pathJoinBuf(sb, current, meta.name)
 				if err := send(meta); err != nil {
 					return err
 				}
@@ -297,7 +302,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 			if contextCanceled(ctx) {
 				return ctx.Err()
 			}
-			meta := metaCacheEntry{name: pathJoinBuf(&sb, current, entry)}
+			meta := metaCacheEntry{name: pathJoinBuf(sb, current, entry)}
 
 			// If directory entry on stack before this, pop it now.
 			for len(dirStack) > 0 && dirStack[len(dirStack)-1] < meta.name {
@@ -323,7 +328,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 			if s.walkReadMu != nil {
 				s.walkReadMu.Lock()
 			}
-			meta.metadata, err = s.readMetadata(ctx, pathJoinBuf(&sb, volumeDir, meta.name, xlStorageFormatFile))
+			meta.metadata, err = s.readMetadata(ctx, pathJoinBuf(sb, volumeDir, meta.name, xlStorageFormatFile))
 			if s.walkReadMu != nil {
 				s.walkReadMu.Unlock()
 			}
@@ -339,7 +344,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 				}
 			case osIsNotExist(err), isSysErrIsDir(err):
 				if legacy {
-					meta.metadata, err = xioutil.ReadFile(pathJoinBuf(&sb, volumeDir, meta.name, xlStorageFormatFileV1))
+					meta.metadata, err = xioutil.ReadFile(pathJoinBuf(sb, volumeDir, meta.name, xlStorageFormatFileV1))
 					diskHealthCheckOK(ctx, err)
 					if err == nil {
 						// It was an object
@@ -353,7 +358,7 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 				// NOT an object, append to stack (with slash)
 				// If dirObject, but no metadata (which is unexpected) we skip it.
 				if !isDirObj {
-					if !isDirEmpty(pathJoinBuf(&sb, volumeDir, meta.name)) {
+					if !isDirEmpty(pathJoinBuf(sb, volumeDir, meta.name)) {
 						dirStack = append(dirStack, meta.name+slashSeparator)
 					}
 				}

cmd/object-api-utils.go

@@ -49,6 +49,7 @@ import (
 	"github.com/minio/minio/internal/logger"
 	"github.com/minio/pkg/trie"
 	"github.com/minio/pkg/wildcard"
+	"github.com/valyala/bytebufferpool"
 	"golang.org/x/exp/slices"
 )
 
@@ -233,25 +234,25 @@ func pathsJoinPrefix(prefix string, elem ...string) (paths []string) {
 
 // pathJoin - like path.Join() but retains trailing SlashSeparator of the last element
 func pathJoin(elem ...string) string {
-	trailingSlash := ""
+	sb := bytebufferpool.Get()
-	if len(elem) > 0 {
+	defer func() {
-		if hasSuffixByte(elem[len(elem)-1], SlashSeparatorChar) {
+		sb.Reset()
-			trailingSlash = SlashSeparator
+		bytebufferpool.Put(sb)
-		}
+	}()
-	}
+
-	return path.Join(elem...) + trailingSlash
+	return pathJoinBuf(sb, elem...)
 }
 
 // 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 {
+func pathJoinBuf(dst *bytebufferpool.ByteBuffer, 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.WriteByte(SlashSeparatorChar)
 			}
 			dst.WriteString(e)
 			added += len(e)
@@ -266,7 +267,7 @@ func pathJoinBuf(dst *bytes.Buffer, elem ...string) string {
 		return s
 	}
 	if trailingSlash {
-		dst.WriteRune(SlashSeparatorChar)
+		dst.WriteByte(SlashSeparatorChar)
 	}
 	return dst.String()
 }

cmd/object-api-utils_test.go

@@ -24,6 +24,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"path"
 	"reflect"
 	"runtime"
 	"strconv"
@@ -36,6 +37,38 @@ import (
 	"github.com/minio/pkg/trie"
 )
 
+func pathJoinOld(elem ...string) string {
+	trailingSlash := ""
+	if len(elem) > 0 {
+		if hasSuffixByte(elem[len(elem)-1], SlashSeparatorChar) {
+			trailingSlash = SlashSeparator
+		}
+	}
+	return path.Join(elem...) + trailingSlash
+}
+
+func BenchmarkPathJoinOld(b *testing.B) {
+	b.Run("PathJoin", func(b *testing.B) {
+		b.ResetTimer()
+		b.ReportAllocs()
+
+		for i := 0; i < b.N; i++ {
+			pathJoinOld("volume", "path/path/path")
+		}
+	})
+}
+
+func BenchmarkPathJoin(b *testing.B) {
+	b.Run("PathJoin", func(b *testing.B) {
+		b.ResetTimer()
+		b.ReportAllocs()
+
+		for i := 0; i < b.N; i++ {
+			pathJoin("volume", "path/path/path")
+		}
+	})
+}
+
 // Wrapper
 func TestPathTraversalExploit(t *testing.T) {
 	if runtime.GOOS != globalWindowsOSName {