fix: return owned update download buffers

Replace bytebufferpool-backed return values with bytes.Buffer storage so
the downloaded and compressed slices remain valid after downloadBinary
returns. Close the zstd encoder on copy failure and propagate final close
errors.

Add round-trip and pool-reuse regression coverage for both returned
buffers.

Co-authored-by: ChatGPT <noreply@openai.com>
Co-authored-by: Claude <noreply@anthropic.com>

cmd/update.go

@@ -19,6 +19,7 @@ package cmd
 
 import (
 	"bufio"
+	"bytes"
 	"crypto"
 	"crypto/tls"
 	"encoding/hex"
@@ -42,7 +43,6 @@ import (
 	xnet "github.com/minio/pkg/v3/net"
 	"github.com/minio/selfupdate"
 	gopsutilcpu "github.com/shirou/gopsutil/v3/cpu"
-	"github.com/valyala/bytebufferpool"
 )
 
 const (
@@ -532,26 +532,21 @@ func downloadBinary(u *url.URL, mode string) (binCompressed []byte, bin []byte,
 	}
 	defer xhttp.DrainBody(reader)
 
-	b := bytebufferpool.Get()
-	bc := bytebufferpool.Get()
-	defer func() {
-		b.Reset()
-		bc.Reset()
+	var b, bc bytes.Buffer
 
-		bytebufferpool.Put(b)
-		bytebufferpool.Put(bc)
-	}()
-
-	w, err := zstd.NewWriter(bc)
+	w, err := zstd.NewWriter(&bc)
 	if err != nil {
 		return nil, nil, err
 	}
 
-	if _, err = io.Copy(w, io.TeeReader(reader, b)); err != nil {
+	if _, err = io.Copy(w, io.TeeReader(reader, &b)); err != nil {
+		_ = w.Close()
 		return nil, nil, err
 	}
 
-	w.Close()
+	if err = w.Close(); err != nil {
+		return nil, nil, err
+	}
 	return bc.Bytes(), b.Bytes(), nil
 }