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_test.go

@@ -18,6 +18,7 @@
 package cmd
 
 import (
+	"bytes"
 	"encoding/hex"
 	"fmt"
 	"net/http"
@@ -28,8 +29,73 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/klauspost/compress/zstd"
+	"github.com/valyala/bytebufferpool"
 )
 
+func TestDownloadBinaryReturnsOwnedBuffers(t *testing.T) {
+	previousMaxProcs := runtime.GOMAXPROCS(1)
+	t.Cleanup(func() {
+		runtime.GOMAXPROCS(previousMaxProcs)
+	})
+
+	payload := []byte("minio update payload")
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write(payload)
+	}))
+	t.Cleanup(server.Close)
+
+	u, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	compressed, downloaded, err := downloadBinary(u, "server")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(downloaded, payload) {
+		t.Fatalf("downloaded binary is %q, want %q", downloaded, payload)
+	}
+	decoder, err := zstd.NewReader(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(decoder.Close)
+	decompressed, err := decoder.DecodeAll(compressed, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(decompressed, payload) {
+		t.Fatalf("decompressed binary is %q, want %q", decompressed, payload)
+	}
+	wantCompressed := bytes.Clone(compressed)
+	wantDownloaded := bytes.Clone(downloaded)
+
+	// Reuse buffers returned to bytebufferpool. downloadBinary's results must
+	// remain valid after the function returns, regardless of later pool users.
+	pooled := make([]*bytebufferpool.ByteBuffer, 8)
+	for i := range pooled {
+		pooled[i] = bytebufferpool.Get()
+		if cap(pooled[i].B) > 0 {
+			pooled[i].B = pooled[i].B[:cap(pooled[i].B)]
+			for j := range pooled[i].B {
+				pooled[i].B[j] = 0xa5
+			}
+		}
+	}
+	for _, b := range pooled {
+		bytebufferpool.Put(b)
+	}
+
+	if !bytes.Equal(compressed, wantCompressed) {
+		t.Fatal("compressed download aliases a buffer returned to bytebufferpool")
+	}
+	if !bytes.Equal(downloaded, wantDownloaded) {
+		t.Fatal("downloaded binary aliases a buffer returned to bytebufferpool")
+	}
+}
+
 func TestMinioVersionToReleaseTime(t *testing.T) {
 	testCases := []struct {
 		version    string