Make Metadata Stringer implementation deterministic

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

metadata/metadata.go

@@ -24,6 +24,7 @@ package metadata // import "google.golang.org/grpc/metadata"
 import (
 	"context"
 	"fmt"
+	"sort"
 	"strings"
 
 	"google.golang.org/grpc/internal"
@@ -91,15 +92,24 @@ func Pairs(kv ...string) MD {
 }
 
 // String implements the Stringer interface for pretty-printing a MD.
-// Ordering of the values is non-deterministic as it ranges over a map.
+// The value returned by this method is recommended only for diagnostic
+// purposes, and users are strongly discouraged from using this for anything
+// mission critical.
 func (md MD) String() string {
 	var sb strings.Builder
 	fmt.Fprintf(&sb, "MD{")
-	for k, v := range md {
-		if sb.Len() > 3 {
+	keys := make([]string, 0, len(md))
+	for k := range md {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	first := true
+	for _, k := range keys {
+		if !first {
 			fmt.Fprintf(&sb, ", ")
 		}
-		fmt.Fprintf(&sb, "%s=[%s]", k, strings.Join(v, ", "))
+		fmt.Fprintf(&sb, "%s=[%s]", k, strings.Join(md[k], ", "))
+		first = false
 	}
 	fmt.Fprintf(&sb, "}")
 	return sb.String()

metadata/metadata_test.go

@@ -22,7 +22,6 @@ import (
 	"context"
 	"reflect"
 	"strconv"
-	"strings"
 	"testing"
 	"time"
 
@@ -342,19 +341,19 @@ func (s) TestAppendToOutgoingContext_FromKVSlice(t *testing.T) {
 func TestStringerMD(t *testing.T) {
 	for _, test := range []struct {
 		md   MD
-		want []string
+		want string
 	}{
-		{MD{}, []string{"MD{}"}},
-		{MD{"k1": []string{}}, []string{"MD{k1=[]}"}},
-		{MD{"k1": []string{"v1", "v2"}}, []string{"MD{k1=[v1, v2]}"}},
-		{MD{"k1": []string{"v1"}}, []string{"MD{k1=[v1]}"}},
-		{MD{"k1": []string{"v1", "v2"}, "k2": []string{}, "k3": []string{"1", "2", "3"}}, []string{"MD{", "k1=[v1, v2]", "k2=[]", "k3=[1, 2, 3]", "}"}},
+		{MD{}, "MD{}"},
+		{MD{"k1": []string{}}, "MD{k1=[]}"},
+		{MD{"k1": []string{"v1", "v2"}}, "MD{k1=[v1, v2]}"},
+		{MD{"k1": []string{"v1"}}, "MD{k1=[v1]}"},
+		{MD{"k1": []string{"v1", "v2"}, "k2": []string{}, "k3": []string{"1", "2", "3"}}, "MD{k1=[v1, v2], k2=[], k3=[1, 2, 3]}"},
+		{MD{"k2": []string{}, "k3": []string{"1", "2", "3"}, "k1": []string{"v1", "v2"}}, "MD{k1=[v1, v2], k2=[], k3=[1, 2, 3]}"},
+		{MD{"k3": []string{"1", "2", "3"}, "k2": []string{}, "k1": []string{"v1", "v2"}}, "MD{k1=[v1, v2], k2=[], k3=[1, 2, 3]}"},
 	} {
 		got := test.md.String()
-		for _, want := range test.want {
-			if !strings.Contains(got, want) {
-				t.Fatalf("Metadata string %q is missing %q", got, want)
-			}
+		if got != test.want {
+			t.Fatalf("Metadata string %q should be %q", got, test.want)
 		}
 	}
 }