fix(v2/callctx): fix SetHeader race by cloning header map

v2/callctx/callctx.go

@@ -74,9 +74,25 @@ func SetHeaders(ctx context.Context, keyvals ...string) context.Context {
 	h, ok := ctx.Value(headerKey).(map[string][]string)
 	if !ok {
 		h = make(map[string][]string)
+	} else {
+		h = cloneHeaders(h)
 	}
+
 	for i := 0; i < len(keyvals); i = i + 2 {
 		h[keyvals[i]] = append(h[keyvals[i]], keyvals[i+1])
 	}
 	return context.WithValue(ctx, headerKey, h)
 }
+
+// cloneHeaders makes a new key-value map while reusing the value slices.
+// As such, new values should be appended to the value slice, and modifying
+// indexed values is not thread safe.
+//
+// TODO: Replace this with maps.Clone when Go 1.21 is the minimum version.
+func cloneHeaders(h map[string][]string) map[string][]string {
+	c := make(map[string][]string, len(h))
+	for k, v := range h {
+		c[k] = v
+	}
+	return c
+}

v2/callctx/callctx_test.go

@@ -31,6 +31,7 @@ package callctx
 
 import (
 	"context"
+	"sync"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -77,3 +78,45 @@ func TestSetHeaders_panics(t *testing.T) {
 	ctx := context.Background()
 	SetHeaders(ctx, "1", "2", "3")
 }
+
+func TestSetHeaders_reuse(t *testing.T) {
+	c := SetHeaders(context.Background(), "key", "value1")
+	v1 := HeadersFromContext(c)
+	c = SetHeaders(c, "key", "value2")
+	v2 := HeadersFromContext(c)
+
+	if cmp.Diff(v2, v1) == "" {
+		t.Errorf("Second header set did not differ from first header set as expected")
+	}
+}
+
+func TestSetHeaders_race(t *testing.T) {
+	key := "key"
+	value := "value"
+	want := map[string][]string{
+		key: []string{value, value},
+	}
+
+	// Init the ctx so a value already exists to be "shared".
+	cctx := SetHeaders(context.Background(), key, value)
+
+	// Reusing the same cctx and adding to the same header key
+	// should *not* produce a race condition when run with -race.
+	var wg sync.WaitGroup
+	for i := 0; i < 3; i++ {
+		wg.Add(1)
+		go func(ctx context.Context) {
+			defer wg.Done()
+			c := SetHeaders(ctx, key, value)
+			h := HeadersFromContext(c)
+
+			// Additionally, if there was a race condition,
+			// we may see that one instance of these headers
+			// contains extra values.
+			if diff := cmp.Diff(h, want); diff != "" {
+				t.Errorf("got(-),want(+):\n%s", diff)
+			}
+		}(cctx)
+	}
+	wg.Wait()
+}