errors: optimize errors.Join for single unwrappable errors

Change-Id: I10bbb782ca7234cda8c82353f2255eec5be588c9
GitHub-Last-Rev: e5ad8fdb802e56bb36c41b4982ed27c1e0809af8
GitHub-Pull-Request: golang/go#70770
Reviewed-on: https://go-review.googlesource.com/c/go/+/635115
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/errors/join.go

@@ -26,6 +26,18 @@ func Join(errs ...error) error {
 	if n == 0 {
 		return nil
 	}
+	if n == 1 {
+		for _, err := range errs {
+			if err != nil {
+				if _, ok := err.(interface {
+					Unwrap() []error
+				}); ok {
+					return err
+				}
+			}
+		}
+	}
+
 	e := &joinError{
 		errs: make([]error, 0, n),
 	}

src/errors/join_test.go

@@ -70,3 +70,37 @@ func TestJoinErrorMethod(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkJoin(b *testing.B) {
+	for _, bb := range []struct {
+		name string
+		errs []error
+	}{
+		{
+			name: "no error",
+		},
+		{
+			name: "single non-nil error",
+			errs: []error{errors.New("err")},
+		},
+		{
+			name: "multiple errors",
+			errs: []error{errors.New("err"), errors.New("newerr"), errors.New("newerr2")},
+		},
+		{
+			name: "unwrappable single error",
+			errs: []error{errors.Join(errors.New("err"))},
+		},
+		{
+			name: "nil first error",
+			errs: []error{nil, errors.New("newerr")},
+		},
+	} {
+		b.Run(bb.name, func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				_ = errors.Join(bb.errs...)
+			}
+		})
+	}
+}