singleflight: add panicError.Unwrap method

Currently when singleflight recovers from a panic, it wraps it with the private
error type panicError. This change adds an `Unwrap` method to panicError to
allow wrapped errors to be returned.

Updates golang/go#62511

Change-Id: Ia510ad7d5881207ef71f9eb89c1766835af19b6b
Reviewed-on: https://go-review.googlesource.com/c/sync/+/526171
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

singleflight/singleflight.go

@@ -31,6 +31,15 @@ func (p *panicError) Error() string {
 	return fmt.Sprintf("%v\n\n%s", p.value, p.stack)
 }
 
+func (p *panicError) Unwrap() error {
+	err, ok := p.value.(error)
+	if !ok {
+		return nil
+	}
+
+	return err
+}
+
 func newPanicError(v interface{}) error {
 	stack := debug.Stack()
 

singleflight/singleflight_test.go

@@ -19,6 +19,69 @@ import (
 	"time"
 )
 
+type errValue struct{}
+
+func (err *errValue) Error() string {
+	return "error value"
+}
+
+func TestPanicErrorUnwrap(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name             string
+		panicValue       interface{}
+		wrappedErrorType bool
+	}{
+		{
+			name:             "panicError wraps non-error type",
+			panicValue:       &panicError{value: "string value"},
+			wrappedErrorType: false,
+		},
+		{
+			name:             "panicError wraps error type",
+			panicValue:       &panicError{value: new(errValue)},
+			wrappedErrorType: false,
+		},
+	}
+
+	for _, tc := range testCases {
+		tc := tc
+
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			var recovered interface{}
+
+			group := &Group{}
+
+			func() {
+				defer func() {
+					recovered = recover()
+					t.Logf("after panic(%#v) in group.Do, recovered %#v", tc.panicValue, recovered)
+				}()
+
+				_, _, _ = group.Do(tc.name, func() (interface{}, error) {
+					panic(tc.panicValue)
+				})
+			}()
+
+			if recovered == nil {
+				t.Fatal("expected a non-nil panic value")
+			}
+
+			err, ok := recovered.(error)
+			if !ok {
+				t.Fatalf("recovered non-error type: %T", recovered)
+			}
+
+			if !errors.Is(err, new(errValue)) && tc.wrappedErrorType {
+				t.Errorf("unexpected wrapped error type %T; want %T", err, new(errValue))
+			}
+		})
+	}
+}
+
 func TestDo(t *testing.T) {
 	var g Group
 	v, err, _ := g.Do("key", func() (interface{}, error) {