Fix StackTracedError logging

When running with `STEPDEBUG=1`, a response with a `StackTracedError`
would result in a nil pointer error. This commit fixes the check and
adds a test case.

api/log/log.go

@@ -39,7 +39,7 @@ func Error(rw http.ResponseWriter, err error) {
 	}
 
 	var st StackTracedError
-	if !errors.As(err, &st) {
+	if errors.As(err, &st) {
 		rl.WithFields(map[string]interface{}{
 			"stack-trace": fmt.Sprintf("%+v", st.StackTrace()),
 		})

api/log/log_test.go

@@ -6,11 +6,16 @@ import (
 	"net/http/httptest"
 	"reflect"
 	"testing"
+	"unsafe"
+
+	pkgerrors "github.com/pkg/errors"
 
 	"github.com/smallstep/certificates/logging"
 )
 
 func TestError(t *testing.T) {
+
+	t.Setenv("STEPDEBUG", "1") // force to end of `Error` function instead of early return
 	theError := errors.New("the error")
 
 	type args struct {
@@ -21,9 +26,10 @@ func TestError(t *testing.T) {
 		name       string
 		args       args
 		withFields bool
+		want       error
 	}{
-		{"normalLogger", args{httptest.NewRecorder(), theError}, false},
+		{"normalLogger", args{httptest.NewRecorder(), theError}, false, theError},
-		{"responseLogger", args{logging.NewResponseLogger(httptest.NewRecorder()), theError}, true},
+		{"responseLogger", args{logging.NewResponseLogger(httptest.NewRecorder()), theError}, true, theError},
 	}
 
 	for _, tt := range tests {
@@ -32,8 +38,61 @@ func TestError(t *testing.T) {
 			if tt.withFields {
 				if rl, ok := tt.args.rw.(logging.ResponseLogger); ok {
 					fields := rl.Fields()
-					if !reflect.DeepEqual(fields["error"], theError) {
+					if !reflect.DeepEqual(fields["error"], tt.want) {
-						t.Errorf("ResponseLogger[\"error\"] = %s, wants %s", fields["error"], theError)
+						t.Errorf("ResponseLogger[\"error\"] = %s, wants %s", fields["error"], tt.want)
+					}
+				} else {
+					t.Error("ResponseWriter does not implement logging.ResponseLogger")
+				}
+			}
+		})
+	}
+}
+
+type mockStackTracedError struct{}
+
+func (t *mockStackTracedError) Error() string {
+	return "a stacktraced error"
+}
+
+func (t *mockStackTracedError) StackTrace() pkgerrors.StackTrace {
+	f := struct{}{}
+	return pkgerrors.StackTrace{ // fake stacktrace
+		pkgerrors.Frame(unsafe.Pointer(&f)),
+		pkgerrors.Frame(unsafe.Pointer(&f)),
+	}
+}
+
+func TestError_StackTracedError(t *testing.T) {
+
+	t.Setenv("STEPDEBUG", "1")
+	aStackTracedError := mockStackTracedError{}
+
+	type args struct {
+		rw  http.ResponseWriter
+		err error
+	}
+	tests := []struct {
+		name       string
+		args       args
+		withFields bool
+		want       error
+	}{
+		{"responseLoggerWithStackTracedError", args{logging.NewResponseLogger(httptest.NewRecorder()), &aStackTracedError}, true, &aStackTracedError},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			Error(tt.args.rw, tt.args.err)
+			if tt.withFields {
+				if rl, ok := tt.args.rw.(logging.ResponseLogger); ok {
+					fields := rl.Fields()
+					if !reflect.DeepEqual(fields["error"], tt.want) {
+						t.Errorf("ResponseLogger[\"error\"] = %s, wants %s", fields["error"], tt.want)
+					}
+					// `stack-trace` expected to be set; not interested in actual output
+					if _, ok := fields["stack-trace"]; !ok {
+						t.Errorf("ResponseLogger[\"stack-trace\"] not set")
 					}
 				} else {
 					t.Error("ResponseWriter does not implement logging.ResponseLogger")