Decode JSON body when Prometheus returns 400 to an api call

400 and 422 are documented error codes from Prometheus, so we should
attempt to parse the error returned for both of them.

Needed to change a test that was requiring the old behaviour - made it
use 500 instead of 400.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

api/prometheus/v1/api.go

@@ -455,6 +455,11 @@ type apiResponse struct {
 	Error     string          `json:"error"`
 }
 
+func apiError(code int) bool {
+	// These are the codes that Prometheus sends when it returns an error.
+	return code == statusAPIError || code == http.StatusBadRequest
+}
+
 func (c apiClient) Do(ctx context.Context, req *http.Request) (*http.Response, []byte, error) {
 	resp, body, err := c.Client.Do(ctx, req)
 	if err != nil {
@@ -463,7 +468,7 @@ func (c apiClient) Do(ctx context.Context, req *http.Request) (*http.Response, [
 
 	code := resp.StatusCode
 
-	if code/100 != 2 && code != statusAPIError {
+	if code/100 != 2 && !apiError(code) {
 		return resp, body, &Error{
 			Type: ErrBadResponse,
 			Msg:  fmt.Sprintf("bad response code %d", resp.StatusCode),
@@ -479,14 +484,14 @@ func (c apiClient) Do(ctx context.Context, req *http.Request) (*http.Response, [
 		}
 	}
 
-	if (code == statusAPIError) != (result.Status == "error") {
+	if apiError(code) != (result.Status == "error") {
 		err = &Error{
 			Type: ErrBadResponse,
 			Msg:  "inconsistent body for response code",
 		}
 	}
 
-	if code == statusAPIError && result.Status == "error" {
+	if apiError(code) && result.Status == "error" {
 		err = &Error{
 			Type: result.ErrorType,
 			Msg:  result.Error,

api/prometheus/v1/api_test.go

@@ -606,7 +606,19 @@ func TestAPIClientDo(t *testing.T) {
 			response: "bad json",
 			err: &Error{
 				Type: ErrBadResponse,
-				Msg:  "bad response code 400",
+				Msg:  "bad response code 500",
+			},
+			code: http.StatusInternalServerError,
+		},
+		{
+			response: &apiResponse{
+				Status:    "error",
+				ErrorType: ErrBadData,
+				Error:     "end timestamp must not be before start time",
+			},
+			err: &Error{
+				Type: ErrBadData,
+				Msg:  "end timestamp must not be before start time",
 			},
 			code: http.StatusBadRequest,
 		},