feat(error): show internal correlation id in error messages (#411)

This commit changes the way error responses from the API are formatted
for display to users. When available, it adds the Correlation (Trace) ID
header to the error string.
    
We often have users that post the error string, but rarely do users have
debug logging active. By adding the correlation ID to error messages, we
can more quickly investigate why something went wrong using the Hetzner
internal tracing system.
    
This is common practice in Web Apps, they usually show a request/trace
ID or an encrypted blob in case of an internal server error.

hcloud/action_test.go

@@ -322,6 +322,7 @@ func TestResourceActionClientAll(t *testing.T) {
 }
 
 func TestActionClientWatchOverallProgress(t *testing.T) {
+	t.Parallel()
 	env := newTestEnv()
 	defer env.Teardown()
 

hcloud/client.go

@@ -379,6 +379,10 @@ func errorFromResponse(resp *Response, body []byte) error {
 	return hcErr
 }
 
+const (
+	headerCorrelationID = "X-Correlation-Id"
+)
+
 // Response represents a response from the API. It embeds http.Response.
 type Response struct {
 	*http.Response
@@ -412,6 +416,12 @@ func (r *Response) readMeta(body []byte) error {
 	return nil
 }
 
+// internalCorrelationID returns the unique ID of the request as set by the API. This ID can help with support requests,
+// as it allows the people working on identify this request in particular.
+func (r *Response) internalCorrelationID() string {
+	return r.Header.Get(headerCorrelationID)
+}
+
 // Meta represents meta information included in an API response.
 type Meta struct {
 	Pagination *Pagination

hcloud/error.go

@@ -100,6 +100,13 @@ type Error struct {
 }
 
 func (e Error) Error() string {
+	if resp := e.Response(); resp != nil {
+		correlationID := resp.internalCorrelationID()
+		if correlationID != "" {
+			// For easier debugging, the error string contains the Correlation ID of the response.
+			return fmt.Sprintf("%s (%s) (Correlation ID: %s)", e.Message, e.Code, correlationID)
+		}
+	}
 	return fmt.Sprintf("%s (%s)", e.Message, e.Code)
 }
 

hcloud/error_test.go

@@ -2,6 +2,7 @@ package hcloud
 
 import (
 	"fmt"
+	"net/http"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -27,6 +28,38 @@ func TestError_Error(t *testing.T) {
 			},
 			want: "unable to authenticate (unauthorized)",
 		},
+		{
+			name: "internal server error with correlation id",
+			fields: fields{
+				Code:    ErrorCodeUnknownError,
+				Message: "Creating image failed because of an unknown error.",
+				response: &Response{
+					Response: &http.Response{
+						StatusCode: http.StatusInternalServerError,
+						Header: func() http.Header {
+							headers := http.Header{}
+							// [http.Header] requires normalized header names, easiest to do by using the Set method
+							headers.Set("X-Correlation-ID", "foobar")
+							return headers
+						}(),
+					},
+				},
+			},
+			want: "Creating image failed because of an unknown error. (unknown_error) (Correlation ID: foobar)",
+		},
+		{
+			name: "internal server error without correlation id",
+			fields: fields{
+				Code:    ErrorCodeUnknownError,
+				Message: "Creating image failed because of an unknown error.",
+				response: &Response{
+					Response: &http.Response{
+						StatusCode: http.StatusInternalServerError,
+					},
+				},
+			},
+			want: "Creating image failed because of an unknown error. (unknown_error)",
+		},
 	}
 
 	for _, tt := range tests {