Fill the errors attribute for HTTP errors (#1528)

Some GCS client libraries (like [this one][1]), are quite strict when it
comes to the set of attributes they expect to find in the error
response. If any of those attributes is missing, they fail to
deserialize the error.

In `fake-gcs-server`, the `httpError.Errors` attribute was always
initialized to `nil`, causing the `error.errors` JSON response attribute
to be omitted and leading to failures in some client libraries.

In this commit we address that issue by always providing content on the
`errors` attribute (`httpError.Errors` field) whenever we return an
error response. The implementation is kept simple without aiming to
return exactly the same values that are [officially documented][2]. The
main goal is to have a sensible response that is _structurally_ correct
for the purposes of deserialization client-side.

[1]: https://crates.io/crates/google-cloud-storage
[2]: https://cloud.google.com/storage/docs/json_api/v1/status-codes

fakestorage/json_response.go

@@ -32,7 +32,7 @@ func jsonToHTTPHandler(h jsonHandler) http.HandlerFunc {
 		status := resp.getStatus()
 		var data any
 		if status > 399 {
-			data = newErrorResponse(status, resp.getErrorMessage(status), nil)
+			data = newErrorResponse(status, resp.getErrorMessage(status), resp.getErrorList(status))
 		} else {
 			data = resp.data
 		}
@@ -59,6 +59,18 @@ func (r *jsonResponse) getErrorMessage(status int) string {
 	return http.StatusText(status)
 }
 
+func (r *jsonResponse) getErrorList(status int) []apiError {
+	if status == http.StatusOK {
+		return nil
+	} else {
+		return []apiError{{
+			Domain:  "global",
+			Reason:  http.StatusText(status),
+			Message: r.getErrorMessage(status),
+		}}
+	}
+}
+
 func errToJsonResponse(err error) jsonResponse {
 	status := 0
 	var pathError *os.PathError

fakestorage/object_test.go

@@ -2028,7 +2028,7 @@ func TestServiceClientComposeObject(t *testing.T) {
 				"files/destination.txt",
 				[]string{"01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "30", "31", "32", "33"},
 				"",
-				"googleapi: Error 400: The number of source components provided (33) exceeds the maximum (32)",
+				"googleapi: Error 400: The number of source components provided (33) exceeds the maximum (32), Bad Request",
 			},
 		}
 		for _, test := range tests {

fakestorage/upload_test.go

@@ -224,7 +224,7 @@ func TestServerClientObjectWriterWithDoesNotExistPrecondition(t *testing.T) {
 		if err == nil {
 			t.Fatal("expected overwriting existing object to fail, but received no error")
 		}
-		if err.Error() != "googleapi: Error 412: Precondition failed" {
+		if err.Error() != "googleapi: Error 412: Precondition failed, Precondition Failed" {
 			t.Errorf("expected HTTP 412 precondition failed error, but got %v", err)
 		}
 
@@ -346,7 +346,7 @@ func TestServerClientObjectOperationsWithIfGenerationNotMatchPrecondition(t *tes
 		if err == nil {
 			t.Fatal("expected overwriting existing object to fail, but received no error")
 		}
-		if err.Error() != "googleapi: Error 412: Precondition failed" {
+		if err.Error() != "googleapi: Error 412: Precondition failed, Precondition Failed" {
 			t.Errorf("expected HTTP 412 precondition failed error, but got %v", err)
 		}