devlxd: Unmask Render errors (#14760)

This implements a better handling of errors during Render on devlxd
handlers, following the same pattern seen on
https://github.com/hamistao/lxd/blob/unmask_write_errors/lxd/daemon.go#L846
When rendering a response (send it to the client) fails, instead of
masking the error we try to send a response using `DevLxdErrorResponse`
to the client communicating the error, since this determines the
response type used for communicating erros on the devlxd API. If that
also fails, an error is logged on the server.

lxd/api.go

@@ -243,7 +243,15 @@ func hoistReqVM(f func(*Daemon, instance.Instance, http.ResponseWriter, *http.Re
 		}
 
 		resp := f(d, inst, w, r)
-		_ = resp.Render(w, r)
+		if resp != nil {
+			err = resp.Render(w, r)
+			if err != nil {
+				writeErr := response.DevLxdErrorResponse(err, true).Render(w, r)
+				if writeErr != nil {
+					logger.Warn("Failed writing error for HTTP response", logger.Ctx{"url": r.URL, "err": err, "writeErr": writeErr})
+				}
+			}
+		}
 	}
 }
 

lxd/devlxd.go

@@ -344,7 +344,15 @@ func hoistReq(f func(*Daemon, instance.Instance, http.ResponseWriter, *http.Requ
 		}
 
 		resp := f(d, c, w, r)
-		_ = resp.Render(w, r)
+		if resp != nil {
+			err = resp.Render(w, r)
+			if err != nil {
+				writeErr := response.DevLxdErrorResponse(err, false).Render(w, r)
+				if writeErr != nil {
+					logger.Warn("Failed writing error for HTTP response", logger.Ctx{"url": r.URL, "err": err, "writeErr": writeErr})
+				}
+			}
+		}
 	}
 }