incusd/response: Abort piped exports that fail mid-stream

The PipeResponse mechanism would always send an early header, then
process the piped data. That's done to avoid hitting a header submission
timeout.

The problem with that is that a failure is now being interpreted as
success by the client as there is no way to go back and change the HTTP
status code.

Instead, use a bit of a hack by aborting the connection entirely.

This is done through a scary panic() call which is intercepted by the Go
HTTP server and will cause the underlying connection to be terminated
without affecting the rest of the daemon.

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>

internal/server/response/response.go

@@ -758,7 +758,14 @@ func (r *pipeResponse) Render(w http.ResponseWriter) error {
 	}
 
 	_, err := util.SafeCopy(w, r.reader)
-	return err
+	if err != nil {
+		// It's too late to send a clean error back to the client, so
+		// instead use the Go HTTP ErrAbortHandler logic to terminate the
+		// connection. This does not cause the daemon itself to panic.
+		panic(http.ErrAbortHandler)
+	}
+
+	return nil
 }
 
 // String returns a quick description of the response.