On Wed, Nov 09, 2016 at 05:18:30PM -0500, David Turner wrote:
> In the event that a HTTP server closes the connection after giving a
> 200 but before giving any packets, we don't want to hang forever
> waiting for a response that will never come. Instead, we should die
> immediately.
I agree we don't want to hang forever, but this leaves open the
question: what is hanging?
My guess is that fetch-pack is waiting for more data from the server,
and remote-curl is waiting for fetch-pack to tell us what to send for
the next request. Neither will make forward progress because they are
effectively waiting on each other.
Which means this is likely a special case of malformed input from the
server. A server which likewise sends a partial response could end up in
the same deadlock, I would think (e.g., a half-finished pktline, or a
pktline but no trailing flush).
That doesn't make it wrong to fix this specific case (especially if it's
a common one), but I wonder if we could do better.
The root of the issue is that only fetch-pack understands the protocol,
and remote-curl is blindly proxying the data. But only remote-curl knows
that the HTTP request has ended, and it doesn't relay that information
to fetch-pack. So I can think of two solutions:
1. Some way of remote-curl communicating the EOF to fetch-pack. It
can't just close the descriptor, since we need to pass more data
over it for the followup requests. You'd need something
out-of-band, or to frame the HTTP data inside another layer of
pktlines, both of which are kind of gross.
2. Have remote-curl understand enough of the protocol that it can
abort rather than hang.
I think that's effectively the approach of your patch, but for one
specific case. But could we, for example, make sure that everything
we proxy is a complete set of pktlines and ends with a flush? And
if not, then we hang up on fetch-pack.
I _think_ that would work, because even the pack is always encased
in pktlines for smart-http.
> @@ -659,6 +662,8 @@ static int post_rpc(struct rpc_state *rpc)
> curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, rpc_in);
> curl_easy_setopt(slot->curl, CURLOPT_FILE, rpc);
>
> +
> + rpc->any_written = 0;
Extra blank line here?
> @@ -667,6 +672,9 @@ static int post_rpc(struct rpc_state *rpc)
> if (err != HTTP_OK)
> err = -1;
>
> + if (!rpc->any_written)
> + err = -1;
> +
I wondered if there were any cases where it was normal for the server to
return zero bytes. Possibly the ref advertisement is one, but this is
_just_ handling post_rpc(), so that's OK. And I think by definition
every response has to at least return a flush packet, or we would make
no forward progress (i.e., the exact case you are dealing with here).
-Peff