[PATCH 5/6] remote-curl: error on incomplete packet

[Denton Liu <liu.denton@xxxxxxxxx>]

Currently, remote-curl acts as a proxy and blindly forwards packets
between an HTTP server and fetch-pack. In the case of a stateless RPC
connection where the connection is terminated with a partially written
packet, remote-curl will blindly send the partially written packet
before waiting on more input from fetch-pack. Meanwhile, fetch-pack will
read the partial packet and continue reading, expecting more input. This
results in a deadlock between the two processes.

Instead of blindly forwarding packets, inspect each packet to ensure
that it is a full packet, erroring out if a partial packet is sent.

Helped-by: Jeff King <peff@xxxxxxxx>
Signed-off-by: Denton Liu <liu.denton@xxxxxxxxx>
---

Notes:
    Unfortunately, I'm not really sure how to test this.

 remote-curl.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/remote-curl.c b/remote-curl.c
index da3e07184a..8b740354e5 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -682,6 +682,8 @@ static curlioerr rpc_ioctl(CURL *handle, int cmd, void *clientp)
 struct rpc_in_data {
 	struct rpc_state *rpc;
 	struct active_request_slot *slot;
+	struct strbuf len_buf;
+	int remaining;
 };
 
 /*
@@ -692,6 +694,7 @@ static size_t rpc_in(char *ptr, size_t eltsize,
 		size_t nmemb, void *buffer_)
 {
 	size_t size = eltsize * nmemb;
+	size_t unwritten = size;
 	struct rpc_in_data *data = buffer_;
 	long response_code;
 
@@ -702,7 +705,42 @@ static size_t rpc_in(char *ptr, size_t eltsize,
 		return size;
 	if (size)
 		data->rpc->any_written = 1;
-	write_or_die(data->rpc->in, ptr, size);
+
+	while (unwritten) {
+		if (!data->remaining) {
+			int digits_remaining = 4 - data->len_buf.len;
+			if (digits_remaining > unwritten)
+				digits_remaining = unwritten;
+			strbuf_add(&data->len_buf, ptr, digits_remaining);
+			ptr += digits_remaining;
+			unwritten -= digits_remaining;
+
+			if (data->len_buf.len == 4) {
+				data->remaining = packet_length(data->len_buf.buf);
+				if (data->remaining < 0) {
+					die(_("remote-curl: bad line length character: %.4s"), data->len_buf.buf);
+				} else if (data->remaining <= 1) {
+					data->remaining = 0;
+				} else if (data->remaining < 4) {
+					die(_("remote-curl: bad line length %d"), data->remaining);
+				} else {
+					data->remaining -= 4;
+				}
+				write_or_die(data->rpc->in, data->len_buf.buf, 4);
+				strbuf_reset(&data->len_buf);
+			}
+		}
+
+		if (data->remaining) {
+			int remaining = data->remaining;
+			if (remaining > unwritten)
+				remaining = unwritten;
+			write_or_die(data->rpc->in, ptr, remaining);
+			ptr += remaining;
+			unwritten -= remaining;
+			data->remaining -= remaining;
+		}
+	}
 	return size;
 }
 
@@ -920,6 +958,8 @@ static int post_rpc(struct rpc_state *rpc, int flush_received)
 	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, rpc_in);
 	rpc_in_data.rpc = rpc;
 	rpc_in_data.slot = slot;
+	strbuf_init(&rpc_in_data.len_buf, 4);
+	rpc_in_data.remaining = 0;
 	curl_easy_setopt(slot->curl, CURLOPT_FILE, &rpc_in_data);
 	curl_easy_setopt(slot->curl, CURLOPT_FAILONERROR, 0);
 
@@ -936,6 +976,9 @@ static int post_rpc(struct rpc_state *rpc, int flush_received)
 	if (!rpc->any_written)
 		err = -1;
 
+	if (rpc_in_data.remaining)
+		err = error(_("%d bytes are still expected"), rpc_in_data.remaining);
+
 	curl_slist_free_all(headers);
 	free(gzip_body);
 	return err;
-- 
2.26.2.706.g87896c9627