[PATCH v2] upload-pack: allow stateless client EOF just prior to haves

From: Daniel Duvall <dan@mutual.io>
Date: Oct 30, 2020, 22:35 UTC
Message-ID: <20201030223504.45978-1-dan@mutual.io>
In-Reply-To: <1604022059-18527-1-git-send-email-dan@mutual.io>

During stateless packfile negotiation where a depth is given, stateless RPC clients (e.g. git-remote-curl) will send multiple upload-pack requests with the first containing only the wants/shallows/deepens/filters and the subsequent containing haves/done.

When upload-pack handles such requests, entering get_common_commits without first whether the client has hung up can result in unexpected EOF during the negotiation loop and a die() with message "fatal: the remote end hung up unexpectedly".

Real world effects include:

 - A client speaking to git-http-backend via a server that doesn't check the
   exit codes of CGIs (e.g. mod_cgi) doesn't know and doesn't care about the
   fatal. It continues to process the response body as normal.

 - A client speaking to a server that does check the exit code and returns an
   errant HTTP status as a result will fail with the message "error: RPC
   failed; HTTP 500 curl 22 The requested URL returned error: 500."

 - Admins running servers that surface the failure must workaround it by
   patching code that handles execution of git-http-backend to ignore exit
   codes or take other heuristic approaches.

 - Admins may have to deal with "hung up unexpectedly" log spam related to the
   failures even in cases where the exit code isn't surfaced as an HTTP
   server-side error status.

To avoid these EOF related fatals, have upload-pack gently peek for an EOF between the sending of shallow/unshallow lines (followed by flush) and the reading of client haves. If the client has hung up at this point, exit normally.

Signed-off-by: Daniel Duvall <dan@mutual.io>

---

Changes in v2:
 - Replaced unconditional flipping (XOR) of PACKET_READ_GENTLE_ON_EOF bit w/
   `&= ~` to flip it back off (as it was when reader was initialized in
   previous clause)
 - Renamed test filename to group with other upload-pack related tests
 - Refactored test using packetize helper
 - Clarified in commit message that file descriptor is still valid but client
   hangup/EOF is the core issue
 - Added possible real-world effects of bug to commit message as suggested

---
 t/t5705-upload-pack-stateless-shallow-eof.sh | 24 ++++++++++++++++++++
 upload-pack.c                                | 13 ++++++++++-
 2 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100755 t/t5705-upload-pack-stateless-shallow-eof.sh

diff --git a/t/t5705-upload-pack-stateless-shallow-eof.sh b/t/t5705-upload-pack-stateless-shallow-eof.sh
new file mode 100755
index 0000000000..cc9d4baa0b
--- /dev/null
+++ b/t/t5705-upload-pack-stateless-shallow-eof.sh
@@ -0,0 +1,24 @@
+#!/bin/sh
+
+test_description='stateless upload-pack gently handles EOF just after want/shallow/depth/flush'
+
+. ./test-lib.sh
+
+test_expect_success 'upload-pack outputs flush and exits ok' '
+	test_commit initial &&
+	head=$(git rev-parse HEAD) &&
+
+	{
+		packetize "want $head" &&
+		packetize "shallow $head" &&
+		packetize "deepen 1" &&
+		printf "0000"
+	} >request &&
+
+	printf "0000" >expect &&
+	git upload-pack --stateless-rpc . <request >actual &&
+
+	test_cmp expect actual
+'
+
+test_done
diff --git a/upload-pack.c b/upload-pack.c
index 3b858eb457..5dc8e1f844 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1344,7 +1344,18 @@ void upload_pack(struct upload_pack_options *options)
 				   PACKET_READ_DIE_ON_ERR_PACKET);
 
 		receive_needs(&data, &reader);
-		if (data.want_obj.nr) {
+
+		/*
+		 * An EOF at this exact point in negotiation should be
+		 * acceptable from stateless clients as they will consume the
+		 * shallow list before doing subsequent rpc with haves/etc.
+		 */
+		if (data.stateless_rpc)
+			reader.options |= PACKET_READ_GENTLE_ON_EOF;
+
+		if (data.want_obj.nr &&
+		    packet_reader_peek(&reader) != PACKET_READ_EOF) {
+			reader.options &= ~PACKET_READ_GENTLE_ON_EOF;
 			get_common_commits(&data, &reader);
 			create_pack_file(&data, NULL);
 		}

-- 
2.29.1.1.ge14d223