[PATCH] upload-pack: fix race condition in error messages

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Aug 10, 2023, 14:40 UTC
Message-ID: <pull.1572.git.1691678450757.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

Test t5516-fetch-push.sh has a test 'deny fetch unreachable SHA1, allowtipsha1inwant=true' that checks stderr for a specific error string from the remote. In some build environments the error sent over the remote connection gets mingled with the error from the die() statement. Since both signals are being output to the same file descriptor (but from parent and child processes), the output we are matching with grep gets split.

To reduce the risk of this failure, follow this process instead:

1. Write an error message to stderr.
2. Write an error message across the connection.
3. exit(1).

This reorders the events so the error is written entirely before the client receives a message from the remote, removing the race condition.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
    upload-pack: fix race condition in error messages
    
    Here is another quick patch that we've been holding in the microsoft/git
    fork for years because it helped prevent some test flakiness, especially
    in our more involved functional test environment.
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1572%2Fderrickstolee%2Fupload-pack-race-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1572/derrickstolee/upload-pack-race-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1572

 upload-pack.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/upload-pack.c b/upload-pack.c
index 94751477ab2..7b25129f0f6 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -801,11 +801,12 @@ error:
 	for (i = 0; i < data->want_obj.nr; i++) {
 		struct object *o = data->want_obj.objects[i].item;
 		if (!is_our_ref(o, data->allow_uor)) {
+			error("git upload-pack: not our ref %s",
+			      oid_to_hex(&o->oid));
 			packet_writer_error(&data->writer,
 					    "upload-pack: not our ref %s",
 					    oid_to_hex(&o->oid));
-			die("git upload-pack: not our ref %s",
-			    oid_to_hex(&o->oid));
+			exit(1);
 		}
 	}
 }

base-commit: a82fb66fed250e16d3010c75404503bea3f0ab61

-- 
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