[PATCH 8/8] http: prompt for credentials on failed POST

All of the smart-http GET requests go through the http_get_* functions, which will prompt for credentials and retry if we see an HTTP 401.

POST requests, however, do not go through any central point. Moreover, it is difficult to retry in the general case; we cannot assume the request body fits in memory or is even seekable, and we don't know how much of it was consumed during the attempt.

Most of the time, this is not a big deal; for both fetching and pushing, we make a GET request before doing any POSTs, so typically we figure out the credentials during the first request, then reuse them during the POST. However, some servers may allow a client to get the list of refs from receive-pack without authentication, and then require authentication when the client actually tries to POST the pack.

This is not ideal, as the client may do a non-trivial amount of work to generate the pack (e.g., delta-compressing objects). However, for a long time it has been the recommended example configuration in git-http-backend(1) for setting up a repository with anonymous fetch and authenticated push. This setup has always been broken without putting a username into the URL. Prior to commit 986bbc0, it did work with a username in the URL, because git would prompt for credentials before making any requests at all. However, post-986bbc0, it is totally broken. Since it has been advertised in the manpage for some time, we should make sure it works.

Unfortunately, it is not as easy as simply calling post_rpc again when it fails, due to the input issue mentioned above. However, we can still make this specific case work by retrying in two specific instances:

  1. If the request is large (bigger than LARGE_PACKET_MAX),
     we will first send a probe request with a single flush
     packet. Since this request is static, we can freely
     retry it.

  2. If the request is small and we are not using gzip, then
     we have the whole thing in-core, and we can freely
     retry.

That means we will not retry in some instances, including:

  1. If we are using gzip. However, we only do so when
     calling git-upload-pack, so it does not apply to
     pushes.

  2. If we have a large request, the probe succeeds, but
     then the real POST wants authentication. This is an
     extremely unlikely configuration and not worth worrying
     about.

While it might be nice to cover those instances, doing so would be significantly more complex for very little real-world gain. In the long run, we will be much better off when curl learns to internally handle authentication as a callback, and we can cleanly handle all cases that way.

Signed-off-by: Jeff King <peff@peff.net>
---
Sorry for the wordy explanation. I really tried to refactor this into a
nice single code path for making both GET and POST requests, but I think
there are just too many corner cases. Suggestions welcome if somebody
has a better idea of how to refactor it (preferably in the form of a
patch).

 remote-curl.c        | 23 +++++++++++++++--------
 t/t5541-http-push.sh |  2 +-
 2 files changed, 16 insertions(+), 9 deletions(-)