receive-pack.c: add protocol support to negotiate atomic-push

This adds support to the protocol between send-pack and receive-pack to
* allow receive-pack to inform the client that it has atomic push capability
* allow send-pack to request atomic push back.

There is currently no setting in send-pack to actually request that atomic
pushes are to be used yet. This only adds protocol capability not ability
for the user to activate it.

igned-off-by: Ronnie Sahlberg <sahlberg@google.com>
send-pack.c: add an --atomic-push command line argument

This adds support to send-pack to to negotiate and use atomic pushes
iff the server supports it. Atomic pushes are activated by a new command
line flag --atomic-push.

In order to do this we also need to change the semantics for send_pack()
slightly. The existing send_pack() function actually don't sent all the
refs back to the server when multiple refs are involved, for example
when using --all. Several of the failure modes for pushes can already be
detected locally in the send_pack client based on the information from the
initial server side list of all the refs as generated by receive-pack.
Any such refs that we thus know would fail to push are thus pruned from
the list of refs we send to the server to update.

For atomic pushes, we have to deal thus with both failures that are detected
locally as well as failures that are reported back from the server. In order
to do so we treat all local failures as push failures too.

We introduce a new status code REF_STATUS_ATOMIC_PUSH_FAILED so we can
flag all refs that we would normally have tried to push to the server
but we did not due to local failures. This is to improve the error message
back to the end user to flag that "these refs failed to update since the
atomic push operation failed."

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
receive-pack.c: use a single transaction when atomic-push is negotiated

Update receive-pack to use an atomic transaction iff the client negotiated
that it wanted atomic-push.
This leaves the default behavior to be the old non-atomic one ref at a
time update. This is to cause as little disruption as possible to existing
clients. It is unknown if there are client scripts that depend on the old
non-atomic behavior so we make it opt-in for now.

Later patch in this series also adds a configuration variable where you can
override the atomic push behavior on the receiving repo and force it
to use atomic updates always.

If it turns out over time that there are no client scripts that depend on the
old behavior we can change git to default to use atomic pushes and instead
offer an opt-out argument for people that do not want atomic pushes.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
push.c: add an --atomic-push argument

Add a command line argument to the git push command to request atomic
pushes.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
t5543-atomic-push.sh: add basic tests for atomic pushes

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
receive-pack.c: add a receive.preferatomicpush configuration variable

Add receive.preferatomicpush setting to receive-pack.c. This is triggers
a new capability "prefer-atomic-push" to be sent back to the send-pack
client, requesting that the client, if it supports it, to request
an atomic push.

This is an alternative way to trigger an atomic push instead of having
to rely of the user using the --atomic-push command line argument.
For backward compatibility, this is only a hint to the client that is should
request atomic pushes if it can. Old clients that do not support atomic pushes
will just ignore this capability and use a normal push.

The reason we need to signal this capability back to the client is due
to that send_pack() has push failure modes where it will detect that
certain refs can not be pushed and fail them early and will later only send
a subset of all refs to the server. For atomic pushes we want the push to fail
if either the refs failed early in the client or if the refs failed when sent
to the server. This means that we can not do the atomic-push logic unilaterally
in the server but need the client to cooperate.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>