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 behaviour 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 behaviour 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 behaviour 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 behaviour 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>
receive-pack.c: add receive.atomicpush configuration option

Add a configuration argument to the receive side to force atomic pushes
for all pushes to the repo.

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

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: add a public is_branch function

Both refs.c and fsck.c have their own private copies of the is_branch function.
Delete the is_branch function from fsck.c and make the version in refs.c
public.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
refs.c: create a public function for is_refname_available

Export a generic is_refname_available() function. We will need this
as a public shared function later when we add additional refs backends
since we want to keep using the same rules for ref naming across
all backends.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs-be-files.c: rename refs to refs-be-files

Rename refs.c to refs-be-files.c to indicate that this file now
holds the implementation for the files based refs backend.
A smaller portion of the code in this file is backend agnostic and will
be moved to a a new refs.c file that will hold all the common refs code
that is shared across all backends.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: add a new refs.c file to hold all common refs code

Create a new erfs.c file that will be used to hold all the refs
code that is backend agnostic and will be shared across all backends.

The reason we renamed everything to refs-be-files.c in the previous patch
and now start moving the common code back to the new refs.c file
instead of the other way around is the etive volumes of code.

With the ref_cache, packed refs and loose ref handling that are all
part of the files based implementation the backend specific part
of the old refs.c file is several times larger than the backend agnostic
part. Therefore it makes more sense to first rename everything to be
part of the files based backend and then move the parts that can be used
as common code back to refs.c.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move update_ref to refs.c

This change moves update_ref() to the refs.c file since this function
does not contain any backend specific code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move delete_ref to the common code

This change moves delete_ref() to the refs.c file since this function
does not contain any backend specific code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move rename_ref to the common code

This change moves rename_ref() to the refs.c file since this function
does not contain any backend specific code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move read_ref_at to the common refs file

This change moves read_ref_at() to the refs.c file since this function
does not contain any backend specific code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move the hidden refs functions to the common code

This change moves the hidden refs functions to the refs.c file since
these functions do not contain any backend specific code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move dwim and friend functions to the common refs code

These functions do not contain any backend specific code so we can move
them to the common code and share across all backends.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move warn_if_dangling_symref* to the common code

These functions do not use any backend specific code so we can move
them to the common code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move read_ref, read_ref_full and ref_exists to the common code

These functions do not depend on the backend implementation so we
can move them to the common code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move resolve_refdup to common

This function can be shared across all refs backends so move it
to the common code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move check_refname_component to the common code

This function does not contain any backend specific code so we
can move it to the common code.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move is_branch to the common code

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move names_conflict to the common code

Move names_conflict to the common code and make it public.
We want to use the same name conflict checks across all backends
so we guarantee compatibility across backends.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: move prettify_refname to the common code

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