refs.c: add a transaction function to append a reflog entry

Define a new transaction update type, UPDATE_LOG, and a new function
transaction_update_reflog. This function will lock the reflog and append
an entry to it during transaction commit. We can pass a flag to this
function, which can truncate the the reflog file before we write the
update.

When performing a reflog transaction update, only write to the reflog iff
msg is non-NULL. This can then be combined with REFLOG_TRUNCATE to perform
an update that only truncates but does not write. This change only affects
whether or not a reflog entry should be generated and written. If msg == NULL
then no such entry will be written.

Orthogonal to this we have a boolean flag REFLOG_TRUNCATE which is used to
tell the transaction system to "truncate the reflog and thus discard all
previous users".

At the current time the only place where we use msg == NULL is also the
place, where we use REFLOG_TRUNCATE. Even though these two settings are
currently only ever used together it still makes sense to have them through
two separate knobs.

This allows future consumers of this API that may want to do things
differently. For example someone can do:
  msg="Reflog truncated by Bob because ..." + REFLOG_TRUNCATE
and have it truncate the log and have it start fresh with an initial message
that explains the log was truncated. This API allows that.

During one transaction we allow to make multiple reflog updates to the
same ref. This means we only need to lock the reflog once, during the first
update that touches the reflog, and that all further updates can just write the
reflog entry since the reflog is already locked.

This allows us to write code such as:

t = transaction_begin()
transaction_reflog_update(t, "foo", REFLOG_TRUNCATE, NULL);
loop-over-something...
   transaction_reflog_update(t, "foo", 0, <message>);
transaction_commit(t)

where we first truncate the reflog and then build the new content one line
at a time.

While this technically looks like O(n2) behavior it is not that bad.
We only do this loop for transactions that cover a single ref during
reflog expire. This means that the linear search inside
transaction_update_reflog() will find the match on the very first entry
thus making it O(1) and not O(n) or our usecases. Thus the whole expire
becomes O(n) instead of O(n2). If in the future we start doing this for many
refs in one single transaction we might want to optimize this.
But there is no need to complexify the code and optimize for future usecases
that might never materialize at this stage.

Instead of recording the line by line reflog updates in memory, use a
tempfile: .git/tmp_reflog_XXXXXX which we write the entries to as the
transaction is built. Then just rename this file onto the destination
reflog file when the transaction is committed.

todo:
This patch needs to add an atexit() thingy as well to ensure that
any remaining files are unlinked on exit, just like the lock_file() thing does.

Change-Id: Ia12213b4ca9fbd0c9b34325d90df95412348515f
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
reflog.c: use a reflog transaction when writing during expire

Use a transaction for all updates during expire_reflog.

Change-Id: Ieb81b2660cefeeecf0bcb3cdbc1ef3cbb86e7eb8
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
refs.c: rename log_ref_setup to create_reflog

log_ref_setup is used to do several semi-related things:
* Sometimes it will create a new reflog including missing parent
  directories and cleaning up any conflicting stale directories
  in the path.
* Fill in a filename buffer for the full path to the reflog.
* Unconditionally re-adjust the permissions for the file.

This function is only called from two places: In checkout.c, where
it is always used to create a reflog and in refs.c log_ref_write,
where it is used to create a reflog sometimes, and sometimes just
to fill in the filename.

Rename log_ref_setup to create_reflog and change it to only take the
refname as an argument to make its signature similar to delete_reflog
and reflog_exists. Change create_reflog to ignore log_all_ref_updates
and "unconditionally" create the reflog when called. Since checkout.c
always wants to create a reflog we can call create_reflog directly and
avoid the temp-and-log_all_ref_update dance.

In log_ref_write, only call create_reflog, iff we want to create a reflog
and if the reflog does not yet exist. This means that for the common case
where the log already exists we now only need to perform a single lstat()
instead of a open(O_CREAT)+lstat()+close().

Change-Id: Ib9493b3b81a97a0e154cc44303a5ed7cdceaaca7
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
refs.c: Remove unlock_ref/close_ref/commit_ref from the refs api

unlock|close|commit_ref can be made static since there are no more external
callers.

Change-Id: I743f9daaa1c101fdc781a76283c2e8b462591d07
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
refs.c: remove lock_any_ref_for_update

No one is using this function so we can delete it.

Change-Id: I524dac239da500e5d4432328b997ed8ab34f6803
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
refs.c: allow deleting refs with a broken sha1

Add back support to make it possible to delete refs that have a broken
sha1.

Add new internal flags REF_ALLOW_BROKEN and RESOLVE_REF_ALLOW_BAD_SHA1
to pass intent from branch.c that we are willing to allow
resolve_ref_unsafe and lock_ref_sha1_basic to allow broken refs.
Since these refs can not actually be resolved to a sha1, they instead resolve
to null_sha1 when these flags are used.

For example, the ref:

   echo "Broken ref" > .git/refs/heads/foo-broken-1

can now be deleted using git branch -d foo-broken-1

Change-Id: I4e744d9e7d8b7e81dde5479965819117d03c98db
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>