git@vger.kernel.org mailing list mirror (one of many)
 help / Atom feed
From: Brandon Williams <bmwill@google.com>
To: Jeff King <peff@peff.net>
Cc: Lars Schneider <larsxschneider@gmail.com>,
	Martin Ågren <martin.agren@gmail.com>,
	Git Users <git@vger.kernel.org>
Subject: Re: [PATCH] config: use a static lock_file struct
Date: Tue, 29 Aug 2017 12:10:51 -0700
Message-ID: <20170829191051.GE131745@google.com> (raw)
In-Reply-To: <20170829190928.GD131745@google.com>

On 08/29, Brandon Williams wrote:
> On 08/29, Jeff King wrote:
> > On Tue, Aug 29, 2017 at 02:53:41PM -0400, Jeff King wrote:
> > 
> > > It looks like the config code has a minor-ish leak. Patch to follow.
> > 
> > Here it is.
> > 
> > -- >8 --
> > Subject: [PATCH] config: use a static lock_file struct
> > 
> > When modifying git config, we xcalloc() a struct lock_file
> > but never free it. This is necessary because the tempfile
> > code (upon which the locking code is built) requires that
> > the resulting struct remain valid through the life of the
> > program. However, it also confuses leak-checkers like
> > valgrind because only the inner "struct tempfile" is still
> > reachable; no pointer to the outer lock_file is kept.
> 
> Is this just due to a limitation in the tempfile code?  Would it be
> possible to improve the tempfile code such that we don't need to require
> that a tempfile, once created, is required to exist for the remaining
> life of the program?
> 
> > 
> > Other code paths solve this by using a single static lock
> > struct. We can do the same here, because we know that we'll
> > only lock and modify one config file at a time (and
> > assertions within the lockfile code will ensure that this
> > remains the case).
> > 
> > That removes a real leak (when we fail to free the struct
> > after locking fails) as well as removes the valgrind false
> > positive. It also means that doing N sequential
> > config-writes will use a constant amount of memory, rather
> > than leaving stale lock_files for each.
> > 
> > Note that since "lock" is no longer a pointer, it can't be
> > NULL anymore. But that's OK. We used that feature only to
> > avoid calling rollback_lock_file() on an already-committed
> > lock. Since the lockfile code keeps its own "active" flag,
> > it's a noop to rollback an inactive lock, and we don't have
> > to worry about this ourselves.
> > 
> > Signed-off-by: Jeff King <peff@peff.net>
> > ---
> > In the long run we may want to drop the "tempfiles must remain forever"
> > rule. This is certainly not the first time it has caused confusion or
> > leaks. And I don't think it's a fundamental issue, just the way the code
> > is written. But in the interim, this fix is probably worth doing.

I didn't read far enough apparently :)  I took a look at this once and
found that the in order to do this we would just need to be careful in
modifying a list of tempfiles.

> > 
> >  config.c | 24 +++++++-----------------
> >  1 file changed, 7 insertions(+), 17 deletions(-)
> > 
> > diff --git a/config.c b/config.c
> > index d0d8ce823a..1603f96e40 100644
> > --- a/config.c
> > +++ b/config.c
> > @@ -2450,7 +2450,7 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
> >  {
> >  	int fd = -1, in_fd = -1;
> >  	int ret;
> > -	struct lock_file *lock = NULL;
> > +	static struct lock_file lock;
> >  	char *filename_buf = NULL;
> >  	char *contents = NULL;
> >  	size_t contents_sz;
> > @@ -2469,8 +2469,7 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
> >  	 * The lock serves a purpose in addition to locking: the new
> >  	 * contents of .git/config will be written into it.
> >  	 */
> > -	lock = xcalloc(1, sizeof(struct lock_file));
> > -	fd = hold_lock_file_for_update(lock, config_filename, 0);
> > +	fd = hold_lock_file_for_update(&lock, config_filename, 0);
> >  	if (fd < 0) {
> >  		error_errno("could not lock config file %s", config_filename);
> >  		free(store.key);
> > @@ -2583,8 +2582,8 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
> >  		close(in_fd);
> >  		in_fd = -1;
> >  
> > -		if (chmod(get_lock_file_path(lock), st.st_mode & 07777) < 0) {
> > -			error_errno("chmod on %s failed", get_lock_file_path(lock));
> > +		if (chmod(get_lock_file_path(&lock), st.st_mode & 07777) < 0) {
> > +			error_errno("chmod on %s failed", get_lock_file_path(&lock));
> >  			ret = CONFIG_NO_WRITE;
> >  			goto out_free;
> >  		}
> > @@ -2639,28 +2638,19 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
> >  		contents = NULL;
> >  	}
> >  
> > -	if (commit_lock_file(lock) < 0) {
> > +	if (commit_lock_file(&lock) < 0) {
> >  		error_errno("could not write config file %s", config_filename);
> >  		ret = CONFIG_NO_WRITE;
> > -		lock = NULL;
> >  		goto out_free;
> >  	}
> >  
> > -	/*
> > -	 * lock is committed, so don't try to roll it back below.
> > -	 * NOTE: Since lockfile.c keeps a linked list of all created
> > -	 * lock_file structures, it isn't safe to free(lock).  It's
> > -	 * better to just leave it hanging around.
> > -	 */
> > -	lock = NULL;
> >  	ret = 0;
> >  
> >  	/* Invalidate the config cache */
> >  	git_config_clear();
> >  
> >  out_free:
> > -	if (lock)
> > -		rollback_lock_file(lock);
> > +	rollback_lock_file(&lock);
> >  	free(filename_buf);
> >  	if (contents)
> >  		munmap(contents, contents_sz);
> > @@ -2669,7 +2659,7 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
> >  	return ret;
> >  
> >  write_err_out:
> > -	ret = write_error(get_lock_file_path(lock));
> > +	ret = write_error(get_lock_file_path(&lock));
> >  	goto out_free;
> >  
> >  }
> > -- 
> > 2.14.1.721.gc5bc1565f1
> > 
> 
> -- 
> Brandon Williams

-- 
Brandon Williams

  reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-27  7:37 [PATCH] pkt-line: re-'static'-ify buffer in packet_write_fmt_1() Martin Ågren
2017-08-27 15:41 ` Jeff King
2017-08-27 18:25   ` Jeff King
2017-08-27 18:21 ` Lars Schneider
2017-08-27 19:09   ` Martin Ågren
2017-08-27 19:15     ` Jeff King
2017-08-27 20:04     ` Lars Schneider
2017-08-27 23:23       ` Jeff King
2017-08-28  4:11         ` Martin Ågren
2017-08-28 17:52           ` Stefan Beller
2017-08-28 17:58             ` Jeff King
2017-09-05 10:03               ` Junio C Hamano
2017-08-29 17:51           ` Lars Schneider
2017-08-29 18:53             ` Jeff King
2017-08-29 18:58               ` [PATCH] config: use a static lock_file struct Jeff King
2017-08-29 19:09                 ` Brandon Williams
2017-08-29 19:10                   ` Brandon Williams [this message]
2017-08-29 19:12                   ` Jeff King
2017-08-30  3:25                     ` Michael Haggerty
2017-08-30  4:31                       ` Jeff King
2017-08-30  4:55                         ` Michael Haggerty
2017-08-30  4:55                         ` Jeff King
2017-08-30  5:55                           ` Jeff King
2017-08-30  7:07                             ` Michael Haggerty
2017-09-02  8:44                               ` Jeff King
2017-09-02 13:50                                 ` Jeff King
2017-08-30  6:55                           ` Michael Haggerty
2017-08-30 19:53                             ` Jeff King
2017-08-30 19:57                               ` Brandon Williams
2017-08-30 20:11                                 ` Jeff King
2017-08-30 21:06                                   ` Brandon Williams
2017-08-31  4:09                                     ` Jeff King
2017-09-06  3:59                 ` Junio C Hamano
2017-09-06 12:41                   ` Jeff King
2017-08-29 19:22               ` [PATCH] pkt-line: re-'static'-ify buffer in packet_write_fmt_1() Martin Ågren
2017-08-29 21:48                 ` Jeff King
2017-08-30  5:31                   ` Jeff King
2017-09-05 10:03                     ` Junio C Hamano
2017-10-10  4:06                       ` [PATCH 0/2] Do not call cmd_*() as a subroutine Junio C Hamano
2017-10-10  4:06                         ` [PATCH 1/2] describe: do not use " Junio C Hamano
2017-10-10  4:06                         ` [PATCH 2/2] merge-ours: " Junio C Hamano

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20170829191051.GE131745@google.com \
    --to=bmwill@google.com \
    --cc=git@vger.kernel.org \
    --cc=larsxschneider@gmail.com \
    --cc=martin.agren@gmail.com \
    --cc=peff@peff.net \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

git@vger.kernel.org mailing list mirror (one of many)

Archives are clonable:
	git clone --mirror https://public-inbox.org/git
	git clone --mirror http://ou63pmih66umazou.onion/git
	git clone --mirror http://czquwvybam4bgbro.onion/git
	git clone --mirror http://hjrcffqmbrq6wope.onion/git

Newsgroups are available over NNTP:
	nntp://news.public-inbox.org/inbox.comp.version-control.git
	nntp://ou63pmih66umazou.onion/inbox.comp.version-control.git
	nntp://czquwvybam4bgbro.onion/inbox.comp.version-control.git
	nntp://hjrcffqmbrq6wope.onion/inbox.comp.version-control.git
	nntp://news.gmane.org/gmane.comp.version-control.git

 note: .onion URLs require Tor: https://www.torproject.org/
       or Tor2web: https://www.tor2web.org/

AGPL code for this site: git clone https://public-inbox.org/ public-inbox