Subject: Re: [PATCH] config: use a static lock_file struct

From: Brandon Williams <[email-masked]>
Date: Tue, 29 Aug 2017 12:10:51 -0700

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 <[email-masked]>
> > ---
> > 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