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@xxxxxxxx>
> > ---
> > 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
