Daniel BarkalowThis takes care of copying the original contents into the replacement file after the lock is held, so that concurrent additions can't miss each other's changes.
Signed-off-by: Daniel Barkalow <barkalow@iabervon.org> --- These two together replace [PATCH 5/11] from my earlier series; the lockfile API turned out not to have anything for O_APPEND (for the simple reason that it always creates a new replacement file). But appending is an obvious logical operation to support internally, especially since it actually has a clear semantics by itself for avoiding races.
cache.h | 1 + lockfile.c | 17 +++++++++++++++++ 2 files changed, 18 insertions(+), 0 deletions(-)
Show changes to 2 files +18 −0
cache.h, lockfile.c
diff --git a/cache.h b/cache.h index 2a1e7ec..4a113b0 100644 --- a/cache.h +++ b/cache.h @@ -391,6 +391,7 @@ struct lock_file { char filename[PATH_MAX]; }; extern int hold_lock_file_for_update(struct lock_file *, const char *path, int); +extern int hold_lock_file_for_append(struct lock_file *, const char *path, int); extern int commit_lock_file(struct lock_file *); extern int hold_locked_index(struct lock_file *, int); diff --git a/lockfile.c b/lockfile.c index 663f18f..95fd3d1 100644 --- a/lockfile.c +++ b/lockfile.c @@ -160,6 +160,23 @@ int hold_lock_file_for_update(struct lock_file *lk, const char *path, int die_on return fd; } +int hold_lock_file_for_append(struct lock_file *lk, const char *path, int die_on_error) +{ + int fd = lock_file(lk, path); + struct stat st; + if (!stat(path, &st)) { + int orig_fd = open(path, O_RDONLY); + size_t mmap_size = xsize_t(st.st_size); + void *mmap = xmmap(NULL, mmap_size, PROT_READ, MAP_PRIVATE, + orig_fd, 0); + write_or_die(fd, mmap, mmap_size); + munmap(mmap, mmap_size); + } + if (fd < 0 && die_on_error) + die("unable to create '%s.lock': %s", path, strerror(errno)); + return fd; +} + int close_lock_file(struct lock_file *lk) { int fd = lk->fd;
-- 1.5.4.3.610.gea6cd