Re: [PATCH 2/8] Add a lockfile function to append to a file

This 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 <redacted>
---
 cache.h    |    1 +
 lockfile.c |   17 +++++++++++++++++
 2 files changed, 18 insertions(+), 0 deletions(-)

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));