The write_message() function safely writes an strbuf to a file.
Sometimes it is inconvenient to require an strbuf, though: the text to
be written may not be stored in a strbuf, or the strbuf should not be
released after writing.

Let's refactor "safely writing string to a file" into
write_with_lock_file(), and make write_message() use it.

The new function will make it easy to create a new convenience function
write_file_gently() that does not die(); as some of the upcoming callers
of this new function will want to append a newline character, we already
add that flag as parameter to write_with_lock_file().

While at it, roll back the locked files in case of failure, as pointed
out by Hannes Sixt.

Signed-off-by: Johannes Schindelin <johannes.schinde...@gmx.de>
---
 sequencer.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 914a576..baccee9 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -234,22 +234,37 @@ static void print_advice(int show_hint, struct 
replay_opts *opts)
        }
 }
 
-static int write_message(struct strbuf *msgbuf, const char *filename)
+static int write_with_lock_file(const char *filename,
+                               const void *buf, size_t len, int append_eol)
 {
        static struct lock_file msg_file;
 
        int msg_fd = hold_lock_file_for_update(&msg_file, filename, 0);
        if (msg_fd < 0)
                return error_errno(_("Could not lock '%s'"), filename);
-       if (write_in_full(msg_fd, msgbuf->buf, msgbuf->len) < 0)
-               return error_errno(_("Could not write to %s"), filename);
-       strbuf_release(msgbuf);
-       if (commit_lock_file(&msg_file) < 0)
+       if (write_in_full(msg_fd, buf, len) < 0) {
+               rollback_lock_file(&msg_file);
+               return error_errno(_("Could not write to '%s'"), filename);
+       }
+       if (append_eol && write(msg_fd, "\n", 1) < 0) {
+               rollback_lock_file(&msg_file);
+               return error_errno(_("Could not write eol to '%s"), filename);
+       }
+       if (commit_lock_file(&msg_file) < 0) {
+               rollback_lock_file(&msg_file);
                return error(_("Error wrapping up %s."), filename);
+       }
 
        return 0;
 }
 
+static int write_message(struct strbuf *msgbuf, const char *filename)
+{
+       int res = write_with_lock_file(filename, msgbuf->buf, msgbuf->len, 0);
+       strbuf_release(msgbuf);
+       return res;
+}
+
 /*
  * Reads a file that was presumably written by a shell script, i.e.
  * with an end-of-line marker that needs to be stripped.
-- 
2.10.1.513.g00ef6dd


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