[PATCH v2 1/3] merge-ll: use strbuf to read back external merge result

[Jeff King <peff@peff.net>]

After the external merge runs, we read the file back into a heap buffer.
This ancient code does it by hand, but these days we can make the code
shorter and less error prone by using strbuf_read_file().

It's not quite a one-liner replacement, because we have to copy the
pointer and size into an mmbuffer_t. Two things to note there:

  1. We can't just pass result->size to strbuf_detach(), since the
     former uses long instead of size_t (something that we'd ideally fix
     in the long run, but is way out of scope here).

  2. We can leave result untouched on error; we zero it at the top of
     the function (confusingly we may still return LL_MERGE_OK and a
     NULL result if we hit an I/O error, but that is how the function
     has always behaved, and callers know to check for NULL).

Signed-off-by: Jeff King <peff@peff.net>
---
Not strictly needed for the rest of the series, but it felt like a
cleanup worth doing, and it conflicts textually.

 merge-ll.c | 22 +++++++---------------
 1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/merge-ll.c b/merge-ll.c
index ef5287dee8..5b6af15e23 100644
--- a/merge-ll.c
+++ b/merge-ll.c
@@ -201,8 +201,8 @@ static enum ll_merge_result ll_ext_merge(const struct ll_merge_driver *fn,
 	struct strbuf cmd = STRBUF_INIT;
 	const char *format = fn->cmdline;
 	struct child_process child = CHILD_PROCESS_INIT;
-	int status, fd, i;
-	struct stat st;
+	int status, i;
+	struct strbuf result_buf = STRBUF_INIT;
 	enum ll_merge_result ret;
 	assert(opts);
 
@@ -241,20 +241,12 @@ static enum ll_merge_result ll_ext_merge(const struct ll_merge_driver *fn,
 	child.use_shell = 1;
 	strvec_push(&child.args, cmd.buf);
 	status = run_command(&child);
-	fd = open(temp[1], O_RDONLY);
-	if (fd < 0)
-		goto bad;
-	if (fstat(fd, &st))
-		goto close_bad;
-	result->size = st.st_size;
-	result->ptr = xmallocz(result->size);
-	if (read_in_full(fd, result->ptr, result->size) != result->size) {
-		FREE_AND_NULL(result->ptr);
-		result->size = 0;
+
+	if (strbuf_read_file(&result_buf, temp[1], 0) >= 0) {
+		result->size = result_buf.len;
+		result->ptr = strbuf_detach(&result_buf, NULL);
 	}
- close_bad:
-	close(fd);
- bad:
+
 	for (i = 0; i < 3; i++)
 		unlink_or_warn(temp[i]);
 	strbuf_release(&cmd);
-- 
2.56.0.rc0.314.g7a874b6915