[PATCHv3 2/2] builtin/commit.c: switch to xstrfmt(), instead of snprintf,

[Elia Pinto <gitter.spiros@gmail.com>]

In this patch, instead of using xnprintf instead of snprintf, which asserts
that we don't truncate, we are switching to dynamic allocation with  xstrfmt(),
, so we can avoid dealing with magic numbers in the code and reduce the cognitive burden from
the programmers, because they no longer have to count bytes needed for static allocation.
As a side effect of this patch we have also reduced the snprintf() calls, that may silently truncate 
results if the programmer is not careful.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net> 
Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
---
This is the third  version of the patch.

Changes from the first version: I have split the original commit in two, as discussed here
http://public-inbox.org/git/20161213132717.42965-1-gitter.spiros@gmail.com/.

Changes from the second version:
- Changed the commit message to clarify the purpose of the patch (
as suggested by Junio)
https://public-inbox.org/git/xmqqtw95mfo3.fsf@gitster.mtv.corp.google.com/T/#m2e6405a8a78a8ca1ed770614c91398290574c4a1



 builtin/commit.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index 09bcc0f13..37228330c 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1526,12 +1526,10 @@ static int git_commit_config(const char *k, const char *v, void *cb)
 static int run_rewrite_hook(const unsigned char *oldsha1,
 			    const unsigned char *newsha1)
 {
-	/* oldsha1 SP newsha1 LF NUL */
-	static char buf[2*40 + 3];
+	char *buf;
 	struct child_process proc = CHILD_PROCESS_INIT;
 	const char *argv[3];
 	int code;
-	size_t n;
 
 	argv[0] = find_hook("post-rewrite");
 	if (!argv[0])
@@ -1547,11 +1545,11 @@ static int run_rewrite_hook(const unsigned char *oldsha1,
 	code = start_command(&proc);
 	if (code)
 		return code;
-	n = snprintf(buf, sizeof(buf), "%s %s\n",
-		     sha1_to_hex(oldsha1), sha1_to_hex(newsha1));
+	buf = xstrfmt("%s %s\n", sha1_to_hex(oldsha1), sha1_to_hex(newsha1));
 	sigchain_push(SIGPIPE, SIG_IGN);
-	write_in_full(proc.in, buf, n);
+	write_in_full(proc.in, buf, strlen(buf));
 	close(proc.in);
+	free(buf);
 	sigchain_pop(SIGPIPE);
 	return finish_command(&proc);
 }
-- 
2.11.0.154.g5f5f154