Re: [PATCH] builtin/commit.c: convert trivial snprintf calls to xsnprintf

On Tue, Dec 13, 2016 at 11:03:53AM -0800, Junio C Hamano wrote:

quoted

quoted

@@ -1525,12 +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])

@@ -1546,34 +1545,33 @@ 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);

Any time you care about the length of the result, I'd generally use an
actual strbuf instead of xstrfmt. The extra strlen isn't a big deal
here, but it somehow seems simpler to me. It probably doesn't matter
much either way, though.

Your justification for this extra allocation was that it is a
heavy-weight operation.  While I agree that the runtime cost of
allocation and deallocation does not matter, I would be a bit
worried about extra cognitive burden to programmers.  They did not
have to worry about leaking because they are writing a fixed length
string.  Now they do, whether they use xstrfmt() or struct strbuf.
When they need to update what they write, they do have to remember
to adjust the size of the "fixed string", and the original is not
free from the "programmers' cognitive cost" point of view, of
course.  Probably use of strbuf/xstrfmt is an overall win.

So I think you are agreeing, but I have a minor nit to pick. :)

The fact that the extra allocation will not hurt performance is
_necessary_, but not _sufficient_. So it's not a justification in
itself, only something we have to check before proceeding.

The only justification here is that magic numbers like "2*40 + 3" are
confusing and a potential maintenance burden. And that's why I suggested
splitting this one out from the other two (whose justification is
"PATH_MAX is sometimes too small").

I agree with you that it's a tradeoff between "magic numbers" versus
"having to free resources". In my opinion it's a net improvement, but I
think it would also be reasonable to switch to xsnprintf() here. Then
the programmer has an automatic check that the buffer size is
sufficient.

-Peff