replace unchecked snprintf calls with heap buffers

We'd prefer to avoid unchecked snprintf calls because
truncation can lead to unexpected results.

These are all cases where truncation shouldn't ever happen,
because the input to snprintf is fixed in size. That makes
them candidates for xsnprintf(), but it's simpler still to
just use the heap, and then nobody has to wonder if "100" is
big enough.

We'll use xstrfmt() where possible, and a strbuf when we need
the resulting size or to reuse the same buffer in a loop.

Signed-off-by: Jeff King <peff@peff.net>

bisect.c

@@ -200,6 +200,7 @@ static struct commit_list *best_bisection_sorted(struct commit_list *list, int n
 {
 	struct commit_list *p;
 	struct commit_dist *array = xcalloc(nr, sizeof(*array));
+	struct strbuf buf = STRBUF_INIT;
 	int cnt, i;
 
 	for (p = list, cnt = 0; p; p = p->next) {
@@ -217,17 +218,18 @@ static struct commit_list *best_bisection_sorted(struct commit_list *list, int n
 	}
 	QSORT(array, cnt, compare_commit_dist);
 	for (p = list, i = 0; i < cnt; i++) {
-		char buf[100]; /* enough for dist=%d */
 		struct object *obj = &(array[i].commit->object);
 
-		snprintf(buf, sizeof(buf), "dist=%d", array[i].distance);
-		add_name_decoration(DECORATION_NONE, buf, obj);
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "dist=%d", array[i].distance);
+		add_name_decoration(DECORATION_NONE, buf.buf, obj);
 
 		p->item = array[i].commit;
 		p = p->next;
 	}
 	if (p)
 		p->next = NULL;
+	strbuf_release(&buf);
 	free(array);
 	return list;
 }

builtin/index-pack.c

@@ -1443,10 +1443,11 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
 	if (!from_stdin) {
 		printf("%s\n", sha1_to_hex(sha1));
 	} else {
-		char buf[48];
-		int len = snprintf(buf, sizeof(buf), "%s\t%s\n",
-				   report, sha1_to_hex(sha1));
-		write_or_die(1, buf, len);
+		struct strbuf buf = STRBUF_INIT;
+
+		strbuf_addf(&buf, "%s\t%s\n", report, sha1_to_hex(sha1));
+		write_or_die(1, buf.buf, buf.len);
+		strbuf_release(&buf);
 
 		/*
 		 * Let's just mimic git-unpack-objects here and write

builtin/notes.c

@@ -554,7 +554,7 @@ static int append_edit(int argc, const char **argv, const char *prefix)
 	struct notes_tree *t;
 	unsigned char object[20], new_note[20];
 	const unsigned char *note;
-	char logmsg[100];
+	char *logmsg;
 	const char * const *usage;
 	struct note_data d = { 0, 0, NULL, STRBUF_INIT };
 	struct option options[] = {
@@ -618,17 +618,16 @@ static int append_edit(int argc, const char **argv, const char *prefix)
 		write_note_data(&d, new_note);
 		if (add_note(t, object, new_note, combine_notes_overwrite))
 			die("BUG: combine_notes_overwrite failed");
-		snprintf(logmsg, sizeof(logmsg), "Notes added by 'git notes %s'",
-			argv[0]);
+		logmsg = xstrfmt("Notes added by 'git notes %s'", argv[0]);
 	} else {
 		fprintf(stderr, _("Removing note for object %s\n"),
 			sha1_to_hex(object));
 		remove_note(t, object);
-		snprintf(logmsg, sizeof(logmsg), "Notes removed by 'git notes %s'",
-			argv[0]);
+		logmsg = xstrfmt("Notes removed by 'git notes %s'", argv[0]);
 	}
 	commit_notes(t, logmsg);
 
+	free(logmsg);
 	free_note_data(&d);
 	free_notes(t);
 	return 0;

builtin/rev-parse.c

@@ -213,13 +213,14 @@ static int show_abbrev(const unsigned char *sha1, void *cb_data)
 
 static void show_datestring(const char *flag, const char *datestr)
 {
-	static char buffer[100];
+	char *buffer;
 
 	/* date handling requires both flags and revs */
 	if ((filter & (DO_FLAGS | DO_REVS)) != (DO_FLAGS | DO_REVS))
 		return;
-	snprintf(buffer, sizeof(buffer), "%s%lu", flag, approxidate(datestr));
+	buffer = xstrfmt("%s%lu", flag, approxidate(datestr));
 	show(buffer);
+	free(buffer);
 }
 
 static int show_file(const char *arg, int output_prefix)