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strbuf: make strbuf_addftime more robust

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The return value of strftime is poorly designed; when it
returns 0, the caller cannot tell if the buffer was not
large enough, or if the output was actually 0 bytes. In the
original implementation of strbuf_addftime, we simply punted
and guessed that our 128-byte hint would be large enough.

We can do better, though, if we're willing to treat strftime
like less of a black box. We can munge the incoming format
to make sure that it never produces 0-length output, and
then "fix" the resulting output.  That lets us reliably grow
the buffer based on strftime's return value.

Clever-idea-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jeff King 2015-06-30 09:26:53 -04:00 committed by Junio C Hamano
parent aa1462cc3d
commit e4f031e34b
2 changed files with 31 additions and 17 deletions
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38
strbuf.c
View file

@ -712,29 +712,33 @@ char *xstrfmt(const char *fmt, ...)
void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm)
{
size_t hint = 128;
size_t len;
/*
* strftime reports "0" if it could not fit the result in the buffer.
* Unfortunately, it also reports "0" if the requested time string
* takes 0 bytes. So if we were to probe and grow, we have to choose
* some arbitrary cap beyond which we guess that the format probably
* just results in a 0-length output. Since we have to choose some
* reasonable cap anyway, and since it is not that big, we may
* as well just grow to their in the first place.
*/
strbuf_grow(sb, 128);
if (!*fmt)
return;
strbuf_grow(sb, hint);
len = strftime(sb->buf + sb->len, sb->alloc - sb->len, fmt, tm);
if (!len) {
/*
* Either we failed, or the format actually produces a 0-length
* output. There's not much we can do, so we leave it blank.
* However, the output array is left in an undefined state, so
* we must re-assert our NUL terminator.
* strftime reports "0" if it could not fit the result in the buffer.
* Unfortunately, it also reports "0" if the requested time string
* takes 0 bytes. So our strategy is to munge the format so that the
* output contains at least one character, and then drop the extra
* character before returning.
*/
sb->buf[sb->len] = '\0';
} else {
sb->len += len;
struct strbuf munged_fmt = STRBUF_INIT;
strbuf_addf(&munged_fmt, "%s ", fmt);
while (!len) {
hint *= 2;
strbuf_grow(sb, hint);
len = strftime(sb->buf + sb->len, sb->alloc - sb->len,
munged_fmt.buf, tm);
}
strbuf_release(&munged_fmt);
len--; /* drop munged space */
}
strbuf_setlen(sb, sb->len + len);
}

10
t/t6300-for-each-ref.sh
View file

@ -235,6 +235,16 @@ test_expect_success 'Check format of strftime date fields' '
test_cmp expected actual
'
test_expect_success 'exercise strftime with odd fields' '
echo >expected &&
git for-each-ref --format="%(authordate:format:)" refs/heads >actual &&
test_cmp expected actual &&
long="long format -- $_z40$_z40$_z40$_z40$_z40$_z40$_z40" &&
echo $long >expected &&
git for-each-ref --format="%(authordate:format:$long)" refs/heads >actual &&
test_cmp expected actual
'
cat >expected <<\EOF
refs/heads/master
refs/remotes/origin/master