On Tue, Jun 30, 2015 at 9:26 AM, Jeff King <peff@xxxxxxxx> wrote:
> On Mon, Jun 29, 2015 at 06:22:47PM -0400, Eric Sunshine wrote:
>
>> Clients of strbuf rightly expect the buffer to grow as needed in
>> order to complete the requested operation. It is, therefore, both
>> weird and expectation-breaking for strbuf_addftime() to lack this
>> behavior. Worse, it doesn't even signal when the format has failed
>> due to insufficient buffer space.
>>
>> How about taking this approach (or something similar), instead, which
>> grows the strbuf as needed?
>
> Here's a patch, on top of jk/date-mode-format (I think it would also be
> fine to just squash into the tip commit; the explanation in the commit
> message is sufficiently mirrored in the code comment).
As a logical change in itself, I could also see introduction of
strbuf_addftime() split out into its own patch (with this patch
squashed in). Either way, it's a welcome improvement over the
original.
> -- >8 --
> Subject: [PATCH] strbuf: make strbuf_addftime more robust
>
> 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@xxxxxxxxxxxxxx>
> Signed-off-by: Jeff King <peff@xxxxxxxx>
> ---
> strbuf.c | 38 +++++++++++++++++++++-----------------
> t/t6300-for-each-ref.sh | 10 ++++++++++
> 2 files changed, 31 insertions(+), 17 deletions(-)
>
> diff --git a/strbuf.c b/strbuf.c
> index a7ba028..e5e7370 100644
> --- a/strbuf.c
> +++ b/strbuf.c
> @@ -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);
> }
> diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
> index c7f368c..7c9bec7 100755
> --- a/t/t6300-for-each-ref.sh
> +++ b/t/t6300-for-each-ref.sh
> @@ -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
> --
> 2.5.0.rc0.336.g8460790
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