From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 638DCC433EF for ; Tue, 2 Nov 2021 01:43:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 42D1160F56 for ; Tue, 2 Nov 2021 01:43:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229699AbhKBBp7 (ORCPT ); Mon, 1 Nov 2021 21:45:59 -0400 Received: from cloud.peff.net ([104.130.231.41]:51228 "EHLO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229479AbhKBBp7 (ORCPT ); Mon, 1 Nov 2021 21:45:59 -0400 Received: (qmail 1265 invoked by uid 109); 2 Nov 2021 01:43:25 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with ESMTP; Tue, 02 Nov 2021 01:43:25 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 29490 invoked by uid 111); 2 Nov 2021 01:43:25 -0000 Received: from coredump.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.2) by peff.net (qpsmtpd/0.94) with (TLS_AES_256_GCM_SHA384 encrypted) ESMTPS; Mon, 01 Nov 2021 21:43:25 -0400 Authentication-Results: peff.net; auth=none Date: Mon, 1 Nov 2021 21:43:23 -0400 From: Jeff King To: Junio C Hamano Cc: Dongsheng Song , Git Mailing List Subject: Re: timezone related bug of git Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Mon, Nov 01, 2021 at 11:18:02AM -0700, Junio C Hamano wrote: > I have to wonder why gm_time_t() needs to use two separate codepaths > for positive and negative tz_offset, while the new code here can get > away without. Does it have something to do with the direction of > truncation during division and modulo operation? Hmm. Unless I am missing something, this part of gm_time_t() is simply over-complicating things: minutes = tz < 0 ? -tz : tz; minutes = (minutes / 100)*60 + (minutes % 100); minutes = tz < 0 ? -minutes : minutes; We switch to doing the computation in absolute-value units, but then restore the sign. But just: minutes = (tz / 100) * 60 + (tz % 100); is equivalent and shorter. If tz is negative, then both terms will be negative, which is what you want (they sum to a larger absolute-value negative number). This comes from f80cd783c6 (date.c: add "show_date()" function., 2005-05-06), so I don't see any sign that there was specific thought given to some obscure handling. And indeed later fixes like fbab835c03 ([PATCH] fix show_date() for positive timezones, 2005-05-18) imply to me that the original was just confused. Later we do: if (minutes > 0) { if (unsigned_add_overflows(time, minutes * 60)) die("Timestamp+tz too large: %"PRItime" +%04d", time, tz); } else if (time < -minutes * 60) die("Timestamp before Unix epoch: %"PRItime" %04d", time, tz); And that does need separate paths for the overflow check, since we're checking different boundaries. I suspect for the strftime() code that we wouldn't need similar checks, because the earlier ones would have caught any problems (i.e., we would not get as far as having a "struct tm" that represented something outside the range of our time_t). -Peff