From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-3.7 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI shortcircuit=no autolearn=ham autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id 52F0D1F404 for ; Fri, 10 Aug 2018 19:47:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727208AbeHJWSi (ORCPT ); Fri, 10 Aug 2018 18:18:38 -0400 Received: from mail-pf1-f195.google.com ([209.85.210.195]:38747 "EHLO mail-pf1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727061AbeHJWSi (ORCPT ); Fri, 10 Aug 2018 18:18:38 -0400 Received: by mail-pf1-f195.google.com with SMTP id x17-v6so4989524pfh.5 for ; Fri, 10 Aug 2018 12:47:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:message-id:in-reply-to:references:from:subject:fcc :content-transfer-encoding:mime-version:to:cc; bh=n8sbfOUAjveeG9L5H4wg9M1dEWXDoj13RsKwshe1Jxs=; b=FDyHuKephWHKEokk8bX5axVTULnGkjxY8wVhPxTh/gXj1grnnONRPoz2nhm/YZFa/v bdtIdSGp2R35CZddrAatEPp4cJaHMVK4Ka2DMi55gkPnP7ccBT8kJnnxtxWOhx2g+6DG yvuQ3e0NwpldIC0Iv4BaZ5MN9tSepucgDcl4/0FJ3kl/GVDP75/Zn8PhjBzih8Y5vB/o 8EnFbNEo0nth2crD2Z1foREi486ek2F3ejbfjIeiH3CuNTNDTmSxgMAqPTFEkupf3wMx e0v51s5GUrRfOAEbneVeaF/Hbmzs+/5noEcKLJiLSLxb0I3wIHl50Y91QoV8guhg5ccx +1Lw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:message-id:in-reply-to:references:from :subject:fcc:content-transfer-encoding:mime-version:to:cc; bh=n8sbfOUAjveeG9L5H4wg9M1dEWXDoj13RsKwshe1Jxs=; b=W9RJkbiFizDims0WYmhlGpoHjTLC4eu0/Lij92pQWcWbLenjDPuWL4HbrtzA/1I8JH fcKIgRpF8fipmUnyxoukJxurzBOYuXloR0Oo5ruOde4zp4nzeIVpIv5Fd+uFd1uj6SHJ +iglhq/ihyrqV9ixg9WJzJL4Kxks8ZrUoA8bcFYgYWwQizTeUR60TDolq8guIna4qv+A AIJO/rCsimCokhRTqMeVYPSHsdZzN/5V9iLyrzXo3wRD3qHrrY1YDrMwnmt6+BCoRXuI WWnlJdTkRofmVhB6zcntxer8HnlkeYkuVesAq41p4n/wvUBCET6c+3L9S6exclHcQin7 vuWA== X-Gm-Message-State: AOUpUlFPmkTHq2qxD13b9Rg+Q7aSKNXv9bdATTA0DgtRLCx9nQQE9GoM o/Rmkf9SZY4mWlG2SX0hHvC7jQGo X-Google-Smtp-Source: AA+uWPyJa9C2nz4pty4iV6DNlZ1wXiNLIF3qXkByfdH75qLd2JOX5eYbai31lhFbDoqThVnle3IRSQ== X-Received: by 2002:a62:23c2:: with SMTP id q63-v6mr8333105pfj.91.1533930441290; Fri, 10 Aug 2018 12:47:21 -0700 (PDT) Received: from [127.0.0.1] ([40.112.142.204]) by smtp.gmail.com with ESMTPSA id o27-v6sm20211995pfj.35.2018.08.10.12.47.20 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 10 Aug 2018 12:47:20 -0700 (PDT) Date: Fri, 10 Aug 2018 12:47:20 -0700 (PDT) X-Google-Original-Date: Fri, 10 Aug 2018 19:47:13 GMT Message-Id: In-Reply-To: References: From: "Johannes Schindelin via GitGitGadget" Subject: [PATCH v2 1/4] Introduce a function to lock/unlock file descriptors when appending Fcc: Sent Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit MIME-Version: 1.0 To: git@vger.kernel.org Cc: Junio C Hamano , Johannes Schindelin Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org From: Johannes Schindelin This function will be used to make write accesses in trace_write() a bit safer. Note: this patch takes a very different approach for cross-platform support than Git is historically taking: the original approach is to first implement everything on Linux, using the functions available on Linux, and then trying to emulate these functions on platforms that do not have those functions such as Windows. This leads to all kinds of undesirable quirks in Git's source code (and performance characteristics) because of the lack of a proper abstraction layer: instead of declaring functions for the functionality we *actually* need, we abuse POSIX functions to say what we need, even if those functions serve much broader purposes (and do not make at all clear what the caller wants of them). For example, when Git wants to determine whether a path refers to a symbolic link, it calls lstat(). That POSIX function has to be emulated on Windows, painstakingly filling all the information lstat() would, only for the caller to throw away everything except that one bit of information, and all of the time figuring out the mtime/atime/ctime and file size and whatnot was spent in vain. If we were to follow that approach, we would extend the fcntl() emulation in compat/mingw.c after this commit, to support the function added in this commit. But fcntl() allows a lot more versatile file region locking that we actually need, so by necessity the fcntl() emulation would be quite complex: To support the `l_whence = SEEK_CUR` (which we would use, if it did not require so much book-keeping due to our writing between locking and unlocking the file), we would have to call `SetFilePointerEx()` (after obtaining the `HANDLE` on which all Win32 API functions work instead of the integer file descriptors used by all POSIX functions). Multiplying the number of Win32 API round-trips. Of course, we could implement an incomplete emulation of fcntl()'s F_WRLCK, but that would be unsatisfying. An alternative approach would be to use the `flock()` function, whose semantics are much closer to existing Win32 API. But `flock()` is not even a POSIX standard, so we would have to implement emulation of `flock()` for *other* platforms. And it would again be the wrong thing to do, as we would once again fail to have an abstraction that clearly says what *exact*functionality we want to use. This commit expressly tries to set a precedent for a better approach: Let's introduce a proper abstraction: a function that says in its name precisely what Git wants it to do (as opposed to *how* it does it on Linux): lock_or_unlock_fd_for_appending(). The next commit will provide a Windows-specific implementation of this function/functionality. Signed-off-by: Johannes Schindelin --- git-compat-util.h | 2 ++ wrapper.c | 16 ++++++++++++++++ 2 files changed, 18 insertions(+) diff --git a/git-compat-util.h b/git-compat-util.h index 9a64998b2..13b83bade 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -1202,6 +1202,8 @@ struct tm *git_gmtime_r(const time_t *, struct tm *); #define getc_unlocked(fh) getc(fh) #endif +extern int lock_or_unlock_fd_for_appending(int fd, int lock_it); + /* * Our code often opens a path to an optional file, to work on its * contents when we can successfully open it. We can ignore a failure diff --git a/wrapper.c b/wrapper.c index e4fa9d84c..5aecbda34 100644 --- a/wrapper.c +++ b/wrapper.c @@ -690,3 +690,19 @@ int xgethostname(char *buf, size_t len) buf[len - 1] = 0; return ret; } + +#ifndef GIT_WINDOWS_NATIVE +int lock_or_unlock_fd_for_appending(int fd, int lock_it) +{ + struct flock flock; + + flock.l_type = lock_it ? F_WRLCK : F_UNLCK; + + /* (un-)lock the whole file */ + flock.l_whence = SEEK_SET; + flock.l_start = 0; + flock.l_len = 0; + + return fcntl(fd, F_SETLKW, &flock); +} +#endif -- gitgitgadget