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[34.168.22.137]) by smtp.gmail.com with ESMTPSA id q17-20020a62ae11000000b0057709fce782sm13860362pff.54.2023.02.02.08.34.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 02 Feb 2023 08:34:44 -0800 (PST) Sender: Junio C Hamano From: Junio C Hamano To: "brian m. carlson" Cc: =?utf-8?B?w4Z2YXIgQXJuZmrDtnLDsA==?= Bjarmason , git@vger.kernel.org, Eli Schwartz , =?utf-8?Q?Ren=C3=A9?= Scharfe , Konstantin Ryabitsev , Michal =?utf-8?Q?Such=C3=A1nek?= , "Raymond E . Pasco" , demerphq , Theodore Ts'o Subject: Re: [PATCH 9/9] git archive docs: document output non-stability References: <230131.86357rrtsg.gmgdl@evledraar.gmail.com> Date: Thu, 02 Feb 2023 08:34:44 -0800 In-Reply-To: (brian m. carlson's message of "Thu, 2 Feb 2023 10:25:08 +0000") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org "brian m. carlson" writes: >> +* We will do our best not to change the "tar" output itself, but won't >> + promise that we're never going to change it. >> ++ >> +If you must avoid using "git" itself for the tree validation, you >> +should be checksumming the uncompressed "tar" output, not e.g. the >> +compressed "tgz" output. >> ++ > > I don't think I want to state this, because it implies that the changes > I made that broke kernel.org (making tar.umask apply to pax headers) > wouldn't have been allowed. We should probably just state that "we > won't promise that the tar output won't change between versions". Maybe, > "We won't change the tar output needlessly, but it may change from time > to time." That is, we won't be "let's change the format just to mix it > up for users", but if there's a valuable patch that could be applied, > then we might well take it. I agree with you. Giving "will do our best not to" is still too strong for that. We won't change the format willy-nilly but when there is a good reason to do so, we should be able to fix or improve the output. >> +While you shouldn't assume that different versions of git will emit >> +the same output, you can assume (e.g. for the purposes of caching) >> +that a given version's output is stable. > > Unfortunately, this isn't actually true if someone uses export-subst. > That's because adding unrelated objects can increase the length of > abbreviations, and then the tar contents can be different. I've > actually seen this in the wild. "subst" is certainly an issue, especially when the substitution is unstable. There shouldn't be cross platform differences to break bit-for-bit stability at least for "tar" format, as we do not rely on any external library. Can we say the same for "zip"? I thought we throw the blob at git_deflate_*() so the exact bitstream is up to the libz implementation?