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[34.168.92.83]) by smtp.gmail.com with ESMTPSA id u14-20020aa7848e000000b0061a829d2679sm281161pfn.37.2023.03.15.12.34.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Mar 2023 12:34:10 -0700 (PDT) Sender: Junio C Hamano From: Junio C Hamano To: Jeff King Cc: Derrick Stolee , Derrick Stolee via GitGitGadget , git@vger.kernel.org, me@ttaylorr.com, vdye@github.com Subject: Re: [PATCH v2 0/8] ref-filter: ahead/behind counting, faster --merged option References: <74656b93-8c67-ffe3-4aea-4c184345384d@github.com> Date: Wed, 15 Mar 2023 12:34:10 -0700 In-Reply-To: (Jeff King's message of "Wed, 15 Mar 2023 13:31:27 -0400") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org Jeff King writes: > E.g., imagine resolving "main" to 1234abcd in step one, then somebody > updates it to 5678cdef, then you run "for-each-ref" to compute > ahead/behind, and now you show an inconsistent result: you say that > "main" points to 1234abcd, but show the wrong ahead/behind information. > > Showing 1234abcd at all is out-of-date, of course, but the real problem > is the lack of atomicity. Most porcelain scripts deal with this by > resolving the refs immediately, assuming object ids are immutable (which > they are modulo games like refs/replace), and then working with them. A really paranoid caller can use %(ahead-behind-detail:refs/heads/main) and get a report on refs/heads/topic, something that conveys refs/heads/topic (at 67f9f40d) is ahead by 2 commits and behind by 4 commits relative to refs/heads/main (at d7c3a768). in a machine readable form. And when the "ahead by 2 commits" disappears, we know 67f9f40d is merged to main sometime before d7c3a768. Then it can say "update-ref -d refs/heads/topic 67f9f40d" to avoid racing with simultanous updaters.