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[104.178.186.189]) by smtp.gmail.com with ESMTPSA id k14sm11048824ili.19.2021.08.31.19.06.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Aug 2021 19:06:27 -0700 (PDT) Date: Tue, 31 Aug 2021 22:06:26 -0400 From: Taylor Blau To: git@vger.kernel.org Cc: peff@peff.net Subject: [PATCH 1/2] pack-write.c: rename `.idx` file into place last 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 We treat the presence of an `.idx` file as the indicator of whether or not it's safe to use a packfile. But `finish_tmp_packfile()` (which is responsible for writing the pack and moving the temporary versions of all of its auxiliary files into place) is inconsistent about the write order. But the `.rev` file is moved into place after the `.idx`, so it's possible for a process to open a pack in between the two (i.e., while the `.idx` file is in place but the `.rev` file is not) and mistakenly fall back to generating the pack's reverse index in memory. Though racy, this amounts to an unnecessary slow-down at worst, and doesn't affect the correctness of the resulting reverse index. Close this race by moving the .rev file into place before moving the .idx file into place. While we're here, only call strbuf_setlen() if we actually modified the buffer by bringing it inside of the same if-statement that calls rename(). Signed-off-by: Taylor Blau --- The diff is a little inscrutable here, since it shows the .idx hunk moving below the .rev hunk (instead of the .rev hunk moving above the .idx one), but the end-result is the same. pack-write.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/pack-write.c b/pack-write.c index f1fc3ecafa..277c60165e 100644 --- a/pack-write.c +++ b/pack-write.c @@ -490,18 +490,18 @@ void finish_tmp_packfile(struct strbuf *name_buffer, strbuf_setlen(name_buffer, basename_len); - strbuf_addf(name_buffer, "%s.idx", hash_to_hex(hash)); - if (rename(idx_tmp_name, name_buffer->buf)) - die_errno("unable to rename temporary index file"); - - strbuf_setlen(name_buffer, basename_len); - if (rev_tmp_name) { strbuf_addf(name_buffer, "%s.rev", hash_to_hex(hash)); if (rename(rev_tmp_name, name_buffer->buf)) die_errno("unable to rename temporary reverse-index file"); + + strbuf_setlen(name_buffer, basename_len); } + strbuf_addf(name_buffer, "%s.idx", hash_to_hex(hash)); + if (rename(idx_tmp_name, name_buffer->buf)) + die_errno("unable to rename temporary index file"); + strbuf_setlen(name_buffer, basename_len); free((void *)idx_tmp_name); -- 2.33.0.96.g73915697e6