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Taylor Blau 95e8383bac midx.c: make changing the preferred pack safe 
The previous patch demonstrates a bug where a MIDX's auxiliary object
order can become out of sync with a MIDX bitmap.

This is because of two confounding factors:

  - First, the object order is stored in a file which is named according
    to the multi-pack index's checksum, and the MIDX does not store the
    object order. This means that the object order can change without
    altering the checksum.

  - But the .rev file is moved into place with finalize_object_file(),
    which link(2)'s the file into place instead of renaming it. For us,
    that means that a modified .rev file will not be moved into place if
    MIDX's checksum was unchanged.

This fix is to force the MIDX's checksum to change when the preferred
pack changes but the set of packs contained in the MIDX does not. In
other words, when the object order changes, the MIDX's checksum needs to
change with it (regardless of whether the MIDX is tracking the same or
different packs).

This prevents a race whereby changing the object order (but not the
packs themselves) enables a reader to see the new .rev file with the old
MIDX, or similarly seeing the new bitmap with the old object order.

But why can't we just stop hardlinking the .rev into place instead
adding additional data to the MIDX? Suppose that's what we did. Then
when we go to generate the new bitmap, we'll load the old MIDX bitmap,
along with the MIDX that it references. That's fine, since the new MIDX
isn't moved into place until after the new bitmap is generated. But the
new object order *has* been moved into place. So we'll read the old
bitmaps in the new order when generating the new bitmap file, meaning
that without this secondary change, bitmap generation itself would
become a victim of the race described here.

This can all be prevented by forcing the MIDX's checksum to change when
the object order does. By embedding the entire object order into the
MIDX, we do just that. That is, the MIDX's checksum will change in
response to any perturbation of the underlying object order. In t5326,
this will cause the MIDX's checksum to update (even without changing the
set of packs in the MIDX), preventing the stale read problem.

Note that this makes it safe to continue to link(2) the MIDX .rev file
into place, since it is now impossible to have a .rev file that is
out-of-sync with the MIDX whose checksum it references. (But we will do
away with MIDX .rev files later in this series anyway, so this is
somewhat of a moot point).

In theory, it is possible to store a "fingerprint" of the full object
order here, so long as that fingerprint changes at least as often as the
full object order does. Some possibilities here include storing the
identity of the preferred pack, along with the mtimes of the
non-preferred packs in a consistent order. But storing a limited part of
the information makes it difficult to reason about whether or not there
are gaps between the two that would cause us to get bitten by this bug
again.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2022-01-27 12:07:52 -08:00
..
config Merge branch 'fs/ssh-signing-other-keytypes' 2021-12-21 15:03:16 -08:00
howto Document how we do embargoed releases 2021-03-27 15:13:12 -07:00
RelNotes The sixth batch 2022-01-03 16:24:15 -08:00
technical midx.c: make changing the preferred pack safe 2022-01-27 12:07:52 -08:00
.gitattributes
.gitignore doc lint: make "lint-docs" non-.PHONY 2021-10-15 10:29:11 -07:00
asciidoc.conf
asciidoctor-extensions.rb
blame-options.txt Merge branch 'bs/doc-blame-color-lines' 2021-10-18 15:47:58 -07:00
build-docdep.perl
cat-texi.perl
cmd-list.perl git.txt: add list of guides 2020-08-04 18:34:02 -07:00
CodingGuidelines CodingGuidelines: document which output goes to stdout vs. stderr 2021-12-04 17:26:41 -08:00
config.txt Merge branch 're/color-default-reset' 2021-12-15 09:39:53 -08:00
date-formats.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
diff-format.txt Documentation/diff-format: state in which cases porcelain status is T 2021-10-04 13:07:18 -07:00
diff-generate-patch.txt doc: point to diff attribute in patch format docs 2021-04-28 13:34:44 +09:00
diff-options.txt Merge branch 'jn/log-m-does-not-imply-p' 2021-08-11 12:36:18 -07:00
doc-diff
docbook-xsl.css
docbook.xsl
everyday.txto
fetch-options.txt fetch: document the --negotiate-only option 2021-06-30 14:57:22 -07:00
fix-texi.perl
git-add.txt add: implement the --sparse option 2021-09-28 10:31:02 -07:00
git-am.txt git-am.txt: clarify --abort behavior 2021-09-10 15:51:14 -07:00
git-annotate.txt
git-apply.txt git-apply: add --allow-empty flag 2021-12-13 14:30:25 -08:00
git-archimport.txt doc: split placeholders as individual tokens 2021-10-28 09:57:09 -07:00
git-archive.txt archive: describe compression level option 2021-10-25 10:08:23 -07:00
git-bisect-lk2009.txt git-bisect-lk2009: make continuation of list indented 2020-10-08 14:01:15 -07:00
git-bisect.txt bisect: swap command-line options in documentation 2020-08-28 14:06:06 -07:00
git-blame.txt Merge branch 'bs/doc-blame-color-lines' 2021-10-18 15:47:58 -07:00
git-branch.txt branch (doc): -m/-c copies config and reflog 2021-10-23 17:12:41 -07:00
git-bugreport.txt Documentation: fix default directory of git bugreport -o 2021-09-07 14:25:13 -07:00
git-bundle.txt git-bundle.txt: add missing words and punctuation 2021-10-27 17:06:12 -07:00
git-cat-file.txt cat-file: disable refs/replace with --batch-all-objects 2021-10-08 15:45:14 -07:00
git-check-attr.txt
git-check-ignore.txt
git-check-mailmap.txt check-mailmap doc: note config options 2021-01-12 14:04:40 -08:00
git-check-ref-format.txt
git-checkout-index.txt
git-checkout.txt Merge branch 'en/zdiff3' 2021-12-15 09:39:47 -08:00
git-cherry-pick.txt doc: express grammar placeholders between angle brackets 2021-11-09 09:39:11 -08:00
git-cherry.txt
git-citool.txt
git-clean.txt
git-clone.txt Merge branch 'en/sparse-checkout-set' 2022-01-03 16:24:15 -08:00
git-column.txt column: fix parsing of the '--nl' option 2021-08-26 14:36:27 -07:00
git-commit-graph.txt Merge branch 'ds/commit-graph-merging-fix' 2020-11-02 13:17:39 -08:00
git-commit-tree.txt
git-commit.txt Document positive variant of commit and merge option "--no-verify" 2021-10-29 11:22:56 -07:00
git-config.txt doc: express grammar placeholders between angle brackets 2021-11-09 09:39:11 -08:00
git-count-objects.txt
git-credential-cache--daemon.txt
git-credential-cache.txt
git-credential-store.txt
git-credential.txt doc: fix git credential synopsis 2021-10-28 09:57:09 -07:00
git-cvsexportcommit.txt doc: express grammar placeholders between angle brackets 2021-11-09 09:39:11 -08:00
git-cvsimport.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
git-cvsserver.txt Documentation: cleanup git-cvsserver 2021-09-16 20:47:48 -07:00
git-daemon.txt
git-describe.txt describe-doc: clarify default length of abbreviation 2021-05-17 15:56:29 +09:00
git-diff-files.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
git-diff-index.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
git-diff-tree.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
git-diff.txt git-diff: fix missing --merge-base docs 2021-07-12 13:55:29 -07:00
git-difftool.txt difftool.c: learn a new way start at specified file 2021-02-22 13:35:49 -08:00
git-fast-export.txt fast-export: allow seeding the anonymized mapping 2020-06-25 14:19:23 -07:00
git-fast-import.txt fast-import: fix typo in documentation 2020-10-04 12:56:29 -07:00
git-fetch-pack.txt
git-fetch.txt Documentation: render special characters correctly 2021-07-30 09:08:12 -07:00
git-filter-branch.txt filter-branch doc: fix filter-repo typo 2020-10-20 12:22:25 -07:00
git-fmt-merge-msg.txt
git-for-each-ref.txt ref-filter: --format=%(raw) support --perl 2021-07-26 12:01:25 -07:00
git-for-each-repo.txt for-each-repo: run subcommands on configured repos 2020-09-25 10:59:44 -07:00
git-format-patch.txt format-patch (doc): clarify --base=auto 2021-10-23 14:33:20 -07:00
git-fsck-objects.txt
git-fsck.txt doc: use three dots for indicating repetition instead of star 2021-11-09 09:39:11 -08:00
git-gc.txt docs: clarify that refs/notes/ do not keep the attached objects alive 2021-02-10 23:43:55 -08:00
git-get-tar-commit-id.txt
git-grep.txt docs: fix linting issues due to incorrect relative section order 2021-04-10 23:36:34 -07:00
git-gui.txt doc: express grammar placeholders between angle brackets 2021-11-09 09:39:11 -08:00
git-hash-object.txt
git-help.txt Merge branch 'ja/doc-cleanup' 2021-12-10 14:35:03 -08:00
git-http-backend.txt docs/http-backend: mention v2 protocol 2021-09-10 15:34:59 -07:00
git-http-fetch.txt doc: uniformize <URL> placeholders' case 2021-11-09 09:39:11 -08:00
git-http-push.txt doc: git-http-push: describe the refs as pattern pairs 2021-11-09 09:39:11 -08:00
git-imap-send.txt git-imap-send.txt: add note about localized Gmail folders 2020-08-31 11:44:33 -07:00
git-index-pack.txt index-pack: add --progress-title option 2021-09-07 10:59:23 -07:00
git-init-db.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
git-init.txt init doc: --shared=0xxx does not give umask but perm bits 2021-11-09 09:39:11 -08:00
git-instaweb.txt
git-interpret-trailers.txt trailer: add new .cmd config option 2021-05-04 12:09:43 +09:00
git-log.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
git-ls-files.txt doc: git-ls-files: express options as optional alternatives 2021-11-09 09:39:11 -08:00
git-ls-remote.txt docs: adjust for the recent rename of pu to seen 2020-06-25 09:18:53 -07:00
git-ls-tree.txt doc: clarify that --abbrev=<n> is about the minimum length 2020-11-04 14:04:44 -08:00
git-mailinfo.txt mailinfo: allow stripping quoted CR without warning 2021-05-10 15:06:22 +09:00
git-mailsplit.txt
git-maintenance.txt maintenance: add support for systemd timers on Linux 2021-09-07 10:57:04 -07:00
git-merge-base.txt
git-merge-file.txt update documentation for new zdiff3 conflictStyle 2021-12-01 14:45:59 -08:00
git-merge-index.txt doc: use three dots for indicating repetition instead of star 2021-11-09 09:39:11 -08:00
git-merge-one-file.txt
git-merge-tree.txt
git-merge.txt update documentation for new zdiff3 conflictStyle 2021-12-01 14:45:59 -08:00
git-mergetool--lib.txt mergetool: break setup_tool out into separate initialization function 2021-02-09 14:09:16 -08:00
git-mergetool.txt doc: describe mergetool configuration in git-mergetool(1) 2021-03-13 15:34:32 -08:00
git-mktag.txt docs: fix linting issues due to incorrect relative section order 2021-04-10 23:36:34 -07:00
git-mktree.txt
git-multi-pack-index.txt git-multi-pack-index.txt: change "folder" to "directory" 2021-10-25 11:06:56 -07:00
git-mv.txt
git-name-rev.txt
git-notes.txt docs: improve the example that illustrates git-notes path names 2020-08-03 12:40:09 -07:00
git-p4.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
git-pack-objects.txt doc: express grammar placeholders between angle brackets 2021-11-09 09:39:11 -08:00
git-pack-redundant.txt doc: express grammar placeholders between angle brackets 2021-11-09 09:39:11 -08:00
git-pack-refs.txt
git-patch-id.txt
git-prune-packed.txt
git-prune.txt
git-pull.txt Merge branch 'js/retire-preserve-merges' 2021-10-18 15:47:56 -07:00
git-push.txt Merge branch 'ds/gender-neutral-doc' 2021-07-16 17:42:53 -07:00
git-quiltimport.txt
git-range-diff.txt Merge branch 'js/range-diff-one-side-only' 2021-02-17 17:21:41 -08:00
git-read-tree.txt Documentation: call out commands that nuke untracked files/directories 2021-09-27 13:38:37 -07:00
git-rebase.txt update documentation for new zdiff3 conflictStyle 2021-12-01 14:45:59 -08:00
git-receive-pack.txt upload-pack: document and rename --advertise-refs 2021-08-05 08:59:37 -07:00
git-reflog.txt doc: express grammar placeholders between angle brackets 2021-11-09 09:39:11 -08:00
git-remote-ext.txt
git-remote-fd.txt
git-remote-helpers.txto
git-remote.txt doc: uniformize <URL> placeholders' case 2021-11-09 09:39:11 -08:00
git-repack.txt builtin/repack.c: make largest pack preferred 2021-09-28 21:20:56 -07:00
git-replace.txt
git-request-pull.txt doc: uniformize <URL> placeholders' case 2021-11-09 09:39:11 -08:00
git-rerere.txt
git-reset.txt Documentation: call out commands that nuke untracked files/directories 2021-09-27 13:38:37 -07:00
git-restore.txt update documentation for new zdiff3 conflictStyle 2021-12-01 14:45:59 -08:00
git-rev-list.txt docs/rev-list: add some examples of --disk-usage 2021-02-17 16:25:29 -08:00
git-rev-parse.txt rev-parse: add option for absolute or relative path formatting 2020-12-12 23:35:51 -08:00
git-revert.txt Documentation: stylistically normalize references to Signed-off-by: 2020-10-20 11:57:40 -07:00
git-rm.txt rm: add --sparse option 2021-09-28 10:31:02 -07:00
git-send-email.txt send-email docs: add format-patch options 2021-10-28 09:06:15 -07:00
git-send-pack.txt send-pack: properly use parse_options() API for usage string 2021-09-12 18:57:30 -07:00
git-sh-i18n--envsubst.txt
git-sh-i18n.txt
git-sh-setup.txt
git-shell.txt
git-shortlog.txt doc: use only hyphens as word separators in placeholders 2021-11-09 09:39:11 -08:00
git-show-branch.txt
git-show-index.txt Documentation: mark --object-format=sha256 as experimental 2020-08-17 10:50:14 -07:00
git-show-ref.txt
git-show.txt doc/git-show: include --diff-merges description 2020-12-21 13:47:32 -08:00
git-sparse-checkout.txt Merge branch 'en/sparse-checkout-set' 2022-01-03 16:24:15 -08:00
git-stage.txt doc: express grammar placeholders between angle brackets 2021-11-09 09:39:11 -08:00
git-stash.txt stash: implement '--staged' option for 'push' and 'save' 2021-10-18 13:09:21 -07:00
git-status.txt status: print stash info with --porcelain=v2 --show-stash 2021-10-21 17:24:30 -07:00
git-stripspace.txt
git-submodule.txt submodule: fall back to remote's HEAD for missing remote.<name>.branch 2020-06-24 09:14:21 -07:00
git-svn.txt Merge branch 'ja/doc-cleanup' 2021-12-10 14:35:03 -08:00
git-switch.txt update documentation for new zdiff3 conflictStyle 2021-12-01 14:45:59 -08:00
git-symbolic-ref.txt
git-tag.txt Documentation: typofix --column description 2021-02-19 19:36:47 -08:00
git-tools.txt
git-unpack-file.txt
git-unpack-objects.txt
git-update-index.txt doc: remove "directory cache" from man pages 2021-01-09 22:57:24 -08:00
git-update-ref.txt update-ref: allow creation of multiple transactions 2020-11-16 13:44:01 -08:00
git-update-server-info.txt
git-upload-archive.txt
git-upload-pack.txt Merge branch 'jk/http-server-protocol-versions' 2021-09-23 13:44:47 -07:00
git-var.txt var: add GIT_DEFAULT_BRANCH variable 2021-11-03 13:25:36 -07:00
git-verify-commit.txt
git-verify-pack.txt
git-verify-tag.txt
git-version.txt documentation: add documentation for 'git version' 2021-09-14 10:05:40 -07:00
git-web--browse.txt doc: split placeholders as individual tokens 2021-10-28 09:57:09 -07:00
git-whatchanged.txt
git-worktree.txt git-worktree.txt: add missing -v to synopsis for worktree list 2021-12-04 23:27:25 -08:00
git-write-tree.txt
git.txt Merge branch 'if/redact-packfile-uri' 2021-12-10 14:35:04 -08:00
gitattributes.txt docs: document symlink restrictions for dot-files 2021-05-04 11:52:03 +09:00
gitcli.txt
gitcore-tutorial.txt
gitcredentials.txt doc: uniformize <URL> placeholders' case 2021-11-09 09:39:11 -08:00
gitcvs-migration.txt
gitdiffcore.txt Merge branch 'en/ort-perf-batch-8' 2021-03-22 14:00:24 -07:00
giteveryday.txt docs: adjust for the recent rename of pu to seen 2020-06-25 09:18:53 -07:00
gitfaq.txt Update docs for change of default merge backend 2021-08-05 15:35:02 -07:00
gitglossary.txt
githooks.txt githooks.txt: clarify documentation on reference-transaction hook 2021-03-01 09:02:01 -08:00
gitignore.txt gitignore.txt: change "folder" to "directory" 2021-10-25 11:06:56 -07:00
gitk.txt doc: log, gitk: move '-L' description to 'line-range-options.txt' 2020-11-01 15:54:14 -08:00
gitmailmap.txt docs: document symlink restrictions for dot-files 2021-05-04 11:52:03 +09:00
gitmodules.txt docs: document symlink restrictions for dot-files 2021-05-04 11:52:03 +09:00
gitnamespaces.txt doc lint: lint and fix missing "GIT" end sections 2021-04-10 23:36:34 -07:00
gitremote-helpers.txt Merge branch 'bc/sha-256-part-2' 2020-07-06 22:09:13 -07:00
gitrepository-layout.txt
gitrevisions.txt
gitsubmodules.txt doc: uniformize <URL> placeholders' case 2021-11-09 09:39:11 -08:00
gittutorial-2.txt
gittutorial.txt Documentation: render special characters correctly 2021-07-30 09:08:12 -07:00
gitweb.conf.txt gitweb: add "e-mail privacy" feature to redact e-mail addresses 2021-04-08 15:54:26 -07:00
gitweb.txt gitweb.txt: change "folder" to "directory" 2021-10-25 11:06:57 -07:00
gitworkflows.txt doc: uniformize <URL> placeholders' case 2021-11-09 09:39:11 -08:00
glossary-content.txt docs: improve fast-forward in glossary content 2021-05-19 21:11:49 +09:00
howto-index.sh
i18n.txt i18n.txt: camel case and monospace "i18n.commitEncoding" 2021-02-24 15:21:25 -08:00
install-doc-quick.sh
install-webdoc.sh
line-range-format.txt doc: add more pointers to gitattributes(5) for userdiff 2020-11-01 15:54:14 -08:00
line-range-options.txt blame-options.txt: also mention 'funcname' in '-L' description 2020-11-01 15:54:14 -08:00
lint-gitlink.perl doc lint: make "lint-docs" non-.PHONY 2021-10-15 10:29:11 -07:00
lint-man-end-blurb.perl doc lint: emit errors on STDERR 2021-10-15 10:16:57 -07:00
lint-man-section-order.perl doc lint: emit errors on STDERR 2021-10-15 10:16:57 -07:00
Makefile Merge branch 'ab/fix-make-lint-docs' 2021-10-29 15:43:13 -07:00
manpage-base-url.xsl.in
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manpage.xsl
merge-options.txt Document positive variant of commit and merge option "--no-verify" 2021-10-29 11:22:56 -07:00
merge-strategies.txt Update docs for change of default merge backend 2021-08-05 15:35:02 -07:00
MyFirstContribution.txt Merge branch 'jc/tutorial-format-patch-base' 2021-11-29 15:41:46 -08:00
MyFirstObjectWalk.txt docs: add headers in MyFirstObjectWalk 2021-10-29 13:02:51 -07:00
object-format-disclaimer.txt Documentation: mark --object-format=sha256 as experimental 2020-08-17 10:50:14 -07:00
pretty-formats.txt Merge branch 'es/pretty-describe-more' 2021-12-15 09:39:48 -08:00
pretty-options.txt log: document --encoding behavior on iconv() failure 2021-10-29 14:35:59 -07:00
pull-fetch-param.txt refspec: add support for negative refspecs 2020-09-30 14:52:00 -07:00
ref-reachability-filters.txt Doc: prefer more specific file name 2020-09-18 15:41:56 -07:00
rev-list-description.txt git-log.txt: include rev-list-description.txt 2020-07-08 22:08:54 -07:00
rev-list-options.txt Merge branch 'jk/strbuf-addftime-seconds-since-epoch' 2021-12-10 14:35:07 -08:00
revisions.txt revisions(7): clarify that most commands take a single revision range 2021-05-18 10:08:03 +09:00
sequencer.txt
signoff-option.txt Documentation: stylistically normalize references to Signed-off-by: 2020-10-20 11:57:40 -07:00
SubmittingPatches SubmittingPatches: fix Asciidoc syntax in "GitHub CI" section 2021-11-13 23:41:54 -08:00
texi.xsl
trace2-target-values.txt
transfer-data-leaks.txt
urls-remotes.txt doc: uniformize <URL> placeholders' case 2021-11-09 09:39:11 -08:00
urls.txt
user-manual.conf
user-manual.txt Update docs for change of default merge backend 2021-08-05 15:35:02 -07:00