maintenance: add prefetch task

When working with very large repositories, an incremental 'git fetch'
command can download a large amount of data. If there are many other
users pushing to a common repo, then this data can rival the initial
pack-file size of a 'git clone' of a medium-size repo.

Users may want to keep the data on their local repos as close as
possible to the data on the remote repos by fetching periodically in
the background. This can break up a large daily fetch into several
smaller hourly fetches.

The task is called "prefetch" because it is work done in advance
of a foreground fetch to make that 'git fetch' command much faster.

However, if we simply ran 'git fetch <remote>' in the background,
then the user running a foreground 'git fetch <remote>' would lose
some important feedback when a new branch appears or an existing
branch updates. This is especially true if a remote branch is
force-updated and this isn't noticed by the user because it occurred
in the background. Further, the functionality of 'git push
--force-with-lease' becomes suspect.

When running 'git fetch <remote> <options>' in the background, use
the following options for careful updating:

1. --no-tags prevents getting a new tag when a user wants to see
   the new tags appear in their foreground fetches.

2. --refmap= removes the configured refspec which usually updates
   refs/remotes/<remote>/* with the refs advertised by the remote.
   While this looks confusing, this was documented and tested by
   b40a50264ac (fetch: document and test --refmap="", 2020-01-21),
   including this sentence in the documentation:

	Providing an empty `<refspec>` to the `--refmap` option
	causes Git to ignore the configured refspecs and rely
	entirely on the refspecs supplied as command-line arguments.

3. By adding a new refspec "+refs/heads/*:refs/prefetch/<remote>/*"
   we can ensure that we actually load the new values somewhere in
   our refspace while not updating refs/heads or refs/remotes. By
   storing these refs here, the commit-graph job will update the
   commit-graph with the commits from these hidden refs.

4. --prune will delete the refs/prefetch/<remote> refs that no
   longer appear on the remote.

5. --no-write-fetch-head prevents updating FETCH_HEAD.

We've been using this step as a critical background job in Scalar
[1] (and VFS for Git). This solved a pain point that was showing up
in user reports: fetching was a pain! Users do not like waiting to
download the data that was created while they were away from their
machines. After implementing background fetch, the foreground fetch
commands sped up significantly because they mostly just update refs
and download a small amount of new data. The effect is especially
dramatic when paried with --no-show-forced-udpates (through
fetch.showForcedUpdates=false).

[1] https://github.com/microsoft/scalar/blob/master/Scalar.Common/Maintenance/FetchStep.cs

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
3 files changed
tree: b0b78b789b6660fd6a31587f913c345757ef51b9
  1. .cirrus.yml
  2. .clang-format
  3. .editorconfig
  4. .gitattributes
  5. .github/
  6. .gitignore
  7. .gitmodules
  8. .mailmap
  9. .travis.yml
  10. .tsan-suppressions
  11. CODE_OF_CONDUCT.md
  12. COPYING
  13. Documentation/
  14. GIT-VERSION-GEN
  15. INSTALL
  16. LGPL-2.1
  17. Makefile
  18. README.md
  19. abspath.c
  20. aclocal.m4
  21. add-interactive.c
  22. add-interactive.h
  23. add-patch.c
  24. advice.c
  25. advice.h
  26. alias.c
  27. alias.h
  28. alloc.c
  29. alloc.h
  30. apply.c
  31. apply.h
  32. archive-tar.c
  33. archive-zip.c
  34. archive.c
  35. archive.h
  36. attr.c
  37. attr.h
  38. banned.h
  39. base85.c
  40. bisect.c
  41. bisect.h
  42. blame.c
  43. blame.h
  44. blob.c
  45. blob.h
  46. block-sha1/
  47. bloom.c
  48. bloom.h
  49. branch.c
  50. branch.h
  51. bugreport.c
  52. builtin.h
  53. builtin/
  54. bulk-checkin.c
  55. bulk-checkin.h
  56. bundle.c
  57. bundle.h
  58. cache-tree.c
  59. cache-tree.h
  60. cache.h
  61. chdir-notify.c
  62. chdir-notify.h
  63. check-builtins.sh
  64. check_bindir
  65. checkout.c
  66. checkout.h
  67. ci/
  68. color.c
  69. color.h
  70. column.c
  71. column.h
  72. combine-diff.c
  73. command-list.txt
  74. commit-graph.c
  75. commit-graph.h
  76. commit-reach.c
  77. commit-reach.h
  78. commit-slab-decl.h
  79. commit-slab-impl.h
  80. commit-slab.h
  81. commit.c
  82. commit.h
  83. common-main.c
  84. compat/
  85. config.c
  86. config.h
  87. config.mak.dev
  88. config.mak.in
  89. config.mak.uname
  90. configure.ac
  91. connect.c
  92. connect.h
  93. connected.c
  94. connected.h
  95. contrib/
  96. convert.c
  97. convert.h
  98. copy.c
  99. credential-cache--daemon.c
  100. credential-cache.c
  101. credential-store.c
  102. credential.c
  103. credential.h
  104. csum-file.c
  105. csum-file.h
  106. ctype.c
  107. daemon.c
  108. date.c
  109. decorate.c
  110. decorate.h
  111. delta-islands.c
  112. delta-islands.h
  113. delta.h
  114. detect-compiler
  115. diff-delta.c
  116. diff-lib.c
  117. diff-no-index.c
  118. diff.c
  119. diff.h
  120. diffcore-break.c
  121. diffcore-delta.c
  122. diffcore-order.c
  123. diffcore-pickaxe.c
  124. diffcore-rename.c
  125. diffcore.h
  126. dir-iterator.c
  127. dir-iterator.h
  128. dir.c
  129. dir.h
  130. editor.c
  131. entry.c
  132. environment.c
  133. ewah/
  134. exec-cmd.c
  135. exec-cmd.h
  136. fast-import.c
  137. fetch-negotiator.c
  138. fetch-negotiator.h
  139. fetch-pack.c
  140. fetch-pack.h
  141. fmt-merge-msg.c
  142. fmt-merge-msg.h
  143. fsck.c
  144. fsck.h
  145. fsmonitor.c
  146. fsmonitor.h
  147. fuzz-commit-graph.c
  148. fuzz-pack-headers.c
  149. fuzz-pack-idx.c
  150. generate-cmdlist.sh
  151. generate-configlist.sh
  152. gettext.c
  153. gettext.h
  154. git-add--interactive.perl
  155. git-archimport.perl
  156. git-bisect.sh
  157. git-compat-util.h
  158. git-cvsexportcommit.perl
  159. git-cvsimport.perl
  160. git-cvsserver.perl
  161. git-difftool--helper.sh
  162. git-filter-branch.sh
  163. git-gui/
  164. git-instaweb.sh
  165. git-merge-octopus.sh
  166. git-merge-one-file.sh
  167. git-merge-resolve.sh
  168. git-mergetool--lib.sh
  169. git-mergetool.sh
  170. git-p4.py
  171. git-parse-remote.sh
  172. git-quiltimport.sh
  173. git-rebase--preserve-merges.sh
  174. git-request-pull.sh
  175. git-send-email.perl
  176. git-sh-i18n.sh
  177. git-sh-setup.sh
  178. git-submodule.sh
  179. git-svn.perl
  180. git-web--browse.sh
  181. git.c
  182. git.rc
  183. gitk-git/
  184. gitweb/
  185. gpg-interface.c
  186. gpg-interface.h
  187. graph.c
  188. graph.h
  189. grep.c
  190. grep.h
  191. hash.h
  192. hashmap.c
  193. hashmap.h
  194. help.c
  195. help.h
  196. hex.c
  197. http-backend.c
  198. http-fetch.c
  199. http-push.c
  200. http-walker.c
  201. http.c
  202. http.h
  203. ident.c
  204. imap-send.c
  205. interdiff.c
  206. interdiff.h
  207. iterator.h
  208. json-writer.c
  209. json-writer.h
  210. khash.h
  211. kwset.c
  212. kwset.h
  213. levenshtein.c
  214. levenshtein.h
  215. line-log.c
  216. line-log.h
  217. line-range.c
  218. line-range.h
  219. linear-assignment.c
  220. linear-assignment.h
  221. list-objects-filter-options.c
  222. list-objects-filter-options.h
  223. list-objects-filter.c
  224. list-objects-filter.h
  225. list-objects.c
  226. list-objects.h
  227. list.h
  228. ll-merge.c
  229. ll-merge.h
  230. lockfile.c
  231. lockfile.h
  232. log-tree.c
  233. log-tree.h
  234. ls-refs.c
  235. ls-refs.h
  236. mailinfo.c
  237. mailinfo.h
  238. mailmap.c
  239. mailmap.h
  240. match-trees.c
  241. mem-pool.c
  242. mem-pool.h
  243. merge-blobs.c
  244. merge-blobs.h
  245. merge-recursive.c
  246. merge-recursive.h
  247. merge.c
  248. mergesort.c
  249. mergesort.h
  250. mergetools/
  251. midx.c
  252. midx.h
  253. name-hash.c
  254. negotiator/
  255. notes-cache.c
  256. notes-cache.h
  257. notes-merge.c
  258. notes-merge.h
  259. notes-utils.c
  260. notes-utils.h
  261. notes.c
  262. notes.h
  263. object-store.h
  264. object.c
  265. object.h
  266. oid-array.c
  267. oid-array.h
  268. oidmap.c
  269. oidmap.h
  270. oidset.c
  271. oidset.h
  272. pack-bitmap-write.c
  273. pack-bitmap.c
  274. pack-bitmap.h
  275. pack-check.c
  276. pack-objects.c
  277. pack-objects.h
  278. pack-revindex.c
  279. pack-revindex.h
  280. pack-write.c
  281. pack.h
  282. packfile.c
  283. packfile.h
  284. pager.c
  285. parse-options-cb.c
  286. parse-options.c
  287. parse-options.h
  288. patch-delta.c
  289. patch-ids.c
  290. patch-ids.h
  291. path.c
  292. path.h
  293. pathspec.c
  294. pathspec.h
  295. perl/
  296. pkt-line.c
  297. pkt-line.h
  298. po/
  299. ppc/
  300. preload-index.c
  301. pretty.c
  302. pretty.h
  303. prio-queue.c
  304. prio-queue.h
  305. progress.c
  306. progress.h
  307. promisor-remote.c
  308. promisor-remote.h
  309. prompt.c
  310. prompt.h
  311. protocol.c
  312. protocol.h
  313. prune-packed.c
  314. prune-packed.h
  315. quote.c
  316. quote.h
  317. range-diff.c
  318. range-diff.h
  319. reachable.c
  320. reachable.h
  321. read-cache.c
  322. rebase-interactive.c
  323. rebase-interactive.h
  324. rebase.c
  325. rebase.h
  326. ref-filter.c
  327. ref-filter.h
  328. reflog-walk.c
  329. reflog-walk.h
  330. refs.c
  331. refs.h
  332. refs/
  333. refspec.c
  334. refspec.h
  335. remote-curl.c
  336. remote-testsvn.c
  337. remote.c
  338. remote.h
  339. replace-object.c
  340. replace-object.h
  341. repo-settings.c
  342. repository.c
  343. repository.h
  344. rerere.c
  345. rerere.h
  346. reset.c
  347. reset.h
  348. resolve-undo.c
  349. resolve-undo.h
  350. revision.c
  351. revision.h
  352. run-command.c
  353. run-command.h
  354. send-pack.c
  355. send-pack.h
  356. sequencer.c
  357. sequencer.h
  358. serve.c
  359. serve.h
  360. server-info.c
  361. setup.c
  362. sh-i18n--envsubst.c
  363. sha1-file.c
  364. sha1-lookup.c
  365. sha1-lookup.h
  366. sha1-name.c
  367. sha1dc/
  368. sha1dc_git.c
  369. sha1dc_git.h
  370. sha256/
  371. shallow.c
  372. shallow.h
  373. shell.c
  374. shortlog.h
  375. sideband.c
  376. sideband.h
  377. sigchain.c
  378. sigchain.h
  379. split-index.c
  380. split-index.h
  381. stable-qsort.c
  382. strbuf.c
  383. strbuf.h
  384. streaming.c
  385. streaming.h
  386. string-list.c
  387. string-list.h
  388. strvec.c
  389. strvec.h
  390. sub-process.c
  391. sub-process.h
  392. submodule-config.c
  393. submodule-config.h
  394. submodule.c
  395. submodule.h
  396. symlinks.c
  397. t/
  398. tag.c
  399. tag.h
  400. tar.h
  401. tempfile.c
  402. tempfile.h
  403. templates/
  404. thread-utils.c
  405. thread-utils.h
  406. tmp-objdir.c
  407. tmp-objdir.h
  408. trace.c
  409. trace.h
  410. trace2.c
  411. trace2.h
  412. trace2/
  413. trailer.c
  414. trailer.h
  415. transport-helper.c
  416. transport-internal.h
  417. transport.c
  418. transport.h
  419. tree-diff.c
  420. tree-walk.c
  421. tree-walk.h
  422. tree.c
  423. tree.h
  424. unicode-width.h
  425. unimplemented.sh
  426. unix-socket.c
  427. unix-socket.h
  428. unpack-trees.c
  429. unpack-trees.h
  430. upload-pack.c
  431. upload-pack.h
  432. url.c
  433. url.h
  434. urlmatch.c
  435. urlmatch.h
  436. usage.c
  437. userdiff.c
  438. userdiff.h
  439. utf8.c
  440. utf8.h
  441. varint.c
  442. varint.h
  443. vcs-svn/
  444. version.c
  445. version.h
  446. versioncmp.c
  447. walker.c
  448. walker.h
  449. wildmatch.c
  450. wildmatch.h
  451. worktree.c
  452. worktree.h
  453. wrap-for-bin.sh
  454. wrapper.c
  455. write-or-die.c
  456. ws.c
  457. wt-status.c
  458. wt-status.h
  459. xdiff-interface.c
  460. xdiff-interface.h
  461. xdiff/
  462. zlib.c
README.md

Build status

Git - fast, scalable, distributed revision control system

Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.

Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.

Please read the file INSTALL for installation instructions.

Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.

See Documentation/gittutorial.txt to get started, then see Documentation/giteveryday.txt for a useful minimum set of commands, and Documentation/git-<commandname>.txt for documentation of each command. If git has been correctly installed, then the tutorial can also be read with man gittutorial or git help tutorial, and the documentation of each command with man git-<commandname> or git help <commandname>.

CVS users may also want to read Documentation/gitcvs-migration.txt (man gitcvs-migration or git help cvs-migration if git is installed).

The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission). To subscribe to the list, send an email with just “subscribe git” in the body to majordomo@vger.kernel.org. The mailing list archives are available at https://lore.kernel.org/git/, http://marc.info/?l=git and other archival sites.

Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.

The maintainer frequently sends the “What's cooking” reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.

The name “git” was given by Linus Torvalds when he wrote the very first version. He described the tool as “the stupid content tracker” and the name as (depending on your mood):

  • random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of “get” may or may not be relevant.
  • stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
  • “global information tracker”: you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
  • “goddamn idiotic truckload of sh*t”: when it breaks