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chromium / external / gitster / git-htmldocs / 683a86135b5ed57740264f0d8f8e59dbd7305d47 / . / technical / racy-git.html
blob: d8a8d16669f67f03ca884531b28d0c2755aa7f36 [file] [log] [blame]
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
2 "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
4<head>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]5<meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" />
6<meta name="generator" content="AsciiDoc 8.6.8" />
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]7<title>Use of index and Racy Git problem</title>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]8<style type="text/css">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]9/* Shared CSS for AsciiDoc xhtml11 and html5 backends */
10
11/* Default font. */
12body {
13 font-family: Georgia,serif;
14}
15
16/* Title font. */
17h1, h2, h3, h4, h5, h6,
18div.title, caption.title,
19thead, p.table.header,
20#toctitle,
21#author, #revnumber, #revdate, #revremark,
22#footer {
23 font-family: Arial,Helvetica,sans-serif;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]24}
25
26body {
27 margin: 1em 5% 1em 5%;
28}
29
30a {
31 color: blue;
32 text-decoration: underline;
33}
34a:visited {
35 color: fuchsia;
36}
37
38em {
39 font-style: italic;
40 color: navy;
41}
42
43strong {
44 font-weight: bold;
45 color: #083194;
46}
47
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]48h1, h2, h3, h4, h5, h6 {
49 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]50 margin-top: 1.2em;
51 margin-bottom: 0.5em;
52 line-height: 1.3;
53}
54
55h1, h2, h3 {
56 border-bottom: 2px solid silver;
57}
58h2 {
59 padding-top: 0.5em;
60}
61h3 {
62 float: left;
63}
64h3 + * {
65 clear: left;
66}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]67h5 {
68 font-size: 1.0em;
69}
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]70
71div.sectionbody {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]72 margin-left: 0;
73}
74
75hr {
76 border: 1px solid silver;
77}
78
79p {
80 margin-top: 0.5em;
81 margin-bottom: 0.5em;
82}
83
84ul, ol, li > p {
85 margin-top: 0;
86}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]87ul > li { color: #aaa; }
88ul > li > * { color: black; }
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]89
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]90.monospaced, code, pre {
91 font-family: "Courier New", Courier, monospace;
92 font-size: inherit;
93 color: navy;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]94 padding: 0;
95 margin: 0;
96}
97
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]98
99#author {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]100 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]101 font-weight: bold;
102 font-size: 1.1em;
103}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]104#email {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]105}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]106#revnumber, #revdate, #revremark {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]107}
108
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]109#footer {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]110 font-size: small;
111 border-top: 2px solid silver;
112 padding-top: 0.5em;
113 margin-top: 4.0em;
114}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]115#footer-text {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]116 float: left;
117 padding-bottom: 0.5em;
118}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]119#footer-badges {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]120 float: right;
121 padding-bottom: 0.5em;
122}
123
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]124#preamble {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]125 margin-top: 1.5em;
126 margin-bottom: 1.5em;
127}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]128div.imageblock, div.exampleblock, div.verseblock,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]129div.quoteblock, div.literalblock, div.listingblock, div.sidebarblock,
130div.admonitionblock {
131 margin-top: 1.0em;
132 margin-bottom: 1.5em;
133}
134div.admonitionblock {
135 margin-top: 2.0em;
136 margin-bottom: 2.0em;
137 margin-right: 10%;
138 color: #606060;
139}
140
141div.content { /* Block element content. */
142 padding: 0;
143}
144
145/* Block element titles. */
146div.title, caption.title {
147 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]148 font-weight: bold;
149 text-align: left;
150 margin-top: 1.0em;
151 margin-bottom: 0.5em;
152}
153div.title + * {
154 margin-top: 0;
155}
156
157td div.title:first-child {
158 margin-top: 0.0em;
159}
160div.content div.title:first-child {
161 margin-top: 0.0em;
162}
163div.content + div.title {
164 margin-top: 0.0em;
165}
166
167div.sidebarblock > div.content {
168 background: #ffffee;
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]169 border: 1px solid #dddddd;
170 border-left: 4px solid #f0f0f0;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]171 padding: 0.5em;
172}
173
174div.listingblock > div.content {
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]175 border: 1px solid #dddddd;
176 border-left: 5px solid #f0f0f0;
177 background: #f8f8f8;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]178 padding: 0.5em;
179}
180
181div.quoteblock, div.verseblock {
182 padding-left: 1.0em;
183 margin-left: 1.0em;
184 margin-right: 10%;
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]185 border-left: 5px solid #f0f0f0;
186 color: #888;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]187}
188
189div.quoteblock > div.attribution {
190 padding-top: 0.5em;
191 text-align: right;
192}
193
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]194div.verseblock > pre.content {
195 font-family: inherit;
196 font-size: inherit;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]197}
198div.verseblock > div.attribution {
199 padding-top: 0.75em;
200 text-align: left;
201}
202/* DEPRECATED: Pre version 8.2.7 verse style literal block. */
203div.verseblock + div.attribution {
204 text-align: left;
205}
206
207div.admonitionblock .icon {
208 vertical-align: top;
209 font-size: 1.1em;
210 font-weight: bold;
211 text-decoration: underline;
212 color: #527bbd;
213 padding-right: 0.5em;
214}
215div.admonitionblock td.content {
216 padding-left: 0.5em;
217 border-left: 3px solid #dddddd;
218}
219
220div.exampleblock > div.content {
221 border-left: 3px solid #dddddd;
222 padding-left: 0.5em;
223}
224
225div.imageblock div.content { padding-left: 0; }
226span.image img { border-style: none; }
227a.image:visited { color: white; }
228
229dl {
230 margin-top: 0.8em;
231 margin-bottom: 0.8em;
232}
233dt {
234 margin-top: 0.5em;
235 margin-bottom: 0;
236 font-style: normal;
237 color: navy;
238}
239dd > *:first-child {
240 margin-top: 0.1em;
241}
242
243ul, ol {
244 list-style-position: outside;
245}
246ol.arabic {
247 list-style-type: decimal;
248}
249ol.loweralpha {
250 list-style-type: lower-alpha;
251}
252ol.upperalpha {
253 list-style-type: upper-alpha;
254}
255ol.lowerroman {
256 list-style-type: lower-roman;
257}
258ol.upperroman {
259 list-style-type: upper-roman;
260}
261
262div.compact ul, div.compact ol,
263div.compact p, div.compact p,
264div.compact div, div.compact div {
265 margin-top: 0.1em;
266 margin-bottom: 0.1em;
267}
268
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]269tfoot {
270 font-weight: bold;
271}
272td > div.verse {
273 white-space: pre;
274}
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]275
276div.hdlist {
277 margin-top: 0.8em;
278 margin-bottom: 0.8em;
279}
280div.hdlist tr {
281 padding-bottom: 15px;
282}
283dt.hdlist1.strong, td.hdlist1.strong {
284 font-weight: bold;
285}
286td.hdlist1 {
287 vertical-align: top;
288 font-style: normal;
289 padding-right: 0.8em;
290 color: navy;
291}
292td.hdlist2 {
293 vertical-align: top;
294}
295div.hdlist.compact tr {
296 margin: 0;
297 padding-bottom: 0;
298}
299
300.comment {
301 background: yellow;
302}
303
304.footnote, .footnoteref {
305 font-size: 0.8em;
306}
307
308span.footnote, span.footnoteref {
309 vertical-align: super;
310}
311
312#footnotes {
313 margin: 20px 0 20px 0;
314 padding: 7px 0 0 0;
315}
316
317#footnotes div.footnote {
318 margin: 0 0 5px 0;
319}
320
321#footnotes hr {
322 border: none;
323 border-top: 1px solid silver;
324 height: 1px;
325 text-align: left;
326 margin-left: 0;
327 width: 20%;
328 min-width: 100px;
329}
330
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]331div.colist td {
332 padding-right: 0.5em;
333 padding-bottom: 0.3em;
334 vertical-align: top;
335}
336div.colist td img {
337 margin-top: 0.3em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]338}
339
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]340@media print {
341 #footer-badges { display: none; }
342}
343
344#toc {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]345 margin-bottom: 2.5em;
346}
347
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]348#toctitle {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]349 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]350 font-size: 1.1em;
351 font-weight: bold;
352 margin-top: 1.0em;
353 margin-bottom: 0.1em;
354}
355
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]356div.toclevel0, div.toclevel1, div.toclevel2, div.toclevel3, div.toclevel4 {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]357 margin-top: 0;
358 margin-bottom: 0;
359}
360div.toclevel2 {
361 margin-left: 2em;
362 font-size: 0.9em;
363}
364div.toclevel3 {
365 margin-left: 4em;
366 font-size: 0.9em;
367}
368div.toclevel4 {
369 margin-left: 6em;
370 font-size: 0.9em;
371}
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]372
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]373span.aqua { color: aqua; }
374span.black { color: black; }
375span.blue { color: blue; }
376span.fuchsia { color: fuchsia; }
377span.gray { color: gray; }
378span.green { color: green; }
379span.lime { color: lime; }
380span.maroon { color: maroon; }
381span.navy { color: navy; }
382span.olive { color: olive; }
383span.purple { color: purple; }
384span.red { color: red; }
385span.silver { color: silver; }
386span.teal { color: teal; }
387span.white { color: white; }
388span.yellow { color: yellow; }
389
390span.aqua-background { background: aqua; }
391span.black-background { background: black; }
392span.blue-background { background: blue; }
393span.fuchsia-background { background: fuchsia; }
394span.gray-background { background: gray; }
395span.green-background { background: green; }
396span.lime-background { background: lime; }
397span.maroon-background { background: maroon; }
398span.navy-background { background: navy; }
399span.olive-background { background: olive; }
400span.purple-background { background: purple; }
401span.red-background { background: red; }
402span.silver-background { background: silver; }
403span.teal-background { background: teal; }
404span.white-background { background: white; }
405span.yellow-background { background: yellow; }
406
407span.big { font-size: 2em; }
408span.small { font-size: 0.6em; }
409
410span.underline { text-decoration: underline; }
411span.overline { text-decoration: overline; }
412span.line-through { text-decoration: line-through; }
413
414div.unbreakable { page-break-inside: avoid; }
415
416
417/*
418 * xhtml11 specific
419 *
420 * */
421
422div.tableblock {
423 margin-top: 1.0em;
424 margin-bottom: 1.5em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]425}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]426div.tableblock > table {
427 border: 3px solid #527bbd;
428}
429thead, p.table.header {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]430 font-weight: bold;
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]431 color: #527bbd;
432}
433p.table {
434 margin-top: 0;
435}
436/* Because the table frame attribute is overriden by CSS in most browsers. */
437div.tableblock > table[frame="void"] {
438 border-style: none;
439}
440div.tableblock > table[frame="hsides"] {
441 border-left-style: none;
442 border-right-style: none;
443}
444div.tableblock > table[frame="vsides"] {
445 border-top-style: none;
446 border-bottom-style: none;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]447}
448
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]449
450/*
451 * html5 specific
452 *
453 * */
454
455table.tableblock {
456 margin-top: 1.0em;
457 margin-bottom: 1.5em;
458}
459thead, p.tableblock.header {
460 font-weight: bold;
461 color: #527bbd;
462}
463p.tableblock {
464 margin-top: 0;
465}
466table.tableblock {
467 border-width: 3px;
468 border-spacing: 0px;
469 border-style: solid;
470 border-color: #527bbd;
471 border-collapse: collapse;
472}
473th.tableblock, td.tableblock {
474 border-width: 1px;
475 padding: 4px;
476 border-style: solid;
477 border-color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]478}
479
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]480table.tableblock.frame-topbot {
481 border-left-style: hidden;
482 border-right-style: hidden;
483}
484table.tableblock.frame-sides {
485 border-top-style: hidden;
486 border-bottom-style: hidden;
487}
488table.tableblock.frame-none {
489 border-style: hidden;
490}
491
492th.tableblock.halign-left, td.tableblock.halign-left {
493 text-align: left;
494}
495th.tableblock.halign-center, td.tableblock.halign-center {
496 text-align: center;
497}
498th.tableblock.halign-right, td.tableblock.halign-right {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]499 text-align: right;
500}
501
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]502th.tableblock.valign-top, td.tableblock.valign-top {
503 vertical-align: top;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]504}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]505th.tableblock.valign-middle, td.tableblock.valign-middle {
506 vertical-align: middle;
507}
508th.tableblock.valign-bottom, td.tableblock.valign-bottom {
509 vertical-align: bottom;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]510}
511
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]512
513/*
514 * manpage specific
515 *
516 * */
517
518body.manpage h1 {
519 padding-top: 0.5em;
520 padding-bottom: 0.5em;
521 border-top: 2px solid silver;
522 border-bottom: 2px solid silver;
523}
524body.manpage h2 {
525 border-style: none;
526}
527body.manpage div.sectionbody {
528 margin-left: 3em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]529}
530
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]531@media print {
532 body.manpage div#toc { display: none; }
533}
534
535
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]536</style>
537<script type="text/javascript">
538/*<![CDATA[*/
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]539var asciidoc = { // Namespace.
540
541/////////////////////////////////////////////////////////////////////
542// Table Of Contents generator
543/////////////////////////////////////////////////////////////////////
544
545/* Author: Mihai Bazon, September 2002
546 * http://students.infoiasi.ro/~mishoo
547 *
548 * Table Of Content generator
549 * Version: 0.4
550 *
551 * Feel free to use this script under the terms of the GNU General Public
552 * License, as long as you do not remove or alter this notice.
553 */
554
555 /* modified by Troy D. Hanson, September 2006. License: GPL */
556 /* modified by Stuart Rackham, 2006, 2009. License: GPL */
557
558// toclevels = 1..4.
559toc: function (toclevels) {
560
561 function getText(el) {
562 var text = "";
563 for (var i = el.firstChild; i != null; i = i.nextSibling) {
564 if (i.nodeType == 3 /* Node.TEXT_NODE */) // IE doesn't speak constants.
565 text += i.data;
566 else if (i.firstChild != null)
567 text += getText(i);
568 }
569 return text;
570 }
571
572 function TocEntry(el, text, toclevel) {
573 this.element = el;
574 this.text = text;
575 this.toclevel = toclevel;
576 }
577
578 function tocEntries(el, toclevels) {
579 var result = new Array;
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]580 var re = new RegExp('[hH]([1-'+(toclevels+1)+'])');
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]581 // Function that scans the DOM tree for header elements (the DOM2
582 // nodeIterator API would be a better technique but not supported by all
583 // browsers).
584 var iterate = function (el) {
585 for (var i = el.firstChild; i != null; i = i.nextSibling) {
586 if (i.nodeType == 1 /* Node.ELEMENT_NODE */) {
587 var mo = re.exec(i.tagName);
588 if (mo && (i.getAttribute("class") || i.getAttribute("className")) != "float") {
589 result[result.length] = new TocEntry(i, getText(i), mo[1]-1);
590 }
591 iterate(i);
592 }
593 }
594 }
595 iterate(el);
596 return result;
597 }
598
599 var toc = document.getElementById("toc");
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]600 if (!toc) {
601 return;
602 }
603
604 // Delete existing TOC entries in case we're reloading the TOC.
605 var tocEntriesToRemove = [];
606 var i;
607 for (i = 0; i < toc.childNodes.length; i++) {
608 var entry = toc.childNodes[i];
609 if (entry.nodeName.toLowerCase() == 'div'
610 && entry.getAttribute("class")
611 && entry.getAttribute("class").match(/^toclevel/))
612 tocEntriesToRemove.push(entry);
613 }
614 for (i = 0; i < tocEntriesToRemove.length; i++) {
615 toc.removeChild(tocEntriesToRemove[i]);
616 }
617
618 // Rebuild TOC entries.
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]619 var entries = tocEntries(document.getElementById("content"), toclevels);
620 for (var i = 0; i < entries.length; ++i) {
621 var entry = entries[i];
622 if (entry.element.id == "")
623 entry.element.id = "_toc_" + i;
624 var a = document.createElement("a");
625 a.href = "#" + entry.element.id;
626 a.appendChild(document.createTextNode(entry.text));
627 var div = document.createElement("div");
628 div.appendChild(a);
629 div.className = "toclevel" + entry.toclevel;
630 toc.appendChild(div);
631 }
632 if (entries.length == 0)
633 toc.parentNode.removeChild(toc);
634},
635
636
637/////////////////////////////////////////////////////////////////////
638// Footnotes generator
639/////////////////////////////////////////////////////////////////////
640
641/* Based on footnote generation code from:
642 * http://www.brandspankingnew.net/archive/2005/07/format_footnote.html
643 */
644
645footnotes: function () {
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]646 // Delete existing footnote entries in case we're reloading the footnodes.
647 var i;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]648 var noteholder = document.getElementById("footnotes");
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]732<body class="article">
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]733<div id="header">
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]734<h1>Use of index and Racy Git problem</h1>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]735</div>
736<div id="content">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]737<div class="sect1">
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]738<h2 id="_background">Background</h2>
739<div class="sectionbody">
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]740<div class="paragraph"><p>The index is one of the most important data structures in Git.
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]741It represents a virtual working tree state by recording list of
742paths and their object names and serves as a staging area to
743write out the next tree object to be committed. The state is
744"virtual" in the sense that it does not necessarily have to, and
745often does not, match the files in the working tree.</p></div>
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]746<div class="paragraph"><p>There are cases Git needs to examine the differences between the
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]747virtual working tree state in the index and the files in the
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]748working tree. The most obvious case is when the user asks <code>git
749diff</code> (or its low level implementation, <code>git diff-files</code>) or
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]750<code>git-ls-files --modified</code>. In addition, Git internally checks
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]751if the files in the working tree are different from what are
752recorded in the index to avoid stomping on local changes in them
753during patch application, switching branches, and merging.</p></div>
754<div class="paragraph"><p>In order to speed up this comparison between the files in the
755working tree and the index entries, the index entries record the
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]756information obtained from the filesystem via <code>lstat(2)</code> system
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]757call when they were last updated. When checking if they differ,
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]758Git first runs <code>lstat(2)</code> on the files and compares the result
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]759with this information (this is what was originally done by the
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]760<code>ce_match_stat()</code> function, but the current code does it in
761<code>ce_match_stat_basic()</code> function). If some of these "cached
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]762stat information" fields do not match, Git can tell that the
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]763files are modified without even looking at their contents.</p></div>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]764<div class="paragraph"><p>Note: not all members in <code>struct stat</code> obtained via <code>lstat(2)</code>
765are used for this comparison. For example, <code>st_atime</code> obviously
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]766is not useful. Currently, Git compares the file type (regular
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]767files vs symbolic links) and executable bits (only for regular
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]768files) from <code>st_mode</code> member, <code>st_mtime</code> and <code>st_ctime</code>
769timestamps, <code>st_uid</code>, <code>st_gid</code>, <code>st_ino</code>, and <code>st_size</code> members.
770With a <code>USE_STDEV</code> compile-time option, <code>st_dev</code> is also
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]771compared, but this is not enabled by default because this member
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]772is not stable on network filesystems. With <code>USE_NSEC</code>
773compile-time option, <code>st_mtim.tv_nsec</code> and <code>st_ctim.tv_nsec</code>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]774members are also compared, but this is not enabled by default
775because in-core timestamps can have finer granularity than
776on-disk timestamps, resulting in meaningless changes when an
777inode is evicted from the inode cache. See commit 8ce13b0
778of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Junio C Hamanod7fccbf2013-07-25 03:24:57[diff] [blame]779([PATCH] Sync in core time granularity with filesystems,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]7802005-01-04).</p></div>
781</div>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]782</div>
783<div class="sect1">
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]784<h2 id="_racy_git">Racy Git</h2>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]785<div class="sectionbody">
786<div class="paragraph"><p>There is one slight problem with the optimization based on the
787cached stat information. Consider this sequence:</p></div>
788<div class="literalblock">
789<div class="content">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]790<pre><code>: modify 'foo'
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]791$ git update-index 'foo'
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]792: modify 'foo' again, in-place, without changing its size</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]793</div></div>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]794<div class="paragraph"><p>The first <code>update-index</code> computes the object name of the
795contents of file <code>foo</code> and updates the index entry for <code>foo</code>
796along with the <code>struct stat</code> information. If the modification
797that follows it happens very fast so that the file&#8217;s <code>st_mtime</code>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]798timestamp does not change, after this sequence, the cached stat
799information the index entry records still exactly match what you
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]800would see in the filesystem, even though the file <code>foo</code> is now
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]801different.
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]802This way, Git can incorrectly think files in the working tree
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]803are unmodified even though they actually are. This is called
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]804the "racy Git" problem (discovered by Pasky), and the entries
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]805that appear clean when they may not be because of this problem
806are called "racily clean".</p></div>
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]807<div class="paragraph"><p>To avoid this problem, Git does two things:</p></div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]808<div class="olist arabic"><ol class="arabic">
809<li>
810<p>
811When the cached stat information says the file has not been
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]812 modified, and the <code>st_mtime</code> is the same as (or newer than)
813 the timestamp of the index file itself (which is the time <code>git
814 update-index foo</code> finished running in the above example), it
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]815 also compares the contents with the object registered in the
816 index entry to make sure they match.
817</p>
818</li>
819<li>
820<p>
821When the index file is updated that contains racily clean
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]822 entries, cached <code>st_size</code> information is truncated to zero
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]823 before writing a new version of the index file.
824</p>
825</li>
826</ol></div>
827<div class="paragraph"><p>Because the index file itself is written after collecting all
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]828the stat information from updated paths, <code>st_mtime</code> timestamp of
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]829it is usually the same as or newer than any of the paths the
830index contains. And no matter how quick the modification that
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]831follows <code>git update-index foo</code> finishes, the resulting
832<code>st_mtime</code> timestamp on <code>foo</code> cannot get a value earlier
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]833than the index file. Therefore, index entries that can be
834racily clean are limited to the ones that have the same
835timestamp as the index file itself.</p></div>
836<div class="paragraph"><p>The callers that want to check if an index entry matches the
837corresponding file in the working tree continue to call
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]838<code>ce_match_stat()</code>, but with this change, <code>ce_match_stat()</code> uses
839<code>ce_modified_check_fs()</code> to see if racily clean ones are
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]840actually clean after comparing the cached stat information using
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]841<code>ce_match_stat_basic()</code>.</p></div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]842<div class="paragraph"><p>The problem the latter solves is this sequence:</p></div>
843<div class="literalblock">
844<div class="content">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]845<pre><code>$ git update-index 'foo'
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]846: modify 'foo' in-place without changing its size
847: wait for enough time
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]848$ git update-index 'bar'</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]849</div></div>
850<div class="paragraph"><p>Without the latter, the timestamp of the index file gets a newer
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]851value, and falsely clean entry <code>foo</code> would not be caught by the
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]852timestamp comparison check done with the former logic anymore.
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]853The latter makes sure that the cached stat information for <code>foo</code>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]854would never match with the file in the working tree, so later
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]855checks by <code>ce_match_stat_basic()</code> would report that the index entry
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]856does not match the file and Git does not have to fall back on more
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]857expensive <code>ce_modified_check_fs()</code>.</p></div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]858</div>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]859</div>
860<div class="sect1">
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]861<h2 id="_runtime_penalty">Runtime penalty</h2>
862<div class="sectionbody">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]863<div class="paragraph"><p>The runtime penalty of falling back to <code>ce_modified_check_fs()</code>
864from <code>ce_match_stat()</code> can be very expensive when there are many
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]865racily clean entries. An obvious way to artificially create
866this situation is to give the same timestamp to all the files in
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]867the working tree in a large project, run <code>git update-index</code> on
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]868them, and give the same timestamp to the index file:</p></div>
869<div class="literalblock">
870<div class="content">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]871<pre><code>$ date &gt;.datestamp
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]872$ git ls-files | xargs touch -r .datestamp
873$ git ls-files | git update-index --stdin
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]874$ touch -r .datestamp .git/index</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]875</div></div>
Junio C Hamanoa1952302013-07-01 21:31:18[diff] [blame]876<div class="paragraph"><p>This will make all index entries racily clean. The linux project, for
877example, there are over 20,000 files in the working tree. On my
878Athlon 64 X2 3800+, after the above:</p></div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]879<div class="literalblock">
880<div class="content">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]881<pre><code>$ /usr/bin/time git diff-files
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]8821.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
8830inputs+0outputs (0major+67111minor)pagefaults 0swaps
884$ git update-index MAINTAINERS
885$ /usr/bin/time git diff-files
8860.02user 0.12system 0:00.14elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]8870inputs+0outputs (0major+935minor)pagefaults 0swaps</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]888</div></div>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]889<div class="paragraph"><p>Running <code>git update-index</code> in the middle checked the racily
890clean entries, and left the cached <code>st_mtime</code> for all the paths
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]891intact because they were actually clean (so this step took about
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]892the same amount of time as the first <code>git diff-files</code>). After
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]893that, they are not racily clean anymore but are truly clean, so
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]894the second invocation of <code>git diff-files</code> fully took advantage
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]895of the cached stat information.</p></div>
896</div>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]897</div>
898<div class="sect1">
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]899<h2 id="_avoiding_runtime_penalty">Avoiding runtime penalty</h2>
900<div class="sectionbody">
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]901<div class="paragraph"><p>In order to avoid the above runtime penalty, post 1.4.2 Git used
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]902to have a code that made sure the index file
903got timestamp newer than the youngest files in the index when
904there are many young files with the same timestamp as the
905resulting index file would otherwise would have by waiting
906before finishing writing the index file out.</p></div>
907<div class="paragraph"><p>I suspected that in practice the situation where many paths in the
908index are all racily clean was quite rare. The only code paths
909that can record recent timestamp for large number of paths are:</p></div>
910<div class="olist arabic"><ol class="arabic">
911<li>
912<p>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]913Initial <code>git add .</code> of a large project.
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]914</p>
915</li>
916<li>
917<p>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]918<code>git checkout</code> of a large project from an empty index into an
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]919 unpopulated working tree.
920</p>
921</li>
922</ol></div>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]923<div class="paragraph"><p>Note: switching branches with <code>git checkout</code> keeps the cached
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]924stat information of existing working tree files that are the
925same between the current branch and the new branch, which are
926all older than the resulting index file, and they will not
927become racily clean. Only the files that are actually checked
928out can become racily clean.</p></div>
929<div class="paragraph"><p>In a large project where raciness avoidance cost really matters,
930however, the initial computation of all object names in the
931index takes more than one second, and the index file is written
932out after all that happens. Therefore the timestamp of the
933index file will be more than one seconds later than the
934youngest file in the working tree. This means that in these
935cases there actually will not be any racily clean entry in
936the resulting index.</p></div>
937<div class="paragraph"><p>Based on this discussion, the current code does not use the
938"workaround" to avoid the runtime penalty that does not exist in
939practice anymore. This was done with commit 0fc82cff on Aug 15,
9402006.</p></div>
941</div>
942</div>
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]943</div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]944<div id="footnotes"><hr /></div>
945<div id="footer">
946<div id="footer-text">
Junio C Hamano683a8612013-08-23 20:50:09[diff] [blame^]947Last updated 2013-08-20 08:40:27 PDT
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]948</div>
949</div>
950</body>
951</html>