hgbook
view en/mq.tex @ 18:e6f4088ebe52
Generate a PDF file with a feedback link on each paragraph.
| author | Bryan O'Sullivan <bos@serpentine.com> |
|---|---|
| date | Tue Jul 04 16:41:31 2006 -0700 (2006-07-04) |
| parents | 2668e15c76e9 |
| children | 187702df428b |
line source
1 \chapter{Managing change with Mercurial Queues}
2 \label{chap:mq}
4 \section{The patch management problem}
5 \label{sec:mq:patch-mgmt}
7 Here is a common scenario: you need to install a software package from
8 source, but you find a bug that you must fix in the source before you
9 can start using the package. You make your changes, forget about the
10 package for a while, and a few months later you need to upgrade to a
11 newer version of the package. If the newer version of the package
12 still has the bug, you must extract your fix from the older source
13 tree and apply it against the newer version. This is a tedious task,
14 and it's easy to make mistakes.
16 This is a simple case of the ``patch management'' problem. You have
17 an ``upstream'' source tree that you can't change; you need to make
18 some local changes on top of the upstream tree; and you'd like to be
19 able to keep those changes separate, so that you can apply them to
20 newer versions of the upstream source.
22 The patch management problem arises in many situations. Probably the
23 most visible is that a user of an open source software project will
24 contribute a bug fix or new feature to the project's maintainers in the
25 form of a patch.
27 Distributors of operating systems that include open source software
28 often need to make changes to the packages they distribute so that
29 they will build properly in their environments.
31 When you have few changes to maintain, it is easy to manage a single
32 patch using the standard \texttt{diff} and \texttt{patch} programs
33 (see section~\ref{sec:mq:patch} for a discussion of these tools).
34 Once the number of changes grows, it starts to makes sense to maintain
35 patches as discrete ``chunks of work,'' so that for example a single
36 patch will contain only one bug fix (the patch might modify several
37 files, but it's doing ``only one thing''), and you may have a number
38 of such patches for different bugs you need fixed and local changes
39 you require. In this situation, if you submit a bug fix patch to the
40 upstream maintainers of a package and they include your fix in a
41 subsequent release, you can simply drop that single patch when you're
42 updating to the newer release.
44 Maintaining a single patch against an upstream tree is a little
45 tedious and error-prone, but not difficult. However, the complexity
46 of the problem grows rapidly as the number of patches you have to
47 maintain increases. With more than a tiny number of patches in hand,
48 understanding which ones you have applied and maintaining them moves
49 from messy to overwhelming.
51 Fortunately, Mercurial includes a powerful extension, Mercurial Queues
52 (or simply ``MQ''), that massively simplifies the patch management
53 problem.
55 \section{The prehistory of Mercurial Queues}
56 \label{sec:mq:history}
58 During the late 1990s, several Linux kernel developers started to
59 maintain ``patch series'' that modified the behaviour of the Linux
60 kernel. Some of these series were focused on stability, some on
61 feature coverage, and others were more speculative.
63 The sizes of these patch series grew rapidly. In 2002, Andrew Morton
64 published some shell scripts he had been using to automate the task of
65 managing his patch queues. Andrew was successfully using these
66 scripts to manage hundreds (sometimes thousands) of patches on top of
67 the Linux kernel.
69 \subsection{A patchwork quilt}
70 \label{sec:mq:quilt}
73 In early 2003, Andreas Gruenbacher and Martin Quinson borrowed the
74 approach of Andrew's scripts and published a tool called ``patchwork
75 quilt''~\cite{web:quilt}, or simply ``quilt''
76 (see~\cite{gruenbacher:2005} for a paper describing it). Because
77 quilt substantially automated patch management, it rapidly gained a
78 large following among open source software developers.
80 Quilt manages a \emph{stack of patches} on top of a directory tree.
81 To begin, you tell quilt to manage a directory tree; it stores away
82 the names and contents of all files in the tree. To fix a bug, you
83 create a new patch (using a single command), edit the files you need
84 to fix, then ``refresh'' the patch.
86 The refresh step causes quilt to scan the directory tree; it updates
87 the patch with all of the changes you have made. You can create
88 another patch on top of the first, which will track the changes
89 required to modify the tree from ``tree with one patch applied'' to
90 ``tree with two patches applied''.
92 You can \emph{change} which patches are applied to the tree. If you
93 ``pop'' a patch, the changes made by that patch will vanish from the
94 directory tree. Quilt remembers which patches you have popped,
95 though, so you can ``push'' a popped patch again, and the directory
96 tree will be restored to contain the modifications in the patch. Most
97 importantly, you can run the ``refresh'' command at any time, and the
98 topmost applied patch will be updated. This means that you can, at
99 any time, change both which patches are applied and what
100 modifications those patches make.
102 Quilt knows nothing about revision control tools, so it works equally
103 well on top of an unpacked tarball or a Subversion repository.
105 \subsection{From patchwork quilt to Mercurial Queues}
106 \label{sec:mq:quilt-mq}
108 In mid-2005, Chris Mason took the features of quilt and wrote an
109 extension that he called Mercurial Queues, which added quilt-like
110 behaviour to Mercurial.
112 The key difference between quilt and MQ is that quilt knows nothing
113 about revision control systems, while MQ is \emph{integrated} into
114 Mercurial. Each patch that you push is represented as a Mercurial
115 changeset. Pop a patch, and the changeset goes away.
117 This integration makes understanding patches and debugging their
118 effects \emph{enormously} easier. Since every applied patch has an
119 associated changeset, you can use \hgcmdargs{log}{\emph{filename}} to
120 see which changesets and patches affected a file. You can use the
121 \hgext{bisect} extension to binary-search through all changesets and
122 applied patches to see where a bug got introduced or fixed. You can
123 use the \hgcmd{annotate} command to see which changeset or patch
124 modified a particular line of a source file. And so on.
126 Because quilt does not care about revision control tools, it is still
127 a tremendously useful piece of software to know about for situations
128 where you cannot use Mercurial and MQ.
129 \section{Getting started with Mercurial Queues}
130 \label{sec:mq:start}
132 Because MQ is implemented as an extension, you must explicitly enable
133 before you can use it. (You don't need to download anything; MQ ships
134 with the standard Mercurial distribution.) To enable MQ, edit your
135 \tildefile{.hgrc} file, and add the lines in figure~\ref{ex:mq:config}.
137 \begin{figure}[ht]
138 \begin{codesample4}
139 [extensions]
140 hgext.mq =
141 \end{codesample4}
142 \label{ex:mq:config}
143 \caption{Contents to add to \tildefile{.hgrc} to enable the MQ extension}
144 \end{figure}
146 Once the extension is enabled, it will make a number of new commands
147 available. To verify that the extension is working, you can use
148 \hgcmd{help} to see if the \hgcmd{qinit} command is now available; see
149 the example in figure~\ref{ex:mq:enabled}.
151 \begin{figure}[ht]
152 \interaction{mq.qinit-help.help}
153 \caption{How to verify that MQ is enabled}
154 \label{ex:mq:enabled}
155 \end{figure}
157 You can use MQ with \emph{any} Mercurial repository, and its commands
158 only operate within that repository. To get started, simply prepare
159 the repository using the \hgcmd{qinit} command (see
160 figure~\ref{ex:mq:qinit}). This command creates an empty directory
161 called \sdirname{.hg/patches}, where MQ will keep its metadata. As
162 with many Mercurial commands, the \hgcmd{qinit} command prints nothing
163 if it succeeds.
165 \begin{figure}[ht]
166 \interaction{mq.tutorial.qinit}
167 \caption{Preparing a repository for use with MQ}
168 \label{ex:mq:qinit}
169 \end{figure}
171 \begin{figure}[ht]
172 \interaction{mq.tutorial.qnew}
173 \caption{Creating a new patch}
174 \label{ex:mq:qnew}
175 \end{figure}
177 \subsection{Creating a new patch}
179 To begin work on a new patch, use the \hgcmd{qnew} command. This
180 command takes one argument, the name of the patch to create. MQ will
181 use this as the name of an actual file in the \sdirname{.hg/patches}
182 directory, as you can see in figure~\ref{ex:mq:qnew}.
184 Also newly present in the \sdirname{.hg/patches} directory are two
185 other files, \sfilename{series} and \sfilename{status}. The
186 \sfilename{series} file lists all of the patches that MQ knows about
187 for this repository, with one patch per line. Mercurial uses the
188 \sfilename{status} file for internal book-keeping; it tracks all of the
189 patches that MQ has \emph{applied} in this repository.
191 \begin{note}
192 You may sometimes want to edit the \sfilename{series} file by hand;
193 for example, to change the sequence in which some patches are
194 applied. However, manually editing the \sfilename{status} file is
195 almost always a bad idea, as it's easy to corrupt MQ's idea of what
196 is happening.
197 \end{note}
199 Once you have created your new patch, you can edit files in the
200 working directory as you usually would. All of the normal Mercurial
201 commands, such as \hgcmd{diff} and \hgcmd{annotate}, work exactly as
202 they did before.
203 \subsection{Refreshing a patch}
205 When you reach a point where you want to save your work, use the
206 \hgcmd{qrefresh} command (figure~\ref{ex:mq:qnew}) to update the patch
207 you are working on. This command folds the changes you have made in
208 the working directory into your patch, and updates its corresponding
209 changeset to contain those changes.
211 \begin{figure}[ht]
212 \interaction{mq.tutorial.qrefresh}
213 \caption{Refreshing a patch}
214 \label{ex:mq:qrefresh}
215 \end{figure}
217 You can run \hgcmd{qrefresh} as often as you like, so it's a good way
218 to ``checkpoint'' your work. Refresh your patch at an opportune
219 time; try an experiment; and if the experiment doesn't work out,
220 \hgcmd{revert} your modifications back to the last time you refreshed.
222 \begin{figure}[ht]
223 \interaction{mq.tutorial.qrefresh2}
224 \caption{Refresh a patch many times to accumulate changes}
225 \label{ex:mq:qrefresh2}
226 \end{figure}
228 \subsection{Stacking and tracking patches}
230 Once you have finished working on a patch, or need to work on another,
231 you can use the \hgcmd{qnew} command again to create a new patch.
232 Mercurial will apply this patch on top of your existing patch. See
233 figure~\ref{ex:mq:qnew2} for an example. Notice that the patch
234 contains the changes in our prior patch as part of its context (you
235 can see this more clearly in the output of \hgcmd{annotate}).
237 \begin{figure}[ht]
238 \interaction{mq.tutorial.qnew2}
239 \caption{Stacking a second patch on top of the first}
240 \label{ex:mq:qnew2}
241 \end{figure}
243 So far, with the exception of \hgcmd{qnew} and \hgcmd{qrefresh}, we've
244 been careful to only use regular Mercurial commands. However, there
245 are more ``natural'' commands you can use when thinking about patches
246 with MQ, as illustrated in figure~\ref{ex:mq:qseries}:
248 \begin{itemize}
249 \item The \hgcmd{qseries} command lists every patch that MQ knows
250 about in this repository, from oldest to newest (most recently
251 \emph{created}).
252 \item The \hgcmd{qapplied} command lists every patch that MQ has
253 \emph{applied} in this repository, again from oldest to newest (most
254 recently applied).
255 \end{itemize}
257 \begin{figure}[ht]
258 \interaction{mq.tutorial.qseries}
259 \caption{Understanding the patch stack with \hgcmd{qseries} and
260 \hgcmd{qapplied}}
261 \label{ex:mq:qseries}
262 \end{figure}
264 \subsection{Manipulating the patch stack}
266 The previous discussion implied that there must be a difference
267 between ``known'' and ``applied'' patches, and there is. MQ can
268 manage a patch without it being applied in the repository.
270 An \emph{applied} patch has a corresponding changeset in the
271 repository, and the effects of the patch and changeset are visible in
272 the working directory. You can undo the application of a patch using
273 the \hgcmd{qpop} command. MQ still \emph{knows about}, or manages, a
274 popped patch, but the patch no longer has a corresponding changeset in
275 the repository, and the working directory does not contain the changes
276 made by the patch. Figure~\ref{fig:mq:stack} illustrates the
277 difference between applied and tracked patches.
279 \begin{figure}[ht]
280 \centering
281 \grafix{mq-stack}
282 \caption{Applied and unapplied patches in the MQ patch stack}
283 \label{fig:mq:stack}
284 \end{figure}
286 You can reapply an unapplied, or popped, patch using the \hgcmd{qpush}
287 command. This creates a new changeset to correspond to the patch, and
288 the patch's changes once again become present in the working
289 directory. See figure~\ref{ex:mq:qpop} for examples of \hgcmd{qpop}
290 and \hgcmd{qpush} in action. Notice that once we have popped a patch
291 or two patches, the output of \hgcmd{qseries} remains the same, while
292 that of \hgcmd{qapplied} has changed.
294 \begin{figure}[ht]
295 \interaction{mq.tutorial.qpop}
296 \caption{Modifying the stack of applied patches}
297 \label{ex:mq:qpop}
298 \end{figure}
300 MQ does not limit you to pushing or popping one patch. You can have
301 no patches, all of them, or any number in between applied at some
302 point in time.
304 \subsection{Working on several patches at once}
306 The \hgcmd{qrefresh} command always refreshes the \emph{topmost}
307 applied patch. This means that you can suspend work on one patch (by
308 refreshing it), pop or push to make a different patch the top, and
309 work on \emph{that} patch for a while.
311 Here's an example that illustrates how you can use this ability.
312 Let's say you're developing a new feature as two patches. The first
313 is a change to the core of your software, and the second---layered on
314 top of the first---changes the user interface to use the code you just
315 added to the core. If you notice a bug in the core while you're
316 working on the UI patch, it's easy to fix the core. Simply
317 \hgcmd{qrefresh} the UI patch to save your in-progress changes, and
318 \hgcmd{qpop} down to the core patch. Fix the core bug,
319 \hgcmd{qrefresh} the core patch, and \hgcmd{qpush} back to the UI
320 patch to continue where you left off.
322 \section{Mercurial Queues and GNU patch}
323 \label{sec:mq:patch}
325 MQ uses the GNU \command{patch} command to apply patches. Because MQ
326 doesn't hide its patch-oriented nature, it is helpful to understand
327 the data that MQ and \command{patch} work with, and a few aspects of
328 how \command{patch} operates.
330 The \command{diff} command generates a list of modifications by
331 comparing two files. The \command{patch} command applies a list of
332 modifications to a file. The kinds of files that \command{diff} and
333 \command{patch} work with are referred to as both ``diffs'' and
334 ``patches;'' there is no difference between a diff and a patch.
336 A patch file can start with arbitrary text; MQ uses this text as the
337 commit message when creating changesets. It treats the first line
338 that starts with the string ``\texttt{diff~-}'' as the separator
339 between header and content.
341 MQ works with \emph{unified} diffs (\command{patch} can accept several
342 other diff formats, but MQ doesn't). A unified diff contains two
343 kinds of header. The \emph{file header} describes the file being
344 modified; it contains the name of the file to modify. When
345 \command{patch} sees a new file header, it looks for a file with that
346 name to start modifying.
348 After the file header comes a series of \emph{hunks}. Each hunk
349 starts with a header; this identifies the range of line numbers within
350 the file that the hunk should modify. Following the header, a hunk
351 starts and ends with a few (usually three) lines of text from the
352 unmodified file; these are called the \emph{context} for the hunk.
353 Each unmodified line begins with a space characters. Within the hunk,
354 a line that begins with ``\texttt{-}'' means ``remove this line,''
355 while a line that begins with ``\texttt{+}'' means ``insert this
356 line.'' For example, a line that is modified is represented by one
357 deletion and one insertion.
359 The \command{diff} command runs hunks together when there's not enough
360 context between modifications to justify
362 When \command{patch} applies a hunk, it tries a handful of
363 successively less accurate strategies to try to make the hunk apply.
364 This falling-back technique often makes it possible to take a patch
365 that was generated against an old version of a file, and apply it
366 against a newer version of that file.
368 First, \command{patch} tries an exact match, where the line numbers,
369 the context, and the text to be modified must apply exactly. If it
370 cannot make an exact match, it tries to find an exact match for the
371 context, without honouring the line numbering information. If this
372 succeeds, it prints a line of output saying that the hunk was applied,
373 but at some \emph{offset} from the original line number.
375 If a context-only match fails, \command{patch} removes the first and
376 last lines of the context, and tries a \emph{reduced} context-only
377 match. If the hunk with reduced context succeeds, it prints a message
378 saying that it applied the hunk with a \emph{fuzz factor} (the number
379 after the fuzz factor indicates how many lines of context
380 \command{patch} had to trim before the patch applied).
382 When neither of these techniques works, \command{patch} prints a
383 message saying that the hunk in question was rejected. It saves
384 rejected hunks (also simply called ``rejects'') to a file with the
385 same name, and an added \sfilename{.rej} extension. It also saves an
386 unmodified copy of the file with a \sfilename{.orig} extension; the
387 copy of the file without any extensions will contain any changes made
388 by hunks that \emph{did} apply cleanly. If you have a patch that
389 modifies \filename{foo} with six hunks, and one of them fails to
390 apply, you will have: an unmodified \filename{foo.orig}, a
391 \filename{foo.rej} containing one hunk, and \filename{foo}, containing
392 the changes made by the five successful five hunks.
394 \subsection{Beware the fuzz}
396 While applying a hunk at an offset, or with a fuzz factor, will often
397 be completely successful, these inexact techniques naturally leave
398 open the possibility of corrupting the patched file. The most common
399 cases typically involve applying a patch twice, or at an incorrect
400 location in the file. If \command{patch} or \hgcmd{qpush} ever
401 mentions an offset or fuzz factor, you should make sure that the
402 modified files are correct afterwards.
404 It's often a good idea to refresh a patch that has applied with an
405 offset or fuzz factor; refreshing the patch generates new context
406 information that will make it apply cleanly. I say ``often,'' not
407 ``always,'' because sometimes refreshing a patch will make it fail to
408 apply against a different revision of the underlying files. In some
409 cases, such as when you're maintaining a patch that must sit on top of
410 multiple versions of a source tree, it's acceptable to have a patch
411 apply with some fuzz, provided you've verified the results of the
412 patching process in such cases.
414 \subsection{Handling rejection}
416 If \hgcmd{qpush} fails to apply a patch, it will print an error
417 message and exit. If it has left \sfilename{.rej} files behind, it is
418 usually best to fix up the rejected hunks before you push more patches
419 or do any further work.
421 If your patch \emph{used to} apply cleanly, and no longer does because
422 you've changed the underlying code that your patches are based on,
423 Mercurial Queues can help; see section~\ref{sec:mq:merge} for details.
425 Unfortunately, there aren't any great techniques for dealing with
426 rejected hunks. Most often, you'll need to view the \sfilename{.rej}
427 file and edit the target file, applying the rejected hunks by hand.
429 If you're feeling adventurous, Neil Brown, a Linux kernel hacker,
430 wrote a tool called \command{wiggle}~\cite{web:wiggle}, which is more
431 vigorous than \command{patch} in its attempts to make a patch apply.
433 Another Linux kernel hacker, Chris Mason (the author of Mercurial
434 Queues), wrote a similar tool called \command{rej}~\cite{web:rej},
435 which takes a simple approach to automating the application of hunks
436 rejected by \command{patch}. \command{rej} can help with four common
437 reasons that a hunk may be rejected:
439 \begin{itemize}
440 \item The context in the middle of a hunk has changed.
441 \item A hunk is missing some context at the beginning or end.
442 \item A large hunk might apply better---either entirely or in
443 part---if it was broken up into smaller hunks.
444 \item A hunk removes lines with slightly different content than those
445 currently present in the file.
446 \end{itemize}
448 If you use \command{wiggle} or \command{rej}, you should be doubly
449 careful to check your results when you're done.
451 \section{Getting the best performance out of MQ}
453 MQ is very efficient at handling a large number of patches. I ran
454 some performance experiments in mid-2006 for a talk that I gave at the
455 2006 EuroPython conference~\cite{web:europython}. I used as my data
456 set the Linux 2.6.17-mm1 patch series, which consists of 1,738
457 patches. I applied thes on top of a Linux kernel repository
458 containing all 27,472 revisions between Linux 2.6.12-rc2 and Linux
459 2.6.17.
461 On my old, slow laptop, I was able to
462 \hgcmdargs{qpush}{\hgopt{qpush}{-a}} all 1,738 patches in 3.5 minutes,
463 and \hgcmdargs{qpop}{\hgopt{qpop}{-a}} them all in 30 seconds. I
464 could \hgcmd{qrefresh} one of the biggest patches (which made 22,779
465 lines of changes to 287 files) in 6.6 seconds.
467 Clearly, MQ is well suited to working in large trees, but there are a
468 few tricks you can use to get the best performance of it.
470 First of all, try to ``batch'' operations together. Every time you
471 run \hgcmd{qpush} or \hgcmd{qpop}, these commands scan the working
472 directory once to make sure you haven't made some changes and then
473 forgotten to run \hgcmd{qrefresh}. On a small tree, the time that
474 this scan takes is unnoticeable. However, on a medium-sized tree
475 (containing tens of thousands of files), it can take a second or more.
477 The \hgcmd{qpush} and \hgcmd{qpop} commands allow you to push and pop
478 multiple patches at a time. You can identify the ``destination
479 patch'' that you want to end up at. When you \hgcmd{qpush} with a
480 destination specified, it will push patches until that patch is at the
481 top of the applied stack. When you \hgcmd{qpop} to a destination, MQ
482 will pop patches until the destination patch \emph{is no longer}
483 applied.
485 You can identify a destination patch using either the name of the
486 patch, or by number. If you use numeric addressing, patches are
487 counted from zero; this means that the first patch is zero, the second
488 is one, and so on.
490 \section{Updating your patches when the underlying code changes}
491 \label{sec:mq:merge}
493 It's common to have a stack of patches on top of an underlying
494 repository that you don't modify directly. If you're working on
495 changes to third-party code, or on a feature that is taking longer to
496 develop than the rate of change of the code beneath, you will often
497 need to sync up with the underlying code, and fix up any hunks in your
498 patches that no longer apply. This is called \emph{rebasing} your
499 patch series.
501 The simplest way to do this is to \hgcmdargs{qpop}{\hgopt{qpop}{-a}}
502 your patches, then \hgcmd{pull} changes into the underlying
503 repository, and finally \hgcmdargs{qpush}{\hgopt{qpop}{-a}} your
504 patches again. MQ will stop pushing any time it runs across a patch
505 that fails to apply during conflicts, allowing you to fix your
506 conflicts, \hgcmd{qrefresh} the affected patch, and continue pushing
507 until you have fixed your entire stack.
509 This approach is easy to use and works well if you don't expect
510 changes to the underlying code to affect how well your patches apply.
511 If your patch stack touches code that is modified frequently or
512 invasively in the underlying repository, however, fixing up rejected
513 hunks by hand quickly becomes tiresome.
515 It's possible to partially automate the rebasing process. If your
516 patches apply cleanly against some revision of the underlying repo, MQ
517 can use this information to help you to resolve conflicts between your
518 patches and a different revision.
520 The process is a little involved.
521 \begin{enumerate}
522 \item To begin, \hgcmdargs{qpush}{-a} all of your patches on top of
523 the revision where you know that they apply cleanly.
524 \item Save a backup copy of your patch directory using
525 \hgcmdargs{qsave}{\hgopt{qsave}{-e} \hgopt{qsave}{-c}}. This prints
526 the name of the directory that it has saved the patches in. It will
527 save the patches to a directory called
528 \sdirname{.hg/patches.\emph{N}}, where \texttt{\emph{N}} is a small
529 integer. It also commits a ``save changeset'' on top of your
530 applied patches; this is for internal book-keeping, and records the
531 states of the \sfilename{series} and \sfilename{status} files.
532 \item Use \hgcmd{pull} to bring new changes into the underlying
533 repository. (Don't run \hgcmdargs{pull}{-u}; see below for why.)
534 \item Update to the new tip revision, using
535 \hgcmdargs{update}{\hgopt{update}{-C}} to override the patches you
536 have pushed.
537 \item Merge all patches using \hgcmdargs{qpush}{\hgopt{qpush}{-m}
538 \hgopt{qpush}{-a}}. The \hgopt{qpush}{-m} option to \hgcmd{qpush}
539 tells MQ to perform a three-way merge if the patch fails to apply.
540 \end{enumerate}
542 During the \hgcmdargs{qpush}{\hgopt{qpush}{-m}}, each patch in the
543 \sfilename{series} file is applied normally. If a patch applies with
544 fuzz or rejects, MQ looks at the queue you \hgcmd{qsave}d, and
545 performs a three-way merge with the corresponding changeset. This
546 merge uses Mercurial's normal merge machinery, so it may pop up a GUI
547 merge tool to help you to resolve problems.
549 When you finish resolving the effects of a patch, MQ refreshes your
550 patch based on the result of the merge.
552 At the end of this process, your repository will have one extra head
553 from the old patch queue, and a copy of the old patch queue will be in
554 \sdirname{.hg/patches.\emph{N}}. You can remove the extra head using
555 \hgcmdargs{qpop}{\hgopt{qpop}{-a} \hgopt{qpop}{-n} patches.\emph{N}}
556 or \hgcmd{strip}. You can delete \sdirname{.hg/patches.\emph{N}} once
557 you are sure that you no longer need it as a backup.
559 \section{Managing patches in a repository}
561 Because MQ's \sdirname{.hg/patches} directory resides outside a
562 Mercurial repository's working directory, the ``underlying'' Mercurial
563 repository knows nothing about the management or presence of patches.
565 This presents the interesting possibility of managing the contents of
566 the patch directory as a Mercurial repository in its own right. This
567 can be a useful way to work. For example, you can work on a patch for
568 a while, \hgcmd{qrefresh} it, then \hgcmd{commit} the current state of
569 the patch. This lets you ``roll back'' to that version of the patch
570 later on.
572 In addition, you can then share different versions of the same patch
573 stack among multiple underlying repositories. I use this when I am
574 developing a Linux kernel feature. I have a pristine copy of my
575 kernel sources for each of several CPU architectures, and a cloned
576 repository under each that contains the patches I am working on. When
577 I want to test a change on a different architecture, I push my current
578 patches to the patch repository associated with that kernel tree, pop
579 and push all of my patches, and build and test that kernel.
581 Managing patches in a repository makes it possible for multiple
582 developers to work on the same patch series without colliding with
583 each other, all on top of an underlying source base that they may or
584 may not control.
586 \subsection{MQ support for patch repositories}
588 MQ helps you to work with the \sdirname{.hg/patches} directory as a
589 repository; when you prepare a repository for working with patches
590 using \hgcmd{qinit}, you can pass the \hgopt{qinit}{-c} option to
591 create the \sdirname{.hg/patches} directory as a Mercurial repository.
593 \begin{note}
594 If you forget to use the \hgopt{qinit}{-c} option, you can simply go
595 into the \sdirname{.hg/patches} directory at any time and run
596 \hgcmd{init}. Don't forget to add an entry for the
597 \sfilename{status} file to the \sfilename{.hgignore} file, though
598 (\hgcmdargs{qinit}{\hgopt{qinit}{-c}} does this for you
599 automatically); you \emph{really} don't want to manage the
600 \sfilename{status} file.
601 \end{note}
603 As a convenience, if MQ notices that the \dirname{.hg/patches}
604 directory is a repository, it will automatically \hgcmd{add} every
605 patch that you create and import.
607 Finally, MQ provides a shortcut command, \hgcmd{qcommit}, that runs
608 \hgcmd{commit} in the \sdirname{.hg/patches} directory. This saves
609 some cumbersome typing.
611 \subsection{A few things to watch out for}
613 MQ's support for working with a repository full of patches is limited
614 in a few small respects.
616 MQ cannot automatically detect changes that you make to the patch
617 directory. If you \hgcmd{pull}, manually edit, or \hgcmd{update}
618 changes to patches or the \sfilename{series} file, you will have to
619 \hgcmdargs{qpop}{\hgopt{qpop}{-a}} and then
620 \hgcmdargs{qpush}{\hgopt{qpush}{-a}} in the underlying repository to
621 see those changes show up there. If you forget to do this, you can
622 confuse MQ's idea of which patches are applied.
624 \section{Commands for working with patches}
626 Once you've been working with patches for a while, you'll find
627 yourself hungry for tools that will help you to understand and
628 manipulate the patches you're dealing with.
630 The \command{diffstat} command~\cite{web:diffstat} generates a
631 histogram of the modifications made to each file in a patch. It
632 provides a good way to ``get a sense of'' a patch---which files it
633 affects, and how much change it introduces to each file and as a
634 whole. (I find that it's a good idea to use \command{diffstat}'s
635 \texttt{-p} option as a matter of course, as otherwise it will try to
636 do clever things with prefixes of file names that inevitably confuse
637 at least me.)
639 The \package{patchutils} package~\cite{web:patchutils} is invaluable.
640 It provides a set of small utilities that follow the ``Unix
641 philosophy;'' each does one useful thing with a patch. The
642 \package{patchutils} command I use most is \command{filterdiff}, which
643 extracts subsets from a patch file. For example, given a patch that
644 modifies hundreds of files across dozens of directories, a single
645 invocation of \command{filterdiff} can generate a smaller patch that
646 only touches files whose names match a particular glob pattern.
648 %%% Local Variables:
649 %%% mode: latex
650 %%% TeX-master: "00book"
651 %%% End: