hgbook
view en/branch.tex @ 296:e0b56f008b09
Fix sample output for 'backout' test.
author | Guido Ostkamp <hg@ostkamp.fastmail.fm> |
---|---|
date | Wed Aug 20 21:51:54 2008 +0200 (2008-08-20) |
parents | a24b370a16ee |
children | c36a6f534b99 |
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1 \chapter{Managing releases and branchy development}
2 \label{chap:branch}
4 Mercurial provides several mechanisms for you to manage a project that
5 is making progress on multiple fronts at once. To understand these
6 mechanisms, let's first take a brief look at a fairly normal software
7 project structure.
9 Many software projects issue periodic ``major'' releases that contain
10 substantial new features. In parallel, they may issue ``minor''
11 releases. These are usually identical to the major releases off which
12 they're based, but with a few bugs fixed.
14 In this chapter, we'll start by talking about how to keep records of
15 project milestones such as releases. We'll then continue on to talk
16 about the flow of work between different phases of a project, and how
17 Mercurial can help you to isolate and manage this work.
19 \section{Giving a persistent name to a revision}
21 Once you decide that you'd like to call a particular revision a
22 ``release'', it's a good idea to record the identity of that revision.
23 This will let you reproduce that release at a later date, for whatever
24 purpose you might need at the time (reproducing a bug, porting to a
25 new platform, etc).
26 \interaction{tag.init}
28 Mercurial lets you give a permanent name to any revision using the
29 \hgcmd{tag} command. Not surprisingly, these names are called
30 ``tags''.
31 \interaction{tag.tag}
33 A tag is nothing more than a ``symbolic name'' for a revision. Tags
34 exist purely for your convenience, so that you have a handy permanent
35 way to refer to a revision; Mercurial doesn't interpret the tag names
36 you use in any way. Neither does Mercurial place any restrictions on
37 the name of a tag, beyond a few that are necessary to ensure that a
38 tag can be parsed unambiguously. A tag name cannot contain any of the
39 following characters:
40 \begin{itemize}
41 \item Colon (ASCII 58, ``\texttt{:}'')
42 \item Carriage return (ASCII 13, ``\Verb+\r+'')
43 \item Newline (ASCII 10, ``\Verb+\n+'')
44 \end{itemize}
46 You can use the \hgcmd{tags} command to display the tags present in
47 your repository. In the output, each tagged revision is identified
48 first by its name, then by revision number, and finally by the unique
49 hash of the revision.
50 \interaction{tag.tags}
51 Notice that \texttt{tip} is listed in the output of \hgcmd{tags}. The
52 \texttt{tip} tag is a special ``floating'' tag, which always
53 identifies the newest revision in the repository.
55 In the output of the \hgcmd{tags} command, tags are listed in reverse
56 order, by revision number. This usually means that recent tags are
57 listed before older tags. It also means that \texttt{tip} is always
58 going to be the first tag listed in the output of \hgcmd{tags}.
60 When you run \hgcmd{log}, if it displays a revision that has tags
61 associated with it, it will print those tags.
62 \interaction{tag.log}
64 Any time you need to provide a revision~ID to a Mercurial command, the
65 command will accept a tag name in its place. Internally, Mercurial
66 will translate your tag name into the corresponding revision~ID, then
67 use that.
68 \interaction{tag.log.v1.0}
70 There's no limit on the number of tags you can have in a repository,
71 or on the number of tags that a single revision can have. As a
72 practical matter, it's not a great idea to have ``too many'' (a number
73 which will vary from project to project), simply because tags are
74 supposed to help you to find revisions. If you have lots of tags, the
75 ease of using them to identify revisions diminishes rapidly.
77 For example, if your project has milestones as frequent as every few
78 days, it's perfectly reasonable to tag each one of those. But if you
79 have a continuous build system that makes sure every revision can be
80 built cleanly, you'd be introducing a lot of noise if you were to tag
81 every clean build. Instead, you could tag failed builds (on the
82 assumption that they're rare!), or simply not use tags to track
83 buildability.
85 If you want to remove a tag that you no longer want, use
86 \hgcmdargs{tag}{--remove}.
87 \interaction{tag.remove}
88 You can also modify a tag at any time, so that it identifies a
89 different revision, by simply issuing a new \hgcmd{tag} command.
90 You'll have to use the \hgopt{tag}{-f} option to tell Mercurial that
91 you \emph{really} want to update the tag.
92 \interaction{tag.replace}
93 There will still be a permanent record of the previous identity of the
94 tag, but Mercurial will no longer use it. There's thus no penalty to
95 tagging the wrong revision; all you have to do is turn around and tag
96 the correct revision once you discover your error.
98 Mercurial stores tags in a normal revision-controlled file in your
99 repository. If you've created any tags, you'll find them in a file
100 named \sfilename{.hgtags}. When you run the \hgcmd{tag} command,
101 Mercurial modifies this file, then automatically commits the change to
102 it. This means that every time you run \hgcmd{tag}, you'll see a
103 corresponding changeset in the output of \hgcmd{log}.
104 \interaction{tag.tip}
106 \subsection{Handling tag conflicts during a merge}
108 You won't often need to care about the \sfilename{.hgtags} file, but
109 it sometimes makes its presence known during a merge. The format of
110 the file is simple: it consists of a series of lines. Each line
111 starts with a changeset hash, followed by a space, followed by the
112 name of a tag.
114 If you're resolving a conflict in the \sfilename{.hgtags} file during
115 a merge, there's one twist to modifying the \sfilename{.hgtags} file:
116 when Mercurial is parsing the tags in a repository, it \emph{never}
117 reads the working copy of the \sfilename{.hgtags} file. Instead, it
118 reads the \emph{most recently committed} revision of the file.
120 An unfortunate consequence of this design is that you can't actually
121 verify that your merged \sfilename{.hgtags} file is correct until
122 \emph{after} you've committed a change. So if you find yourself
123 resolving a conflict on \sfilename{.hgtags} during a merge, be sure to
124 run \hgcmd{tags} after you commit. If it finds an error in the
125 \sfilename{.hgtags} file, it will report the location of the error,
126 which you can then fix and commit. You should then run \hgcmd{tags}
127 again, just to be sure that your fix is correct.
129 \subsection{Tags and cloning}
131 You may have noticed that the \hgcmd{clone} command has a
132 \hgopt{clone}{-r} option that lets you clone an exact copy of the
133 repository as of a particular changeset. The new clone will not
134 contain any project history that comes after the revision you
135 specified. This has an interaction with tags that can surprise the
136 unwary.
138 Recall that a tag is stored as a revision to the \sfilename{.hgtags}
139 file, so that when you create a tag, the changeset in which it's
140 recorded necessarily refers to an older changeset. When you run
141 \hgcmdargs{clone}{-r foo} to clone a repository as of tag
142 \texttt{foo}, the new clone \emph{will not contain the history that
143 created the tag} that you used to clone the repository. The result
144 is that you'll get exactly the right subset of the project's history
145 in the new repository, but \emph{not} the tag you might have expected.
147 \subsection{When permanent tags are too much}
149 Since Mercurial's tags are revision controlled and carried around with
150 a project's history, everyone you work with will see the tags you
151 create. But giving names to revisions has uses beyond simply noting
152 that revision \texttt{4237e45506ee} is really \texttt{v2.0.2}. If
153 you're trying to track down a subtle bug, you might want a tag to
154 remind you of something like ``Anne saw the symptoms with this
155 revision''.
157 For cases like this, what you might want to use are \emph{local} tags.
158 You can create a local tag with the \hgopt{tag}{-l} option to the
159 \hgcmd{tag} command. This will store the tag in a file called
160 \sfilename{.hg/localtags}. Unlike \sfilename{.hgtags},
161 \sfilename{.hg/localtags} is not revision controlled. Any tags you
162 create using \hgopt{tag}{-l} remain strictly local to the repository
163 you're currently working in.
165 \section{The flow of changes---big picture vs. little}
167 To return to the outline I sketched at the beginning of a chapter,
168 let's think about a project that has multiple concurrent pieces of
169 work under development at once.
171 There might be a push for a new ``main'' release; a new minor bugfix
172 release to the last main release; and an unexpected ``hot fix'' to an
173 old release that is now in maintenance mode.
175 The usual way people refer to these different concurrent directions of
176 development is as ``branches''. However, we've already seen numerous
177 times that Mercurial treats \emph{all of history} as a series of
178 branches and merges. Really, what we have here is two ideas that are
179 peripherally related, but which happen to share a name.
180 \begin{itemize}
181 \item ``Big picture'' branches represent the sweep of a project's
182 evolution; people give them names, and talk about them in
183 conversation.
184 \item ``Little picture'' branches are artefacts of the day-to-day
185 activity of developing and merging changes. They expose the
186 narrative of how the code was developed.
187 \end{itemize}
189 \section{Managing big-picture branches in repositories}
191 The easiest way to isolate a ``big picture'' branch in Mercurial is in
192 a dedicated repository. If you have an existing shared
193 repository---let's call it \texttt{myproject}---that reaches a ``1.0''
194 milestone, you can start to prepare for future maintenance releases on
195 top of version~1.0 by tagging the revision from which you prepared
196 the~1.0 release.
197 \interaction{branch-repo.tag}
198 You can then clone a new shared \texttt{myproject-1.0.1} repository as
199 of that tag.
200 \interaction{branch-repo.clone}
202 Afterwards, if someone needs to work on a bug fix that ought to go
203 into an upcoming~1.0.1 minor release, they clone the
204 \texttt{myproject-1.0.1} repository, make their changes, and push them
205 back.
206 \interaction{branch-repo.bugfix}
207 Meanwhile, development for the next major release can continue,
208 isolated and unabated, in the \texttt{myproject} repository.
209 \interaction{branch-repo.new}
211 \section{Don't repeat yourself: merging across branches}
213 In many cases, if you have a bug to fix on a maintenance branch, the
214 chances are good that the bug exists on your project's main branch
215 (and possibly other maintenance branches, too). It's a rare developer
216 who wants to fix the same bug multiple times, so let's look at a few
217 ways that Mercurial can help you to manage these bugfixes without
218 duplicating your work.
220 In the simplest instance, all you need to do is pull changes from your
221 maintenance branch into your local clone of the target branch.
222 \interaction{branch-repo.pull}
223 You'll then need to merge the heads of the two branches, and push back
224 to the main branch.
225 \interaction{branch-repo.merge}
227 \section{Naming branches within one repository}
229 In most instances, isolating branches in repositories is the right
230 approach. Its simplicity makes it easy to understand; and so it's
231 hard to make mistakes. There's a one-to-one relationship between
232 branches you're working in and directories on your system. This lets
233 you use normal (non-Mercurial-aware) tools to work on files within a
234 branch/repository.
236 If you're more in the ``power user'' category (\emph{and} your
237 collaborators are too), there is an alternative way of handling
238 branches that you can consider. I've already mentioned the
239 human-level distinction between ``small picture'' and ``big picture''
240 branches. While Mercurial works with multiple ``small picture''
241 branches in a repository all the time (for example after you pull
242 changes in, but before you merge them), it can \emph{also} work with
243 multiple ``big picture'' branches.
245 The key to working this way is that Mercurial lets you assign a
246 persistent \emph{name} to a branch. There always exists a branch
247 named \texttt{default}. Even before you start naming branches
248 yourself, you can find traces of the \texttt{default} branch if you
249 look for them.
251 As an example, when you run the \hgcmd{commit} command, and it pops up
252 your editor so that you can enter a commit message, look for a line
253 that contains the text ``\texttt{HG: branch default}'' at the bottom.
254 This is telling you that your commit will occur on the branch named
255 \texttt{default}.
257 To start working with named branches, use the \hgcmd{branches}
258 command. This command lists the named branches already present in
259 your repository, telling you which changeset is the tip of each.
260 \interaction{branch-named.branches}
261 Since you haven't created any named branches yet, the only one that
262 exists is \texttt{default}.
264 To find out what the ``current'' branch is, run the \hgcmd{branch}
265 command, giving it no arguments. This tells you what branch the
266 parent of the current changeset is on.
267 \interaction{branch-named.branch}
269 To create a new branch, run the \hgcmd{branch} command again. This
270 time, give it one argument: the name of the branch you want to create.
271 \interaction{branch-named.create}
273 After you've created a branch, you might wonder what effect the
274 \hgcmd{branch} command has had. What do the \hgcmd{status} and
275 \hgcmd{tip} commands report?
276 \interaction{branch-named.status}
277 Nothing has changed in the working directory, and there's been no new
278 history created. As this suggests, running the \hgcmd{branch} command
279 has no permanent effect; it only tells Mercurial what branch name to
280 use the \emph{next} time you commit a changeset.
282 When you commit a change, Mercurial records the name of the branch on
283 which you committed. Once you've switched from the \texttt{default}
284 branch to another and committed, you'll see the name of the new branch
285 show up in the output of \hgcmd{log}, \hgcmd{tip}, and other commands
286 that display the same kind of output.
287 \interaction{branch-named.commit}
288 The \hgcmd{log}-like commands will print the branch name of every
289 changeset that's not on the \texttt{default} branch. As a result, if
290 you never use named branches, you'll never see this information.
292 Once you've named a branch and committed a change with that name,
293 every subsequent commit that descends from that change will inherit
294 the same branch name. You can change the name of a branch at any
295 time, using the \hgcmd{branch} command.
296 \interaction{branch-named.rebranch}
297 In practice, this is something you won't do very often, as branch
298 names tend to have fairly long lifetimes. (This isn't a rule, just an
299 observation.)
301 \section{Dealing with multiple named branches in a repository}
303 If you have more than one named branch in a repository, Mercurial will
304 remember the branch that your working directory on when you start a
305 command like \hgcmd{update} or \hgcmdargs{pull}{-u}. It will update
306 the working directory to the tip of this branch, no matter what the
307 ``repo-wide'' tip is. To update to a revision that's on a different
308 named branch, you may need to use the \hgopt{update}{-C} option to
309 \hgcmd{update}.
311 This behaviour is a little subtle, so let's see it in action. First,
312 let's remind ourselves what branch we're currently on, and what
313 branches are in our repository.
314 \interaction{branch-named.parents}
315 We're on the \texttt{bar} branch, but there also exists an older
316 \hgcmd{foo} branch.
318 We can \hgcmd{update} back and forth between the tips of the
319 \texttt{foo} and \texttt{bar} branches without needing to use the
320 \hgopt{update}{-C} option, because this only involves going backwards
321 and forwards linearly through our change history.
322 \interaction{branch-named.update-switchy}
324 If we go back to the \texttt{foo} branch and then run \hgcmd{update},
325 it will keep us on \texttt{foo}, not move us to the tip of
326 \texttt{bar}.
327 \interaction{branch-named.update-nothing}
329 Committing a new change on the \texttt{foo} branch introduces a new
330 head.
331 \interaction{branch-named.foo-commit}
332 We can no longer update from \texttt{foo} to \texttt{bar} without
333 going ``sideways'' in history, so Mercurial forces us to provide the
334 \hgopt{update}{-C} option to \hgcmd{update}.
335 \interaction{branch-named.update-bar}
337 \section{Branch names and merging}
339 As you've probably noticed, merges in Mercurial are not symmetrical.
340 Let's say our repository has two heads, 17 and 23. If I
341 \hgcmd{update} to 17 and then \hgcmd{merge} with 23, Mercurial records
342 17 as the first parent of the merge, and 23 as the second. Whereas if
343 I \hgcmd{update} to 23 and then \hgcmd{merge} with 17, it records 23
344 as the first parent, and 17 as the second.
346 This affects Mercurial's choice of branch name when you merge. After
347 a merge, Mercurial will retain the branch name of the first parent
348 when you commit the result of the merge. If your first parent's
349 branch name is \texttt{foo}, and you merge with \texttt{bar}, the
350 branch name will still be \texttt{foo} after you merge.
352 It's not unusual for a repository to contain multiple heads, each with
353 the same branch name. Let's say I'm working on the \texttt{foo}
354 branch, and so are you. We commit different changes; I pull your
355 changes; I now have two heads, each claiming to be on the \texttt{foo}
356 branch. The result of a merge will be a single head on the
357 \texttt{foo} branch, as you might hope.
359 But if I'm working on the \texttt{bar} branch, and I merge work from
360 the \texttt{foo} branch, the result will remain on the \texttt{bar}
361 branch.
362 \interaction{branch-named.merge}
364 To give a more concrete example, if I'm working on the
365 \texttt{bleeding-edge} branch, and I want to bring in the latest fixes
366 from the \texttt{stable} branch, Mercurial will choose the ``right''
367 (\texttt{bleeding-edge}) branch name when I pull and merge from
368 \texttt{stable}.
370 \section{Branch naming is generally useful}
372 You shouldn't think of named branches as applicable only to situations
373 where you have multiple long-lived branches cohabiting in a single
374 repository. They're very useful even in the one-branch-per-repository
375 case.
377 In the simplest case, giving a name to each branch gives you a
378 permanent record of which branch a changeset originated on. This
379 gives you more context when you're trying to follow the history of a
380 long-lived branchy project.
382 If you're working with shared repositories, you can set up a
383 \hook{pretxnchangegroup} hook on each that will block incoming changes
384 that have the ``wrong'' branch name. This provides a simple, but
385 effective, defence against people accidentally pushing changes from a
386 ``bleeding edge'' branch to a ``stable'' branch. Such a hook might
387 look like this inside the shared repo's \hgrc.
388 \begin{codesample2}
389 [hooks]
390 pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch
391 \end{codesample2}
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