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author dukebody <dukebody@gmail.com>
date Mon Oct 26 14:12:27 2009 +0100 (2009-10-26)
parents 477d6a3e5023
children
rev   line source
bos@559 1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
bos@559 2
bos@559 3 <chapter id="chap:branch">
bos@572 4 <?dbhtml filename="managing-releases-and-branchy-development.html"?>
bos@559 5 <title>Managing releases and branchy development</title>
bos@559 6
bos@584 7 <para id="x_369">Mercurial provides several mechanisms for you to manage a
bos@559 8 project that is making progress on multiple fronts at once. To
bos@559 9 understand these mechanisms, let's first take a brief look at a
bos@559 10 fairly normal software project structure.</para>
bos@559 11
bos@584 12 <para id="x_36a">Many software projects issue periodic <quote>major</quote>
bos@559 13 releases that contain substantial new features. In parallel, they
bos@559 14 may issue <quote>minor</quote> releases. These are usually
bos@559 15 identical to the major releases off which they're based, but with
bos@559 16 a few bugs fixed.</para>
bos@559 17
bos@584 18 <para id="x_36b">In this chapter, we'll start by talking about how to keep
bos@559 19 records of project milestones such as releases. We'll then
bos@559 20 continue on to talk about the flow of work between different
bos@559 21 phases of a project, and how Mercurial can help you to isolate and
bos@559 22 manage this work.</para>
bos@559 23
bos@559 24 <sect1>
bos@559 25 <title>Giving a persistent name to a revision</title>
bos@559 26
bos@584 27 <para id="x_36c">Once you decide that you'd like to call a particular
bos@559 28 revision a <quote>release</quote>, it's a good idea to record
bos@559 29 the identity of that revision. This will let you reproduce that
bos@559 30 release at a later date, for whatever purpose you might need at
bos@559 31 the time (reproducing a bug, porting to a new platform, etc).
bos@567 32 &interaction.tag.init;</para>
bos@559 33
bos@584 34 <para id="x_36d">Mercurial lets you give a permanent name to any revision
bos@559 35 using the <command role="hg-cmd">hg tag</command> command. Not
bos@567 36 surprisingly, these names are called <quote>tags</quote>.</para>
bos@567 37
bos@567 38 &interaction.tag.tag;
bos@559 39
bos@584 40 <para id="x_36e">A tag is nothing more than a <quote>symbolic name</quote>
bos@559 41 for a revision. Tags exist purely for your convenience, so that
bos@559 42 you have a handy permanent way to refer to a revision; Mercurial
bos@559 43 doesn't interpret the tag names you use in any way. Neither
bos@559 44 does Mercurial place any restrictions on the name of a tag,
bos@559 45 beyond a few that are necessary to ensure that a tag can be
bos@559 46 parsed unambiguously. A tag name cannot contain any of the
bos@559 47 following characters:</para>
bos@559 48 <itemizedlist>
bos@584 49 <listitem><para id="x_36f">Colon (ASCII 58,
bos@559 50 <quote><literal>:</literal></quote>)</para>
bos@559 51 </listitem>
bos@584 52 <listitem><para id="x_370">Carriage return (ASCII 13,
bos@559 53 <quote><literal>\r</literal></quote>)</para>
bos@559 54 </listitem>
bos@584 55 <listitem><para id="x_371">Newline (ASCII 10,
bos@559 56 <quote><literal>\n</literal></quote>)</para>
bos@559 57 </listitem></itemizedlist>
bos@559 58
bos@584 59 <para id="x_372">You can use the <command role="hg-cmd">hg tags</command>
bos@559 60 command to display the tags present in your repository. In the
bos@559 61 output, each tagged revision is identified first by its name,
bos@559 62 then by revision number, and finally by the unique hash of the
bos@567 63 revision.</para>
bos@567 64
bos@567 65 &interaction.tag.tags;
bos@567 66
bos@584 67 <para id="x_373">Notice that <literal>tip</literal> is listed in the output
bos@567 68 of <command role="hg-cmd">hg tags</command>. The
bos@567 69 <literal>tip</literal> tag is a special <quote>floating</quote>
bos@567 70 tag, which always identifies the newest revision in the
bos@567 71 repository.</para>
bos@559 72
bos@584 73 <para id="x_374">In the output of the <command role="hg-cmd">hg
bos@559 74 tags</command> command, tags are listed in reverse order, by
bos@559 75 revision number. This usually means that recent tags are listed
bos@559 76 before older tags. It also means that <literal>tip</literal> is
bos@559 77 always going to be the first tag listed in the output of
bos@559 78 <command role="hg-cmd">hg tags</command>.</para>
bos@559 79
bos@584 80 <para id="x_375">When you run <command role="hg-cmd">hg log</command>, if it
bos@559 81 displays a revision that has tags associated with it, it will
bos@567 82 print those tags.</para>
bos@567 83
bos@567 84 &interaction.tag.log;
bos@559 85
bos@584 86 <para id="x_376">Any time you need to provide a revision ID to a Mercurial
bos@559 87 command, the command will accept a tag name in its place.
bos@559 88 Internally, Mercurial will translate your tag name into the
bos@567 89 corresponding revision ID, then use that.</para>
bos@567 90
bos@567 91 &interaction.tag.log.v1.0;
bos@559 92
bos@584 93 <para id="x_377">There's no limit on the number of tags you can have in a
bos@559 94 repository, or on the number of tags that a single revision can
bos@559 95 have. As a practical matter, it's not a great idea to have
bos@559 96 <quote>too many</quote> (a number which will vary from project
bos@559 97 to project), simply because tags are supposed to help you to
bos@559 98 find revisions. If you have lots of tags, the ease of using
bos@559 99 them to identify revisions diminishes rapidly.</para>
bos@559 100
bos@584 101 <para id="x_378">For example, if your project has milestones as frequent as
bos@559 102 every few days, it's perfectly reasonable to tag each one of
bos@559 103 those. But if you have a continuous build system that makes
bos@559 104 sure every revision can be built cleanly, you'd be introducing a
bos@559 105 lot of noise if you were to tag every clean build. Instead, you
bos@559 106 could tag failed builds (on the assumption that they're rare!),
bos@559 107 or simply not use tags to track buildability.</para>
bos@559 108
bos@584 109 <para id="x_379">If you want to remove a tag that you no longer want, use
bos@567 110 <command role="hg-cmd">hg tag --remove</command>.</para>
bos@567 111
bos@567 112 &interaction.tag.remove;
bos@567 113
bos@584 114 <para id="x_37a">You can also modify a tag at any time, so that it identifies
bos@567 115 a different revision, by simply issuing a new <command
bos@567 116 role="hg-cmd">hg tag</command> command. You'll have to use the
bos@567 117 <option role="hg-opt-tag">-f</option> option to tell Mercurial
bos@567 118 that you <emphasis>really</emphasis> want to update the
bos@567 119 tag.</para>
bos@567 120
bos@567 121 &interaction.tag.replace;
bos@567 122
bos@584 123 <para id="x_37b">There will still be a permanent record of the previous
bos@567 124 identity of the tag, but Mercurial will no longer use it.
bos@567 125 There's thus no penalty to tagging the wrong revision; all you
bos@567 126 have to do is turn around and tag the correct revision once you
bos@567 127 discover your error.</para>
bos@559 128
bos@584 129 <para id="x_37c">Mercurial stores tags in a normal revision-controlled file
bos@559 130 in your repository. If you've created any tags, you'll find
bos@675 131 them in a file in the root of your repository named <filename
bos@559 132 role="special">.hgtags</filename>. When you run the <command
bos@559 133 role="hg-cmd">hg tag</command> command, Mercurial modifies
bos@559 134 this file, then automatically commits the change to it. This
bos@559 135 means that every time you run <command role="hg-cmd">hg
bos@559 136 tag</command>, you'll see a corresponding changeset in the
bos@567 137 output of <command role="hg-cmd">hg log</command>.</para>
bos@567 138
bos@567 139 &interaction.tag.tip;
bos@559 140
bos@559 141 <sect2>
bos@559 142 <title>Handling tag conflicts during a merge</title>
bos@559 143
bos@584 144 <para id="x_37d">You won't often need to care about the <filename
bos@559 145 role="special">.hgtags</filename> file, but it sometimes
bos@559 146 makes its presence known during a merge. The format of the
bos@559 147 file is simple: it consists of a series of lines. Each line
bos@559 148 starts with a changeset hash, followed by a space, followed by
bos@559 149 the name of a tag.</para>
bos@559 150
bos@584 151 <para id="x_37e">If you're resolving a conflict in the <filename
bos@559 152 role="special">.hgtags</filename> file during a merge,
bos@559 153 there's one twist to modifying the <filename
bos@559 154 role="special">.hgtags</filename> file: when Mercurial is
bos@559 155 parsing the tags in a repository, it
bos@559 156 <emphasis>never</emphasis> reads the working copy of the
bos@559 157 <filename role="special">.hgtags</filename> file. Instead, it
bos@559 158 reads the <emphasis>most recently committed</emphasis>
bos@559 159 revision of the file.</para>
bos@559 160
bos@584 161 <para id="x_37f">An unfortunate consequence of this design is that you
bos@559 162 can't actually verify that your merged <filename
bos@559 163 role="special">.hgtags</filename> file is correct until
bos@559 164 <emphasis>after</emphasis> you've committed a change. So if
bos@559 165 you find yourself resolving a conflict on <filename
bos@559 166 role="special">.hgtags</filename> during a merge, be sure to
bos@559 167 run <command role="hg-cmd">hg tags</command> after you commit.
bos@559 168 If it finds an error in the <filename
bos@559 169 role="special">.hgtags</filename> file, it will report the
bos@559 170 location of the error, which you can then fix and commit. You
bos@559 171 should then run <command role="hg-cmd">hg tags</command>
bos@559 172 again, just to be sure that your fix is correct.</para>
bos@559 173 </sect2>
bos@675 174
bos@559 175 <sect2>
bos@559 176 <title>Tags and cloning</title>
bos@559 177
bos@584 178 <para id="x_380">You may have noticed that the <command role="hg-cmd">hg
bos@559 179 clone</command> command has a <option
bos@559 180 role="hg-opt-clone">-r</option> option that lets you clone
bos@559 181 an exact copy of the repository as of a particular changeset.
bos@559 182 The new clone will not contain any project history that comes
bos@559 183 after the revision you specified. This has an interaction
bos@559 184 with tags that can surprise the unwary.</para>
bos@559 185
bos@701 186 <para id="x_381">Recall that a tag is stored as a revision to
bos@701 187 the <filename role="special">.hgtags</filename> file. When you
bos@701 188 create a tag, the changeset in which its recorded refers to an
bos@701 189 older changeset. When you run <command role="hg-cmd">hg clone
bos@701 190 -r foo</command> to clone a repository as of tag
bos@701 191 <literal>foo</literal>, the new clone <emphasis>will not
bos@701 192 contain any revision newer than the one the tag refers to,
bos@701 193 including the revision where the tag was created</emphasis>.
bos@701 194 The result is that you'll get exactly the right subset of the
bos@559 195 project's history in the new repository, but
bos@559 196 <emphasis>not</emphasis> the tag you might have
bos@559 197 expected.</para>
bos@559 198 </sect2>
bos@675 199
bos@559 200 <sect2>
bos@559 201 <title>When permanent tags are too much</title>
bos@559 202
bos@584 203 <para id="x_382">Since Mercurial's tags are revision controlled and carried
bos@559 204 around with a project's history, everyone you work with will
bos@559 205 see the tags you create. But giving names to revisions has
bos@559 206 uses beyond simply noting that revision
bos@559 207 <literal>4237e45506ee</literal> is really
bos@559 208 <literal>v2.0.2</literal>. If you're trying to track down a
bos@559 209 subtle bug, you might want a tag to remind you of something
bos@559 210 like <quote>Anne saw the symptoms with this
bos@559 211 revision</quote>.</para>
bos@559 212
bos@584 213 <para id="x_383">For cases like this, what you might want to use are
bos@559 214 <emphasis>local</emphasis> tags. You can create a local tag
bos@559 215 with the <option role="hg-opt-tag">-l</option> option to the
bos@559 216 <command role="hg-cmd">hg tag</command> command. This will
bos@559 217 store the tag in a file called <filename
bos@559 218 role="special">.hg/localtags</filename>. Unlike <filename
bos@559 219 role="special">.hgtags</filename>, <filename
bos@559 220 role="special">.hg/localtags</filename> is not revision
bos@559 221 controlled. Any tags you create using <option
bos@559 222 role="hg-opt-tag">-l</option> remain strictly local to the
bos@559 223 repository you're currently working in.</para>
bos@559 224 </sect2>
bos@559 225 </sect1>
bos@675 226
bos@559 227 <sect1>
bos@559 228 <title>The flow of changes&emdash;big picture vs. little</title>
bos@559 229
bos@701 230 <para id="x_384">To return to the outline I sketched at the
bos@701 231 beginning of the chapter, let's think about a project that has
bos@701 232 multiple concurrent pieces of work under development at
bos@701 233 once.</para>
bos@559 234
bos@584 235 <para id="x_385">There might be a push for a new <quote>main</quote> release;
bos@559 236 a new minor bugfix release to the last main release; and an
bos@559 237 unexpected <quote>hot fix</quote> to an old release that is now
bos@559 238 in maintenance mode.</para>
bos@559 239
bos@584 240 <para id="x_386">The usual way people refer to these different concurrent
bos@559 241 directions of development is as <quote>branches</quote>.
bos@559 242 However, we've already seen numerous times that Mercurial treats
bos@559 243 <emphasis>all of history</emphasis> as a series of branches and
bos@559 244 merges. Really, what we have here is two ideas that are
bos@559 245 peripherally related, but which happen to share a name.</para>
bos@559 246 <itemizedlist>
bos@584 247 <listitem><para id="x_387"><quote>Big picture</quote> branches represent
bos@559 248 the sweep of a project's evolution; people give them names,
bos@559 249 and talk about them in conversation.</para>
bos@559 250 </listitem>
bos@584 251 <listitem><para id="x_388"><quote>Little picture</quote> branches are
bos@559 252 artefacts of the day-to-day activity of developing and
bos@559 253 merging changes. They expose the narrative of how the code
bos@559 254 was developed.</para>
bos@559 255 </listitem></itemizedlist>
bos@675 256 </sect1>
bos@675 257
bos@559 258 <sect1>
bos@559 259 <title>Managing big-picture branches in repositories</title>
bos@559 260
bos@584 261 <para id="x_389">The easiest way to isolate a <quote>big picture</quote>
bos@559 262 branch in Mercurial is in a dedicated repository. If you have
bos@559 263 an existing shared repository&emdash;let's call it
bos@559 264 <literal>myproject</literal>&emdash;that reaches a
bos@559 265 <quote>1.0</quote> milestone, you can start to prepare for
bos@559 266 future maintenance releases on top of version 1.0 by tagging the
bos@567 267 revision from which you prepared the 1.0 release.</para>
bos@567 268
bos@567 269 &interaction.branch-repo.tag;
bos@567 270
bos@584 271 <para id="x_38a">You can then clone a new shared
bos@567 272 <literal>myproject-1.0.1</literal> repository as of that
bos@567 273 tag.</para>
bos@567 274
bos@567 275 &interaction.branch-repo.clone;
bos@559 276
bos@584 277 <para id="x_38b">Afterwards, if someone needs to work on a bug fix that ought
bos@559 278 to go into an upcoming 1.0.1 minor release, they clone the
bos@559 279 <literal>myproject-1.0.1</literal> repository, make their
bos@567 280 changes, and push them back.</para>
bos@567 281
bos@567 282 &interaction.branch-repo.bugfix;
bos@567 283
bos@584 284 <para id="x_38c">Meanwhile, development for
bos@559 285 the next major release can continue, isolated and unabated, in
bos@567 286 the <literal>myproject</literal> repository.</para>
bos@567 287
bos@567 288 &interaction.branch-repo.new;
bos@675 289 </sect1>
bos@675 290
bos@559 291 <sect1>
bos@559 292 <title>Don't repeat yourself: merging across branches</title>
bos@559 293
bos@584 294 <para id="x_38d">In many cases, if you have a bug to fix on a maintenance
bos@559 295 branch, the chances are good that the bug exists on your
bos@559 296 project's main branch (and possibly other maintenance branches,
bos@559 297 too). It's a rare developer who wants to fix the same bug
bos@559 298 multiple times, so let's look at a few ways that Mercurial can
bos@559 299 help you to manage these bugfixes without duplicating your
bos@559 300 work.</para>
bos@559 301
bos@584 302 <para id="x_38e">In the simplest instance, all you need to do is pull changes
bos@559 303 from your maintenance branch into your local clone of the target
bos@567 304 branch.</para>
bos@567 305
bos@567 306 &interaction.branch-repo.pull;
bos@567 307
bos@584 308 <para id="x_38f">You'll then need to merge the heads of the two branches, and
bos@567 309 push back to the main branch.</para>
bos@567 310
bos@567 311 &interaction.branch-repo.merge;
bos@675 312 </sect1>
bos@675 313
bos@559 314 <sect1>
bos@559 315 <title>Naming branches within one repository</title>
bos@559 316
bos@584 317 <para id="x_390">In most instances, isolating branches in repositories is the
bos@559 318 right approach. Its simplicity makes it easy to understand; and
bos@559 319 so it's hard to make mistakes. There's a one-to-one
bos@559 320 relationship between branches you're working in and directories
bos@559 321 on your system. This lets you use normal (non-Mercurial-aware)
bos@559 322 tools to work on files within a branch/repository.</para>
bos@559 323
bos@584 324 <para id="x_391">If you're more in the <quote>power user</quote> category
bos@559 325 (<emphasis>and</emphasis> your collaborators are too), there is
bos@559 326 an alternative way of handling branches that you can consider.
bos@559 327 I've already mentioned the human-level distinction between
bos@559 328 <quote>small picture</quote> and <quote>big picture</quote>
bos@559 329 branches. While Mercurial works with multiple <quote>small
bos@559 330 picture</quote> branches in a repository all the time (for
bos@559 331 example after you pull changes in, but before you merge them),
bos@559 332 it can <emphasis>also</emphasis> work with multiple <quote>big
bos@559 333 picture</quote> branches.</para>
bos@559 334
bos@584 335 <para id="x_392">The key to working this way is that Mercurial lets you
bos@559 336 assign a persistent <emphasis>name</emphasis> to a branch.
bos@559 337 There always exists a branch named <literal>default</literal>.
bos@559 338 Even before you start naming branches yourself, you can find
bos@559 339 traces of the <literal>default</literal> branch if you look for
bos@559 340 them.</para>
bos@559 341
bos@584 342 <para id="x_393">As an example, when you run the <command role="hg-cmd">hg
bos@559 343 commit</command> command, and it pops up your editor so that
bos@559 344 you can enter a commit message, look for a line that contains
bos@559 345 the text <quote><literal>HG: branch default</literal></quote> at
bos@559 346 the bottom. This is telling you that your commit will occur on
bos@559 347 the branch named <literal>default</literal>.</para>
bos@559 348
bos@584 349 <para id="x_394">To start working with named branches, use the <command
bos@559 350 role="hg-cmd">hg branches</command> command. This command
bos@559 351 lists the named branches already present in your repository,
bos@567 352 telling you which changeset is the tip of each.</para>
bos@567 353
bos@567 354 &interaction.branch-named.branches;
bos@567 355
bos@584 356 <para id="x_395">Since you haven't created any named branches yet, the only
bos@567 357 one that exists is <literal>default</literal>.</para>
bos@559 358
bos@584 359 <para id="x_396">To find out what the <quote>current</quote> branch is, run
bos@559 360 the <command role="hg-cmd">hg branch</command> command, giving
bos@559 361 it no arguments. This tells you what branch the parent of the
bos@567 362 current changeset is on.</para>
bos@567 363
bos@567 364 &interaction.branch-named.branch;
bos@559 365
bos@584 366 <para id="x_397">To create a new branch, run the <command role="hg-cmd">hg
bos@559 367 branch</command> command again. This time, give it one
bos@567 368 argument: the name of the branch you want to create.</para>
bos@567 369
bos@567 370 &interaction.branch-named.create;
bos@559 371
bos@584 372 <para id="x_398">After you've created a branch, you might wonder what effect
bos@559 373 the <command role="hg-cmd">hg branch</command> command has had.
bos@559 374 What do the <command role="hg-cmd">hg status</command> and
bos@567 375 <command role="hg-cmd">hg tip</command> commands report?</para>
bos@567 376
bos@567 377 &interaction.branch-named.status;
bos@567 378
bos@584 379 <para id="x_399">Nothing has changed in the
bos@559 380 working directory, and there's been no new history created. As
bos@559 381 this suggests, running the <command role="hg-cmd">hg
bos@559 382 branch</command> command has no permanent effect; it only
bos@559 383 tells Mercurial what branch name to use the
bos@559 384 <emphasis>next</emphasis> time you commit a changeset.</para>
bos@559 385
bos@584 386 <para id="x_39a">When you commit a change, Mercurial records the name of the
bos@559 387 branch on which you committed. Once you've switched from the
bos@559 388 <literal>default</literal> branch to another and committed,
bos@559 389 you'll see the name of the new branch show up in the output of
bos@559 390 <command role="hg-cmd">hg log</command>, <command
bos@559 391 role="hg-cmd">hg tip</command>, and other commands that
bos@567 392 display the same kind of output.</para>
bos@567 393
bos@567 394 &interaction.branch-named.commit;
bos@567 395
bos@584 396 <para id="x_39b">The <command role="hg-cmd">hg log</command>-like commands
bos@567 397 will print the branch name of every changeset that's not on the
bos@559 398 <literal>default</literal> branch. As a result, if you never
bos@559 399 use named branches, you'll never see this information.</para>
bos@559 400
bos@584 401 <para id="x_39c">Once you've named a branch and committed a change with that
bos@559 402 name, every subsequent commit that descends from that change
bos@559 403 will inherit the same branch name. You can change the name of a
bos@559 404 branch at any time, using the <command role="hg-cmd">hg
bos@567 405 branch</command> command.</para>
bos@567 406
bos@567 407 &interaction.branch-named.rebranch;
bos@567 408
bos@584 409 <para id="x_39d">In practice, this is something you won't do very often, as
bos@567 410 branch names tend to have fairly long lifetimes. (This isn't a
bos@567 411 rule, just an observation.)</para>
bos@675 412 </sect1>
bos@675 413
bos@559 414 <sect1>
bos@559 415 <title>Dealing with multiple named branches in a
bos@559 416 repository</title>
bos@559 417
bos@584 418 <para id="x_39e">If you have more than one named branch in a repository,
bos@559 419 Mercurial will remember the branch that your working directory
bos@701 420 is on when you start a command like <command role="hg-cmd">hg
bos@559 421 update</command> or <command role="hg-cmd">hg pull
bos@559 422 -u</command>. It will update the working directory to the tip
bos@559 423 of this branch, no matter what the <quote>repo-wide</quote> tip
bos@559 424 is. To update to a revision that's on a different named branch,
bos@559 425 you may need to use the <option role="hg-opt-update">-C</option>
bos@559 426 option to <command role="hg-cmd">hg update</command>.</para>
bos@559 427
bos@672 428 <para id="x_39f">This behavior is a little subtle, so let's see it in
bos@559 429 action. First, let's remind ourselves what branch we're
bos@567 430 currently on, and what branches are in our repository.</para>
bos@567 431
bos@567 432 &interaction.branch-named.parents;
bos@567 433
bos@584 434 <para id="x_3a0">We're on the <literal>bar</literal> branch, but there also
bos@567 435 exists an older <command role="hg-cmd">hg foo</command>
bos@567 436 branch.</para>
bos@559 437
bos@584 438 <para id="x_3a1">We can <command role="hg-cmd">hg update</command> back and
bos@559 439 forth between the tips of the <literal>foo</literal> and
bos@559 440 <literal>bar</literal> branches without needing to use the
bos@559 441 <option role="hg-opt-update">-C</option> option, because this
bos@559 442 only involves going backwards and forwards linearly through our
bos@567 443 change history.</para>
bos@567 444
bos@567 445 &interaction.branch-named.update-switchy;
bos@559 446
bos@584 447 <para id="x_3a2">If we go back to the <literal>foo</literal> branch and then
bos@559 448 run <command role="hg-cmd">hg update</command>, it will keep us
bos@559 449 on <literal>foo</literal>, not move us to the tip of
bos@567 450 <literal>bar</literal>.</para>
bos@567 451
bos@567 452 &interaction.branch-named.update-nothing;
bos@559 453
bos@584 454 <para id="x_3a3">Committing a new change on the <literal>foo</literal> branch
bos@567 455 introduces a new head.</para>
bos@567 456
bos@567 457 &interaction.branch-named.foo-commit;
bos@675 458 </sect1>
bos@675 459
bos@559 460 <sect1>
bos@559 461 <title>Branch names and merging</title>
bos@559 462
bos@584 463 <para id="x_3a4">As you've probably noticed, merges in Mercurial are not
bos@559 464 symmetrical. Let's say our repository has two heads, 17 and 23.
bos@559 465 If I <command role="hg-cmd">hg update</command> to 17 and then
bos@559 466 <command role="hg-cmd">hg merge</command> with 23, Mercurial
bos@559 467 records 17 as the first parent of the merge, and 23 as the
bos@559 468 second. Whereas if I <command role="hg-cmd">hg update</command>
bos@559 469 to 23 and then <command role="hg-cmd">hg merge</command> with
bos@559 470 17, it records 23 as the first parent, and 17 as the
bos@559 471 second.</para>
bos@559 472
bos@584 473 <para id="x_3a5">This affects Mercurial's choice of branch name when you
bos@559 474 merge. After a merge, Mercurial will retain the branch name of
bos@559 475 the first parent when you commit the result of the merge. If
bos@559 476 your first parent's branch name is <literal>foo</literal>, and
bos@559 477 you merge with <literal>bar</literal>, the branch name will
bos@559 478 still be <literal>foo</literal> after you merge.</para>
bos@559 479
bos@584 480 <para id="x_3a6">It's not unusual for a repository to contain multiple heads,
bos@559 481 each with the same branch name. Let's say I'm working on the
bos@559 482 <literal>foo</literal> branch, and so are you. We commit
bos@559 483 different changes; I pull your changes; I now have two heads,
bos@559 484 each claiming to be on the <literal>foo</literal> branch. The
bos@559 485 result of a merge will be a single head on the
bos@559 486 <literal>foo</literal> branch, as you might hope.</para>
bos@559 487
bos@584 488 <para id="x_3a7">But if I'm working on the <literal>bar</literal> branch, and
bos@559 489 I merge work from the <literal>foo</literal> branch, the result
bos@567 490 will remain on the <literal>bar</literal> branch.</para>
bos@567 491
bos@567 492 &interaction.branch-named.merge;
bos@559 493
bos@584 494 <para id="x_3a8">To give a more concrete example, if I'm working on the
bos@559 495 <literal>bleeding-edge</literal> branch, and I want to bring in
bos@559 496 the latest fixes from the <literal>stable</literal> branch,
bos@559 497 Mercurial will choose the <quote>right</quote>
bos@559 498 (<literal>bleeding-edge</literal>) branch name when I pull and
bos@559 499 merge from <literal>stable</literal>.</para>
bos@675 500 </sect1>
bos@675 501
bos@559 502 <sect1>
bos@559 503 <title>Branch naming is generally useful</title>
bos@559 504
bos@584 505 <para id="x_3a9">You shouldn't think of named branches as applicable only to
bos@559 506 situations where you have multiple long-lived branches
bos@559 507 cohabiting in a single repository. They're very useful even in
bos@559 508 the one-branch-per-repository case.</para>
bos@559 509
bos@584 510 <para id="x_3aa">In the simplest case, giving a name to each branch gives you
bos@559 511 a permanent record of which branch a changeset originated on.
bos@559 512 This gives you more context when you're trying to follow the
bos@559 513 history of a long-lived branchy project.</para>
bos@559 514
bos@584 515 <para id="x_3ab">If you're working with shared repositories, you can set up a
bos@559 516 <literal role="hook">pretxnchangegroup</literal> hook on each
bos@559 517 that will block incoming changes that have the
bos@559 518 <quote>wrong</quote> branch name. This provides a simple, but
bos@559 519 effective, defence against people accidentally pushing changes
bos@559 520 from a <quote>bleeding edge</quote> branch to a
bos@559 521 <quote>stable</quote> branch. Such a hook might look like this
bos@559 522 inside the shared repo's <filename role="special">
bos@559 523 /.hgrc</filename>.</para>
bos@580 524 <programlisting>[hooks]
bos@580 525 pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch</programlisting>
bos@559 526 </sect1>
bos@559 527 </chapter>
bos@559 528
bos@559 529 <!--
bos@559 530 local variables:
bos@559 531 sgml-parent-document: ("00book.xml" "book" "chapter")
bos@559 532 end:
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