Here ends the automated magic. Sooner or later, once you get the hang of branching and merging, you're going to have to ask Subversion to merge specific changes from one place to another. To do this, you're going to have to start passing more complicated arguments to svn merge. The next section describes the fully expanded syntax of the command and discusses a number of common scenarios that require it.
Just as the term “changeset” is often used in version control systems, so is the term cherrypicking. This word refers to the act of choosing one specific changeset from a branch and replicating it to another. Cherrypicking may also refer to the act of duplicating a particular set of (not necessarily contiguous!) changesets from one branch to another. This is in contrast to more typical merging scenarios, where the “next” contiguous range of revisions is duplicated automatically.
Why would people want to replicate just a single change?
It comes up more often than you'd think. For example, let's
go back in time and imagine that you haven't yet merged your
private feature branch back to the trunk. At the
water cooler, you get word that Sally made an interesting
integer.c on the trunk.
Looking over the history of commits to the trunk, you see that
in revision 355 she fixed a critical bug that directly
impacts the feature you're working on. You might not be ready
to merge all the trunk changes to your branch just yet, but
you certainly need that particular bug fix in order to continue
$ svn diff -c 355 ^/calc/trunk Index: integer.c =================================================================== --- integer.c (revision 354) +++ integer.c (revision 355) @@ -147,7 +147,7 @@ case 6: sprintf(info->operating_system, "HPFS (OS/2 or NT)"); break; case 7: sprintf(info->operating_system, "Macintosh"); break; case 8: sprintf(info->operating_system, "Z-System"); break; - case 9: sprintf(info->operating_system, "CP/MM"); + case 9: sprintf(info->operating_system, "CP/M"); break; case 10: sprintf(info->operating_system, "TOPS-20"); break; case 11: sprintf(info->operating_system, "NTFS (Windows NT)"); break; case 12: sprintf(info->operating_system, "QDOS"); break;
Just as you used svn diff in the prior example to examine revision 355, you can pass the same option to svn merge:
$ svn merge -c 355 ^/calc/trunk --- Merging r355 into '.': U integer.c --- Recording mergeinfo for merge of r355 into '.': U . $ svn status M integer.c
You can now go through the usual testing procedures before committing this change to your branch. After the commit, Subversion marks r355 as having been merged to the branch so that future “magic” merges that synchronize your branch with the trunk know to skip over r355. (Merging the same change to the same branch almost always results in a conflict!)
$ cd my-calc-branch $ svn propget svn:mergeinfo . /trunk:341-349,355 # Notice that r355 isn't listed as "eligible" to merge, because # it's already been merged. $ svn mergeinfo ^/calc/trunk --show-revs eligible r350 r351 r352 r353 r354 r356 r357 r358 r359 r360 $ svn merge ^/calc/trunk --- Merging r350 through r354 into '.': U . U integer.c U Makefile --- Merging r356 through r360 into '.': U . U integer.c U button.c --- Recording mergeinfo for merge of r350 through r360 into '.': U .
This use case of replicating (or backporting) bug fixes from one branch to another is perhaps the most popular reason for cherrypicking changes; it comes up all the time, for example, when a team is maintaining a “release branch” of software. (We discuss this pattern in the section called “Release Branches”.)
Did you notice how, in the last example, the merge invocation merged two distinct ranges? The svn merge command applied two independent patches to your working copy to skip over changeset 355, which your branch already contained. There's nothing inherently wrong with this, except that it has the potential to make conflict resolution trickier. If the first range of changes creates conflicts, you must resolve them interactively for the merge process to continue and apply the second range of changes. If you postpone a conflict from the first wave of changes, the whole merge command will bail out with an error message.
A word of warning: while svn diff and svn merge are very similar in concept, they do have different syntax in many cases. Be sure to read about them in Chapter 9, Subversion Complete Reference for details, or ask svn help. For example, svn merge requires a working copy path as a target, that is, a place where it should apply the generated patch. If the target isn't specified, it assumes you are trying to perform one of the following common operations:
You want to merge directory changes into your current working directory.
You want to merge the changes in a specific file into a file by the same name that exists in your current working directory.
If you are merging a directory and haven't specified a target path, svn merge assumes the first case and tries to apply the changes into your current directory. If you are merging a file, and that file (or a file by the same name) exists in your current working directory, svn merge assumes the second case and tries to apply the changes to a local file with the same name.
You've now seen some examples of the svn merge command, and you're about to see several more. If you're feeling confused about exactly how merging works, you're not alone. Many users (especially those new to version control) are initially perplexed about the proper syntax of the command and about how and when the feature should be used. But fear not, this command is actually much simpler than you think! There's a very easy technique for understanding exactly how svn merge behaves.
The main source of confusion is the name of the command. The term “merge” somehow denotes that branches are combined together, or that some sort of mysterious blending of data is going on. That's not the case. A better name for the command might have been svn diff-and-apply, because that's all that happens: two repository trees are compared, and the differences are applied to a working copy.
If you're using svn merge to do basic copying of changes between branches, it will generally do the right thing automatically. For example, a command such as the following:
$ svn merge ^/calc/branches/some-branch
will attempt to duplicate any changes made
some-branch into your current working
directory, which is presumably a working copy that shares some
historical connection to the branch. The command is smart
enough to only duplicate changes that your working copy
doesn't yet have. If you repeat this command once a week, it
will only duplicate the “newest” branch changes
that happened since you last merged.
If you choose to use the svn merge command in all its full glory by giving it specific revision ranges to duplicate, the command takes three main arguments:
An initial repository tree (often called the left side of the comparison)
A final repository tree (often called the right side of the comparison)
A working copy to accept the differences as local changes (often called the target of the merge)
Once these three arguments are specified, then the two trees are compared and the differences applied to the target working copy as local modifications. When the command is done, the results are no different than if you had hand-edited the files or run various svn add or svn delete commands yourself. If you like the results, you can commit them. If you don't like the results, you can simply svn revert all of the changes.
The syntax of svn merge allows you to specify the three necessary arguments rather flexibly. Here are some examples:
$ svn merge http://svn.example.com/repos/branch1@150 \ http://svn.example.com/repos/branch2@212 \ my-working-copy $ svn merge -r 100:200 http://svn.example.com/repos/trunk my-working-copy $ svn merge -r 100:200 http://svn.example.com/repos/trunk
The first syntax lays out all three arguments explicitly, naming each tree in the form URL@REV and naming the working copy target. The second syntax is used as a shorthand for situations when you're comparing two different revisions of the same URL. The last syntax shows how the working copy argument is optional; if omitted, it defaults to the current directory.
While the first example shows the “full”
syntax of svn merge, use it
very carefully; it can result in merges which do not record
svn:mergeinfo metadata at all. The
next section talks a bit more about this.
Subversion tries to generate merge metadata whenever it
can, to make future invocations of svn
merge smarter. There are still situations, however,
svn:mergeinfo data is not created or
changed. Remember to be a bit wary of these scenarios:
- Merging unrelated sources
If you ask svn merge to compare two URLs that aren't related to each other, a patch is still generated and applied to your working copy, but no merging metadata is created. There's no common history between the two sources, and future “smart” merges depend on that common history.
- Merging from foreign repositories
While it's possible to run a command such as
svn merge -r 100:200, the resultant patch also lacks any historical merge metadata. At the time of this writing, Subversion has no way of representing different repository URLs within the
If this option is passed to svn merge, it causes the merging logic to mindlessly generate differences the same way that svn diff does, ignoring any historical relationships. We discuss this later in this chapter in the section called “Noticing or Ignoring Ancestry”.
- Applying reverse merges from a target's natural history
Earlier in this chapter (the section called “Undoing Changes”) we discussed how to use svn merge to apply a “reverse patch” as a way of rolling back changes. If this technique is used to undo a change to an object's personal history (e.g., commit r5 to the trunk, then immediately roll back r5 using
svn merge . -c -5), this sort of merge doesn't affect the recorded mergeinfo.
Just like the svn update command, svn merge applies changes to your working copy. And therefore it's also capable of creating conflicts. The conflicts produced by svn merge, however, are sometimes different, and this section explains those differences.
To begin with, assume that your working copy has no local edits. When you svn update to a particular revision, the changes sent by the server always apply “cleanly” to your working copy. The server produces the delta by comparing two trees: a virtual snapshot of your working copy, and the revision tree you're interested in. Because the left hand side of the comparison is exactly equal to what you already have, the delta is guaranteed to correctly convert your working copy into the right hand tree.
But svn merge has no such guarantees and can be much more chaotic: the advanced user can ask the server to compare any two trees at all, even ones that are unrelated to the working copy! This means there's large potential for human error. Users will sometimes compare the wrong two trees, creating a delta that doesn't apply cleanly. The svn merge subcommand does its best to apply as much of the delta as possible, but some parts may be impossible. A common sign that you merged the wrong delta is unexpected tree conflicts:
$ svn merge -r 1288:1351 http://svn.example.com/myrepos/branch --- Merging r1289 through r1351 into '.': C bar.c C foo.c C docs --- Recording mergeinfo for merge of r1289 through r1351 into '.': U . Summary of conflicts: Tree conflicts: 3 $ svn st ! C bar.c > local missing, incoming edit upon merge ! C foo.c > local missing, incoming edit upon merge ! C docs > local delete, incoming edit upon merge
In the previous example, it might be the case that
docs all exist in both snapshots of the
branch being compared. The resultant delta wants to change
the contents of the corresponding paths in your working copy,
but those paths don't exist in the working copy. Whatever the
case, the preponderance of tree conflicts most likely means that
the user compared the wrong two trees; it's a classic
sign of user error. When this happens, it's easy to
recursively revert all the changes created by the merge
svn revert . --recursive), delete any
unversioned files or directories left behind after the
revert, and rerun svn merge with the
Also keep in mind that a merge into a working copy with no local edits can still produce text conflicts.
$ svn merge -c 1701 http://svn.example.com/myrepos/branchX --accept postpone --- Merging r1701 into '.': C glub.c C sputter.c --- Recording mergeinfo for merge of r1701 into '.': U . Summary of conflicts: Text conflicts: 2 C:\SVN\src-branch-1.7.x>svn st M . ? glub.c.merge-left.r1700 ? glub.c.merge-right.r1701 C glub.c ? glub.c.working ? sputter.c.merge-left.r1700 ? sputter.c.merge-right.r1701 C sputter.c ? sputter.c.working Summary of conflicts: Text conflicts: 2
How can a conflict possibly happen? Again, because the user can request svn merge to define and apply any old delta to the working copy, that delta may contain textual changes that don't cleanly apply to a working file, even if the file has no local modifications.
Another small difference between svn
update and svn merge is the names
of the full-text files created when a conflict happens. In
the section called “Resolve Any Conflicts”, we saw that an
update produces files named
filename.rNEWREV. When svn
merge produces a conflict, though, it creates three
filename.merge-right.rNEWREV. In this case,
the terms “merge-left” and “merge-right”
are describing which side of the double-tree comparison the file
came from, “rOLDREV” describes the revision of the
left side, and “rNEWREV” the revision of the right
side. In any case, these differing names help you distinguish
between conflicts that happened as a result of an update and
ones that happened as a result of a merge.
Sometimes there's a particular changeset that you don't
want automatically merged. For example, perhaps your
team's policy is to do new development work on
/trunk, but is more conservative about
backporting changes to a stable branch you use for releasing
to the public. On one extreme, you can manually cherrypick
single changesets from the trunk to the branch—just the
changes that are stable enough to pass muster. Maybe things
aren't quite that strict, though; perhaps most of the time
you just let svn merge
automatically merge most changes from trunk to branch. In
this case, you want a way to mask a few specific changes
out, that is, prevent them from ever being automatically
Through Subversion 1.7, the only way to block a changeset
is to make the system believe that the change has
already been merged. To do this, invoke
the merge subcommand with the
$ cd my-calc-branch $ svn propget svn:mergeinfo . /trunk:1680-3305 # Let's make the metadata list r3328 as already merged. $ svn merge -c 3328 --record-only ^/calc/trunk --- Recording mergeinfo for merge of r3328 into '.': U . $ svn status M . $ svn propget svn:mergeinfo . /trunk:1680-3305,3328 $ svn commit -m "Block r3328 from being merged to the branch." …
Beginning with Subversion 1.7,
merges are transitive. This means that, in addition to recording
mergeinfo describing the blocked revision(s), any
svn:mergeinfo property differences in the
merge source are also applied. For example, let's say we want to
block the 'frazzle' feature from ever being merged from
^/trunk to our
^/branches/proj-X branch. We know that all
the frazzle work was done on its own branch, which was
trunk in revision 1055:
$ svn log -v ^/trunk -r 1055 ------------------------------------------------------------------------ r1055 | francesca | 2011-09-22 07:40:06 -0400 (Thu, 22 Sep 2011) | 3 lines Changed paths: M /trunk M /trunk/src/frazzle.c Reintegrate the frazzle-feature-branch to trunk.
Because revision 1055 was a reintegrate merge we know that mergeinfo was recorded describing the merge:
$ svn diff ^/trunk -c 1055 --depth empty Index: . =================================================================== --- . (revision 1054) +++ . (revision 1055) Property changes on: . ___________________________________________________________________ Modified: svn:mergeinfo Merged /branches/frazzle-feature-branch:r997-1003
Now simply blocking merges of revision 1055 from
^/trunk isn't foolproof since someone could
merge r996:1003 directly from
Fortunately the transitive nature
--record-only merges in Subversion 1.7
prevents this; the
svn:mergeinfo diff from
revision 1055, thus blocking merges directly from the frazzle
branch and as it has always done prior to
Subversion 1.7, it blocks merges of revision 1055 directly
$ cd branches/proj-X $ svn merge ^/trunk . -c 1055 --record-only --- Merging r1055 into '.': G . --- Recording mergeinfo for merge of r1055 into '.': G . $ svn diff --depth empty . Index: . =================================================================== --- . (revision 1070) +++ . (working copy) Property changes on: . ___________________________________________________________________ Modified: svn:mergeinfo Merged /trunk:r1055 Merged /branches/frazzle-feature-branch:r997-1003
Blocking changes with
works, but it's also a little bit
dangerous. The main problem is that we're not clearly
differentiating between the ideas of “I already have
this change” and “I don't have this change, but
don't currently want it.” We're effectively lying to
the system, making it think that the change was previously
merged. This puts the responsibility on you—the
user—to remember that the change wasn't actually merged,
it just wasn't wanted. There's no way to ask Subversion for a
list of “blocked changelists.” If you want to
track them (so that you can unblock them someday) you'll need
to record them in a text file somewhere, or perhaps in an
There is an alternative to destroying and re-creating a branch after reintegration. To understand why it works you need to understand why the branch is initially unfit for further use after it has been reintegrated.
Let's assume you created your branch in revision A. While working on your branch, you created one or more revisions which made changes to the branch. Before reintegrating your branch back to trunk, you made a final merge from trunk to your branch, and committed the result of this merge as revision B.
When reintegrating your branch into the trunk, you create a new revision X which changes the trunk. The changes made to trunk in this revision X are semantically equivalent to the changes you made to your branch between revisions A and B.
If you now try to merge outstanding changes from trunk to your branch, Subversion will consider changes made in revision X as eligible for merging into the branch. However, since your branch already contains all the changes made in revision X, merging these changes can result in spurious conflicts! These conflicts are often tree conflicts, especially if renames were made on the branch or the trunk while the branch was in development.
So what can be done about this? We need to make sure that
Subversion does not try to merge revision X
into the branch. This is done using the
--record-only merge option, which was introduced
in the section called “Blocking Changes”.
To carry out the record-only merge, get a working copy of
the branch which was just reintegrated in revision
X, and merge just revision X
from trunk into your branch, making sure to use the
This merge uses the cherry-picking merge syntax, which was introduced in the section called “Cherrypicking”. Continuing with the running example from the section called “Reintegrating a Branch”, where revision X was revision 391:
$ cd my-calc-branch $ svn update Updating '.': Updated to revision 393. $ svn merge --record-only -c 391 ^/calc/trunk --- Recording mergeinfo for merge of r391 into '.': U . $ svn commit -m "Block revision 391 from being merged into my-calc-branch." Sending . Committed revision 394.
Now your branch is ready to soak up changes from the trunk again. After another sync of your branch to the trunk, you can even reintegrate the branch a second time. If necessary, you can do another record-only merge to keep the branch alive. Rinse and repeat.
It should now also be apparent why deleting the branch and re-creating it has the same effect as doing the above record-only merge. Because revision X is part of the natural history (see the sidebar Natural History and Implicit Mergeinfo) of the newly created branch, Subversion will never attempt to merge revision X into the branch, avoiding spurious conflicts.
One of the main features of any version control system is to keep track of who changed what, and when they did it. The svn log and svn blame subcommands are just the tools for this: when invoked on individual files, they show not only the history of changesets that affected the file, but also exactly which user wrote which line of code, and when she did it.
When changes start getting replicated between branches, however, things start to get complicated. For example, if you were to ask svn log about the history of your feature branch, it would show exactly every revision that ever affected the branch:
$ cd my-calc-branch $ svn log -q ------------------------------------------------------------------------ r390 | user | 2002-11-22 11:01:57 -0600 (Fri, 22 Nov 2002) ------------------------------------------------------------------------ r388 | user | 2002-11-21 05:20:00 -0600 (Thu, 21 Nov 2002) ------------------------------------------------------------------------ r381 | user | 2002-11-20 15:07:06 -0600 (Wed, 20 Nov 2002) ------------------------------------------------------------------------ r359 | user | 2002-11-19 19:19:20 -0600 (Tue, 19 Nov 2002) ------------------------------------------------------------------------ r357 | user | 2002-11-15 14:29:52 -0600 (Fri, 15 Nov 2002) ------------------------------------------------------------------------ r343 | user | 2002-11-07 13:50:10 -0600 (Thu, 07 Nov 2002) ------------------------------------------------------------------------ r341 | user | 2002-11-03 07:17:16 -0600 (Sun, 03 Nov 2002) ------------------------------------------------------------------------ r303 | sally | 2002-10-29 21:14:35 -0600 (Tue, 29 Oct 2002) ------------------------------------------------------------------------ r98 | sally | 2002-02-22 15:35:29 -0600 (Fri, 22 Feb 2002) ------------------------------------------------------------------------
But is this really an accurate picture of all the changes that happened on the branch? What's left out here is the fact that revisions 390, 381, and 357 were actually the results of merging changes from the trunk. If you look at one of these logs in detail, the multiple trunk changesets that comprised the branch change are nowhere to be seen:
$ svn log -v -r 390 ------------------------------------------------------------------------ r390 | user | 2002-11-22 11:01:57 -0600 (Fri, 22 Nov 2002) | 1 line Changed paths: M /branches/my-calc-branch/button.c M /branches/my-calc-branch/README Final merge of trunk changes to my-calc-branch.
We happen to know that this merge to the branch was
nothing but a merge of trunk changes. How can we see those
trunk changes as well? The answer is to use the
option. This option expands those “child”
changes that were part of the merge.
$ svn log -v -r 390 -g ------------------------------------------------------------------------ r390 | user | 2002-11-22 11:01:57 -0600 (Fri, 22 Nov 2002) | 1 line Changed paths: M /branches/my-calc-branch/button.c M /branches/my-calc-branch/README Final merge of trunk changes to my-calc-branch. ------------------------------------------------------------------------ r383 | sally | 2002-11-21 03:19:00 -0600 (Thu, 21 Nov 2002) | 2 lines Changed paths: M /branches/my-calc-branch/button.c Merged via: r390 Fix inverse graphic error on button. ------------------------------------------------------------------------ r382 | sally | 2002-11-20 16:57:06 -0600 (Wed, 20 Nov 2002) | 2 lines Changed paths: M /branches/my-calc-branch/README Merged via: r390 Document my last fix in README.
By making the log operation use merge history, we see not just the revision we queried (r390), but also the two revisions that came along on the ride with it—a couple of changes made by Sally to the trunk. This is a much more complete picture of history!
The svn blame command also takes the
option. If this option is neglected, somebody looking at
a line-by-line annotation of
get the mistaken impression that you were responsible for the
lines that fixed a certain error:
$ svn blame button.c … 390 user retval = inverse_func(button, path); 390 user return retval; 390 user } …
And while it's true that you did actually commit those three lines in revision 390, two of them were actually written by Sally back in revision 383:
$ svn blame button.c -g … G 383 sally retval = inverse_func(button, path); G 383 sally return retval; 390 user } …
Now we know who to really blame for those two lines of code!
When conversing with a Subversion developer, you might very likely hear reference to the term ancestry. This word is used to describe the relationship between two objects in a repository: if they're related to each other, one object is said to be an ancestor of the other.
For example, suppose you commit revision 100, which
includes a change to a file
foo.c@99 is an
On the other hand, suppose you commit the deletion of
foo.c in revision 101, and then add a
new file by the same name in revision 102. In this case,
foo.c@102 may appear to be related
(they have the same path), but in fact are completely
different objects in the repository. They share no history
The reason for bringing this up is to point out an
important difference between svn diff and
svn merge. The former command ignores
ancestry, while the latter command is quite sensitive to it.
For example, if you asked svn diff to
compare revisions 99 and 102 of
you would see line-based diffs; the diff
command is blindly comparing two paths. But if you asked
svn merge to compare the same two objects,
it would notice that they're unrelated and first attempt to
delete the old file, then add the new file; the output would
indicate a deletion followed by an add:
D foo.c A foo.c
Most merges involve comparing trees that are ancestrally
related to one another; therefore, svn
merge defaults to this behavior. Occasionally,
however, you may want the merge command to
compare two unrelated trees. For example, you may have
imported two source-code trees representing different vendor
releases of a software project (see
the section called “Vendor Branches”). If you ask
svn merge to compare the two trees, you'd
see the entire first tree being deleted, followed by an add
of the entire second tree! In these situations, you'll want
svn merge to do a path-based comparison
only, ignoring any relations between files and directories.
--ignore-ancestry option to your
merge command, and it will behave just
like svn diff. (And conversely, the
--notice-ancestry option will cause
svn diff to behave like the
svn merge command.)
--ignore-ancestry option also disables
This means that
svn:mergeinfo is not considered
when svn merge is determining what revisions
to merge, nor is
svn:mergeinfo recorded to
describe the merge.
A common desire is to refactor source code, especially in Java-based software projects. Files and directories are shuffled around and renamed, often causing great disruption to everyone working on the project. Sounds like a perfect case to use a branch, doesn't it? Just create a branch, shuffle things around, and then merge the branch back to the trunk, right?
Alas, this scenario doesn't work so well right now and is considered one of Subversion's current weak spots. The problem is that Subversion's svn update command isn't as robust as it should be, particularly when dealing with copy and move operations.
When you use svn copy to duplicate a file, the repository remembers where the new file came from, but it fails to transmit that information to the client which is running svn update or svn merge. Instead of telling the client, “Copy that file you already have to this new location,” it sends down an entirely new file. This can lead to problems, especially because the same thing happens with renamed files. A lesser-known fact about Subversion is that it lacks “true renames”—the svn move command is nothing more than an aggregation of svn copy and svn delete.
For example, suppose that while working on your private
branch, you rename
whole.c. Effectively you've created
a new file in your branch that is a copy of the original
file, and deleted the original file. Meanwhile, back
trunk, Sally has committed some
integer.c. Now you
decide to merge your branch to the trunk:
$ cd calc/trunk $ svn merge --reintegrate ^/calc/branches/my-calc-branch --- Merging differences between repository URLs into '.': D integer.c A whole.c U . --- Recording mergeinfo for merge between repository URLs into '.': U .
This doesn't look so bad at first glance, but it's also
probably not what you or Sally expected. The merge operation
has deleted the latest version of
integer.c file (the one containing
Sally's latest changes), and blindly added your
whole.c file—which is a
duplicate of the older version
integer.c. The net effect is that
merging your “rename” to the trunk has removed
Sally's recent changes from the latest revision!
This isn't true data loss. Sally's changes are still in the repository's history, but it may not be immediately obvious that this has happened. The moral of this story is that until Subversion improves, be very careful about merging copies and renames from one branch to another.
If you've just upgraded your server to Subversion 1.5 or
later, there's a risk that pre-1.5 Subversion
clients can cause problems with
This is because pre-1.5 clients don't support this feature;
when one of these older clients performs svn
merge, it doesn't modify the value of
svn:mergeinfo property at all. So the
subsequent commit, despite being the result of a merge,
doesn't tell the repository about the duplicated
changes—that information is lost. Later on,
when “merge-aware” clients attempt automatic
merging, they're likely to run into all sorts of conflicts
resulting from repeated merges.
If you and your team are relying on the merge-tracking
features of Subversion, you may want to configure your
repository to prevent older clients from committing changes.
The easy way to do this is by inspecting
the “capabilities” parameter in
start-commit hook script. If the
client reports itself as having
capabilities, the hook script can allow the commit to start.
If the client doesn't report that capability, have the hook
deny the commit.
Example 4.1, “Merge-tracking gatekeeper start-commit hook script” gives an
example of such a hook script:
Example 4.1. Merge-tracking gatekeeper start-commit hook script
#!/usr/bin/env python import sys # The start-commit hook is invoked before a Subversion txn is created # in the process of doing a commit. Subversion runs this hook # by invoking a program (script, executable, binary, etc.) named # 'start-commit' (for which this file is a template) # with the following ordered arguments: # #  REPOS-PATH (the path to this repository) #  USER (the authenticated user attempting to commit) #  CAPABILITIES (a colon-separated list of capabilities reported # by the client; see note below) capabilities = sys.argv.split(':') if "mergeinfo" not in capabilities: sys.stderr.write("Commits from merge-tracking-unaware clients are " "not permitted. Please upgrade to Subversion 1.5 " "or newer.\n") sys.exit(1) sys.exit(0)
For more information about hook scripts, see the section called “Implementing Repository Hooks”.
The bottom line is that Subversion's merge-tracking
feature has an extremely complex internal implementation, and
svn:mergeinfo property is the only
window the user has into the machinery.
Sometimes mergeinfo will appear on paths that you didn't expect to be touched by an operation. Sometimes mergeinfo won't be generated at all, when you expect it to. Furthermore, the management of mergeinfo metadata has a whole set of taxonomies and behaviors around it, such as “explicit” versus “implicit” mergeinfo, “operative” versus “inoperative” revisions, specific mechanisms of mergeinfo “elision,” and even “inheritance” from parent to child directories.
We've chosen to only briefly cover, if at all, these detailed topics for a couple of reasons. First, the level of detail is absolutely overwhelming for a typical user. Second, and more importantly, the typical user shouldn't have to understand these concepts; they should typically remain in the background as pesky implementation details. All that said, if you enjoy this sort of thing, you can get a fantastic overview in a paper posted at CollabNet's website: http://www.collab.net/community/subversion/articles/merge-info.html.
For now, if you want to steer clear of the complexities of merge tracking, we recommend that you follow these simple best practices:
For short-term feature branches, follow the simple procedure described throughout the section called “Basic Merging”.
Avoid subtree merges and subtree mergeinfo, perform merges only on the root of your branches, not on subdirectories or files (see Subtree Merges and Subtree Mergeinfo) .
Don't ever edit the
svn:mergeinfoproperty directly; use svn merge with the
--record-onlyoption to effect a desired change to the metadata (as demonstrated in the section called “Blocking Changes”).
Your merge target should be a working copy which represents the root of a complete tree representing a single location in the repository at a single point in time:
Don't use the
--allow-mixed-revisionsoption to merge into mixed-revision working copies.
Don't merge to targets with “switched” subdirectories (as described next in the section called “Traversing Branches”).
Avoid merges to targets with sparse directories. Likewise, don't merge to depths other than
Be sure you have read access to all of the merge source and read/write access to all of the merge target.
 At least, this is true in Subversion 1.7 at the time of this writing. This behavior may improve in future versions of Subversion.
 Interestingly, after rolling
back a revision like this, we wouldn't be able to
reapply the revision using
svn merge . -c
5, since the mergeinfo would already list r5
as being applied. We would have to use
--ignore-ancestry option to make
the merge command ignore the existing