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
        change to 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
        your work.
$ 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”.)
| ![[Warning]](images/warning.png)  | Warning | 
|---|---|
| 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.[32] | 
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
        on 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
        any 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,
        where svn:mergeinfo data is not created or
        changed.  Remember to be a bit wary of these scenarios:
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.
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 http://svn.foreignproject.com/repos/trunksvn:mergeinfo property.
--ignore-ancestry
            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”.
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.[33]
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
        bar.c, foo.c, and
        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
        correct arguments.
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.mine,
        filename.rOLDREV, and
        filename.rNEWREV.  When svn
        merge produces a conflict, though, it creates three
        files named filename.working,
        filename.merge-left.rOLDREV, and
        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
        merged.
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 --record-only
        option:
$ 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, --record-only
        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
        reintegrated to 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
        ^/branches/frazzle-feature-branch.
        Fortunately the transitive nature
        of --record-only merges in Subversion 1.7
        prevents this; the --record-only merge
        applies 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
        from ^/trunk:
$ 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 --record-only
        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
        invented property.
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
        --record-only option.
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
        --use-merge-history (-g)
        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
        --use-merge-history (-g)
        option.  If this option is neglected, somebody looking at
        a line-by-line annotation of button.c may
        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.
        Then foo.c@99 is an
        “ancestor” of foo.c@100.
        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@99 and
        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
        or “ancestry.”
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 foo.c,
        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.
        Add the --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.)
| ![[Tip]](images/tip.png)  | Tip | 
|---|---|
| The  | 
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 integer.c
        to 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
        on trunk, Sally has committed some
        improvements to 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
        the integer.c file (the one containing
        Sally's latest changes), and blindly added your
        new whole.c file—which is a
        duplicate of the older version
        of 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
        Merge Tracking.
        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
        the 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
        the start-commit hook script.  If the
        client reports itself as having mergeinfo
        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:
#
#   [1] REPOS-PATH   (the path to this repository)
#   [2] USER         (the authenticated user attempting to commit)
#   [3] CAPABILITIES (a colon-separated list of capabilities reported
#                     by the client; see note below)
capabilities = sys.argv[3].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
        the 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:mergeinfo
            property directly; use svn
            merge with the --record-only option
            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-revisions
                option 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
                  --depth=infinity
Be sure you have read access to all of the merge source and read/write access to all of the merge target.
[32] At least, this is true in Subversion 1.7 at the time of this writing. This behavior may improve in future versions of Subversion.
[33] 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
              the --ignore-ancestry option to make
              the merge command ignore the existing
              mergeinfo!