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Developer guide to breezy transports
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This guide describes the `Transport` classes that Breezy uses for most
local and remote file access.  (Working tree files are the major
exception (`bug 606249 <https://bugs.launchpad.net/bzr/+bug/606249>`).


Handling symlinks
#################

A symlink creates an alias where files or directories can be accessed by a
different name.  Symlinks are useful but raise a few annoying cases for
bzr.

It's important to have tests for symlinks but these tests can't run on
Windows, so you need eg ::

    _test_needs_features = [tests.SymlinkFeature]

or ::

    self.requireFeature(tests.SymlinkFeature)

Breezy versions symlinks as objects in their own right, whose content is
the path they point to.  Breezy doesn't care whether a versioned
symlink is absolute or relative; or whether it points inside or outside
the working tree; or whether its referent exists or not.  In Unix the
target of a symlink is a byte string; Breezy treats this as a Unicode string
in the filesystem encoding (`sys.getfilesystemencoding()`).

So when we say ``brz add symlink``, this should always add the symlink to
its containing working tree, and never dereference the symlink.

However, ``brz add symlink/file`` shouldn't add ``file`` as a child of
``symlink``.  (Symlinks don't have files underneath them: they may point to
a directory which contains children, but if the symlink was pointed
somewhere else those children would be unaffected.)  This could either add
the file in its containing working tree, or fail outright.

One interesting case for this is ::

    brz add ~/dev/bug123/a.c

where ``~/dev`` is actually a symlink to ``/srv/dev/joe/``.  In this case
clearly the user does want us to follow the symlink to open the tree.

As of bzr2.2, when we open a `WorkingTree`, we typically immediately
compute its real path and store that as ``.basedir``, but `BzrDir` stores
its apparent path.  (This may not be the best thing.)


Useful functions
----------------

`breezy.osutils.dereference_path` does the commonly useful operation of
resolving the directory part of a path, but leaving the filename
untouched.  In other words ::

  ln -s x a
  ln -s y x/b
  dereference_path('a/b') => 'x/b'


Relative paths beyond symlinks
------------------------------

Another interesting case is when a control directory contains a relative
path, perhaps from a branch to its master or from a working tree to its
branch.  If it contains ``../`` parts as it typically will, these may have
different effects depending on whether they're looked up relative to the
real path or the apparent path given by the user.  It may be that some
users expect different behaviours at different times.  

Resolving the path relative to the real directory makes it somewhat more
consistent with what you would see by in a shell entering that directory
and then opening the given name.  It may also make things more consistent
when there are multiple links to the same bzrdir.  However it may cause
problems when using a transport that hides symlinks.

We could possibly handle this by doing less path arithmetic and asking the
OS or server to open the path including ``..`` and other relative
elements, but that might cause other problems.  HTTP servers may do their
own path arithmetic before passing it to the OS.


Transports that hide symlinks
-----------------------------

On local, SFTP and bzr+ssh transports, we can directly see symlinks as
symlinks.  Over HTTP (and FTP?) they're expanded by the server and we
cannot detect them.  This can cause problems when Breezy follows relative
paths because typically we will join the paths, and we may do this
inconsistently with how the server, which can see the symlinks, would do.


Symlinks and ChrootTransports
-----------------------------

Breezy has an internal concept of a `ChrootTransport` that locks access into
a particular directory.  Symlinks should not break out of a chroot jail
which implies they should be expanded and checked within breezy.
(At least as long as the transport lets us see the symlink; otherwise it
may not be possible.) 


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