Piston is a utility that enables merge tracking of remote repositories.
This is similar to <tt>svn:externals</tt>, except you have a local copy of
the files, which you can modify at will.  As long as the changes are
mergeable, you should have no problems.

This tool has a similar purpose than svnmerge.py which you can find in the
contrib/client-side folder of the main Subversion repository at
http://svn.collab.net/repos/svn/trunk/contrib/client-side/svnmerge.py.
The main difference is that Piston is designed to work with remote
repositories.  Another tool you might want to look at, SVK, situated at
http://svk.elixus.org.

From Wikipedia's Piston page (http://en.wikipedia.org/wiki/Piston):
  In general, a piston is a sliding plug that fits closely inside the bore
  of a cylinder.

  Its purpose is either to change the volume enclosed by the cylinder, or
  to exert a force on a fluid inside the cylinder.

For this utility, I retain the second meaning, "to exert a force on a fluid
inside the cylinder."  Piston forces the content of a remote repository
location back into our own.

= Installation

Nothing could be simpler:

 $ gem install --include-dependencies piston


= Usage

First, you need to import the remote repository location:

 $ piston import http://dev.rubyonrails.org/svn/rails/trunk vendor/rails
 Exported r4720 from 'http://dev.rubyonrails.org/svn/rails/trunk' to 'vendor/rails'

 $ svn commit -m "Importing local copy of Rails"

When you want to get the latest changes from the remote repository location:

 $ piston update vendor/rails
 Updated 'vendor/rails' to r4720.

 $ svn commit -m "Updates vendor/rails to the latest revision"

You can prevent a local Piston-managed folder from updating by using the
+lock+ subcommand:

 $ piston lock vendor/rails
 'vendor/rails' locked at r4720.

When you want to update again, you unlock:

 $ piston unlock vendor/rails
 'vendor/rails' unlocked.


= Contributions

== Bash Shell Completion Script

Michael Schuerig contributed a Bash shell completion script.  You should copy
+contrib/piston+ from your gem repository to the appropriate folder.  Michael
said:

  I've put together a bash completion function for piston. On Debian, I
  just put it in /etc/bash_completion.d, alternatively, the contents can
  be copied to ~/.bash_completion. I don't know how things are organized
  on other Unix/Linux systems.


= Caveats

== Speed

This tool is SLOW.  The update process particularly so.  I use a brute force
approach.  Subversion cannot merge from remote repositories, so instead I
checkout the folder at the initial revision, and then run svn update and
parse the results of that to determine what changes have occured.

If a local copy of a file was changed, it's changes will be merged back in.
If that introduces a conflict, Piston will not detect it.  The commit will be
rejected by Subversion anyway.

== Copies / Renames

Piston *does not* track copies.  Since Subversion does renames in two
phases (copy + delete), that is what Piston does.

== Local Operations Only

Piston only works if you have a working copy.  It also never commits your
working copy directly.  You are responsible for reviewing the changes and
applying any pending fixes.

== Remote Repository UUID

Piston caches the remote repository UUID, allowing it to know if the remote
repos is still the same.  Piston refuses to work against a different
repository than the one we checked out from originally.


= Subversion Properties Used

* <tt>piston:uuid</tt>: The remote repository's UUID, which we always confirm
  before doing any operations.
* <tt>piston:root</tt>: The repository root URL from which this Piston folder
  was exported from.
* <tt>piston:remote-revision</tt>: The <tt>Last Changed Rev</tt> of the remote
  repository.
* <tt>piston:local-revision</tt>: The <tt>Last Changed Rev</tt> of the Piston
  managed folder, to enable us to know if we need to do any merging.
* <tt>piston:locked</tt>: The revision at which this folder is locked.  If
  this property is set and non-blank, Piston will skip the folder with
  an appropriate message.


= Dependencies

Piston depends on the following libraries:

* yaml
* getoptlong
* uri
* fileutils

These dependencies are all included in a stock 1.8.4 Ruby distribution.