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Version Control with Subversion
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Version Control with Subversion - Locking - Lock Communication

Lock Communication

We've seen how svn lock and svn unlock can be used to create, release, break, and steal locks. This satisfies the goal of serializing commit access to a file. But what about the larger problem of preventing wasted time?

For example, suppose Harry locks an image file and then begins editing it. Meanwhile, miles away, Sally wants to do the same thing. She doesn't think to run svn status --show-updates , so she has no idea that Harry has already locked the file. She spends hours editing the file, and when she tries to commit her change, she discovers that either the file is locked or that she's out-of-date. Regardless, her changes aren't mergeable with Harry's. One of these two people has to throw away their work, and a lot of time has been wasted.

Subversion's solution to this problem is provide a mechanism to remind users that a file ought to be locked before the editing begins.

The mechanism is a special property, svn:needs-lock. If the property is attached to a file (the value is irrelevant), then the file will have read-only permissions. When the user locks the file and receives a lock token, the file becomes read-write. When the lock is released—either explicitly unlocked, or released via commit—the file returns to read-only again.

The theory, then, is that if the image file has this property attached, then Sally would immediately notice something is strange when she opens the file for editing. Her application would be unable to save changes, or (better yet) tell her that the file is read-only. This reminds her to lock the file before editing, whereby she discovers the pre-existing lock:

$ /usr/local/bin/gimp raisin.jpg
gimp: error: file is read-only!

$ ls -l raisin.jpg
-r--r--r--   1 sally   sally   215589 Jun  8 19:23 raisin.jpg

$ svn lock raisin.jpg
svn: Lock request failed: 423 Locked (https://svn.example.com)

$ svn info https://svn.example.com/repos/project/raisin.jpg | grep Lock
Lock Token: opaquelocktoken:fc2b4dee-98f9-0310-abf3-653ff3226e6b
Lock Owner: harry
Lock Created: 2005-06-08 07:29:18 -0500 (Thu, 08 June 2005)
Lock Comment (1 line):
Making some tweaks.  Locking for the next two hours.

As a matter of “best practice”, both users and administrators are encouraged to attach the svn:needs-lock property to any file which cannot be contextually merged. It's the main technique for encouraging good locking habits and preventing wasted effort.

Note that this property is a communication tool which works independently from the locking system. In other words, any file can be locked, whether or not this property is present. And conversely, the presence of this property doesn't make the repository require a lock when committing.

The system isn't flawless, either. It's possible that even when a file has the property, the read-only reminder won't always work. Sometimes applications misbehave and “hijack” the read-only file, silently allowing users to edit and save the file anyway. Unfortunately, there's not much Subversion can do about this.


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Version Control with Subversion
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