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<sect1 id="tsvn-dug-locking">
  <title>Locking</title>
  <indexterm>
    <primary>locking</primary>
  </indexterm>
  <indexterm>
    <primary>readonly</primary>
  </indexterm>
  <para>
    Subversion generally works best without locking, using the
    <quote>Copy-Modify-Merge</quote> methods described earlier in
    <xref linkend="tsvn-basics-versioning-copymodifymerge"/>.
    However there are a few instances when you may need to implement
    some form of locking policy.
    <itemizedlist>
      <listitem>
        <para>
          You are using <quote>unmergeable</quote> files,
          for example, graphics files. If two people change
          the same file, merging is not possible, so one of you
          will lose their changes.
        </para>
      </listitem>
      <listitem>
        <para>
          Your company has always used a locking revision control system in the past
          and there has been a management decision that
          <quote>locking is best</quote>.
        </para>
      </listitem>
    </itemizedlist>
  </para>
  <para>
    Firstly you need to ensure that your Subversion server is upgraded to at
    least version 1.2. Earlier versions do not support locking at all. If you
    are using <literal>file://</literal> access, then of course only your
    client needs to be updated.
  </para>
  <important>
    <title>
      The Three Meanings of <quote>Lock</quote>
    </title>
    <para>
      In this section, and almost everywhere in this book, the
      words <quote>lock</quote> and <quote>locking</quote> describe
      a mechanism for mutual exclusion between users to avoid
      clashing commits. Unfortunately, there are two other sorts
      of <quote>lock</quote> with which Subversion, and therefore
      this book, sometimes needs to be concerned.
    </para>
    <para>
      The second is <literal>working copy locks</literal>,
      used internally by Subversion to prevent clashes between
      multiple Subversion clients operating on the same working
      copy. Usually you get these locks whenever a command
      like update/commit/... is interrupted due to an error.
      These locks can be removed by running the cleanup
      command on the working copy, as described in <xref
            linkend="tsvn-dug-cleanup" />.
    </para>
    <para>
      And third, files and folders can get locked if they're in
      use by another process, for example if you have a word
      document opened in Word, that file is locked and can not
      be accessed by TortoiseSVN.
    </para>
    <para>
      You can generally forget about these other kinds of locks
      until something goes wrong that requires you to care about
      them.  In this book, <quote>lock</quote> means the first sort
      unless the contrary is either clear from context or explicitly
      stated.
    </para>
  </important>
  <sect2 id="tsvn-dug-locking-how">
    <title>How Locking Works in Subversion</title>
    <para>
      By default, nothing is locked and anyone who has commit access can
      commit changes to any file at any time. Others will update their
      working copies periodically and changes in the repository will be
      merged with local changes.
    </para>
    <para>
      If you <firstterm>Get a Lock</firstterm> on a file, then only
      you can commit that file. Commits by all other users will be
      blocked until you release the lock. A locked file cannot be
      modified in any way in the repository, so it cannot be deleted
      or renamed either, except by the lock owner.
    </para>
    <important>
      <para>
        A lock is not assigned to a specific user, but to a specific
        user and a working copy. Having a lock in one working copy
        also prevents the same user from committing the locked file
        from another working copy.
      </para>
      <para>
        As an example, imagine that user Jon has a working copy on
        his office PC. There he starts working on an image, and therefore
        acquires a lock on that file. When he leaves his office he's not
        finished yet with that file, so he doesn't release that lock.
        Back at home Jon also has a working copy and decides to work
        a little more on the project. But he can't modify or commit
        that same image file, because the lock for that file resides
        in his working copy in the office.
      </para>
    </important>
    <para>
      However, other users will not necessarily know that you have taken
      out a lock. Unless they check the lock status regularly, the first
      they will know about it is when their commit fails, which in most
      cases is not very useful. To make it easier to manage locks, there
      is a new Subversion property
      <literal>svn:needs-lock</literal>.
      When this property is set (to any value) on a file, whenever the
      file is checked out or updated, the local copy is made read-only
      <emphasis>unless</emphasis> that working copy holds a lock for
      the file. This acts as a warning that you should not edit that
      file unless you have first acquired a lock.
      Files which are versioned and read-only are marked with a
      special overlay in TortoiseSVN to indicate that you need to
      acquire a lock before editing.
    </para>
    <para>
      Locks are recorded by working copy location as well as by owner.
      If you have several working copies (at home, at work) then you
      can only hold a lock in <emphasis>one</emphasis> of those
      working copies.
    </para>
    <para>
      If one of your co-workers acquires a lock and then goes on holiday
      without releasing it, what do you do? Subversion provides a means
      to force locks. Releasing a lock held by someone else is referred
      to as <firstterm>Breaking</firstterm> the lock, and forcibly
      acquiring a lock which someone else already holds is referred to
      as <firstterm>Stealing</firstterm> the lock. Naturally these are
      not things you should do lightly if you want to remain friends with
      your co-workers.
    </para>
    <para>
      Locks are recorded in the repository, and a lock token is created
      in your local working copy. If there is a discrepancy, for example
      if someone else has broken the lock, the local lock token becomes
      invalid. The repository is always the definitive reference.
    </para>
  </sect2>
  <sect2 id="tsvn-dug-locking-lock">
    <?dbhh topicname="HIDD_LOCK"?>
    <title>Getting a Lock</title>
    <para>
      Select the file(s) in your working copy for which you want to
      acquire a lock, then select the command
      <menuchoice>
        <guimenu>TortoiseSVN</guimenu>
        <guimenuitem>Get Lock...</guimenuitem>
      </menuchoice>.
      <figure id="tsvn-dug-locking-dia-lock">
        <title>The Locking Dialog</title>
        <graphic fileref="../images/GetLock.png"/>
      </figure>
      A dialog appears, allowing you to enter a comment, so others
      can see why you have locked the file. The comment is optional
      and currently only used with Svnserve
      based repositories.
      If (and <emphasis>only</emphasis> if) you need to steal the
      lock from someone else, check the
      <guilabel>Steal lock</guilabel> box, then click on
      <guibutton>OK</guibutton>.
    </para>
    <para>
      You can set the project property <literal>tsvn:logtemplatelock</literal>
      to provide a message template for users to fill in as the lock message.
      Refer to <xref linkend="tsvn-dug-propertypage"/> for instructions on how to set properties.
    </para>
    <para>
      If you select a folder and then use
      <menuchoice>
        <guimenu>TortoiseSVN</guimenu>
        <guimenuitem>Get Lock...</guimenuitem>
      </menuchoice>
      the lock dialog will open with <emphasis>every</emphasis> file
      in <emphasis>every</emphasis> sub-folder selected for locking.
      If you really want to lock an entire hierarchy, that is the way
      to do it, but you could become very unpopular with your co-workers
      if you lock them out of the whole project. Use with care ...
    </para>
  </sect2>
  <sect2 id="tsvn-dug-locking-unlock">
    <title>Releasing a Lock</title>
    <?dbhh topicname="HIDD_UNLOCK"?>
    <para>
      To make sure you don't forget to release a lock you don't need
      any more, locked files are shown in the commit dialog and selected
      by default. If you continue with the commit, locks you hold on the
      selected files are removed, even if the files haven't been modified.
      If you don't want to release a lock on certain files, you can
      uncheck them (if they're not modified).
      If you want to keep a lock on a file you've modified, you have
      to enable the <guilabel>Keep locks</guilabel> checkbox before
      you commit your changes.
    </para>
    <para>
      To release a lock manually, select the file(s) in your working copy
      for which you want to release the lock, then select the command
      <menuchoice>
        <guimenu>TortoiseSVN</guimenu>
        <guimenuitem>Release Lock</guimenuitem>
      </menuchoice>
      There is nothing further to enter so TortoiseSVN will contact
      the repository and release the locks. You can also use this
      command on a folder to release all locks recursively.
    </para>
  </sect2>
  <sect2 id="tsvn-dug-locking-status">
    <title>Checking Lock Status</title>
    <para>
      <figure id="tsvn-dug-locking-dia-status">
        <title>The Check for Modifications Dialog</title>
        <graphic fileref="../images/RepoStatus.png"/>
      </figure>
      To see what locks you and others hold, you can use
      <menuchoice>
        <guimenu>TortoiseSVN</guimenu>
        <guimenuitem>Check for Modifications...</guimenuitem>
      </menuchoice>.
      Locally held lock tokens show up immediately. To check for
      locks held by others (and to see if any of your locks are
      broken or stolen) you need to click on
      <guibutton>Check Repository</guibutton>.
    </para>
    <para>
      From the context menu here, you can also get and release locks,
      as well as breaking and stealing locks held by others.
    </para>
    <caution>
      <title>Avoid Breaking and Stealing Locks</title>
      <para>
        If you break or steal someone else's lock without telling
        them, you could potentially cause loss of work. If you
        are working with unmergeable file types and you steal
        someone else's lock, once you release the lock they
        are free to check in their changes and overwrite yours.
        Subversion doesn't lose data, but you have lost the
        team-working protection that locking gave you.
      </para>
    </caution>
  </sect2>
  <sect2 id="tsvn-dug-locking-needs-lock">
    <title>Making Non-locked Files Read-Only</title>
    <para>
      As mentioned above, the most effective way to use locking is to set
      the <literal>svn:needs-lock</literal> property on files. Refer to
      <xref linkend="tsvn-dug-propertypage"/>
      for instructions on how to set properties.
      Files with this property set will always be checked out and updated
      with the read-only flag set unless your working copy holds a lock.
      <graphic fileref="../images/ReadOnlyIcon.png"/>
      As a reminder, TortoiseSVN uses a special overlay to indicate this.
    </para>
    <para>
      If you operate a policy where every file has to be locked then
      you may find it easier to use Subversion's auto-props feature
      to set the property automatically every time you add new files.
      Read <xref linkend="tsvn-dug-propertypage-auto-props"/> for
      further information.
    </para>
  </sect2>
  <sect2 id="tsvn-dug-locking-hooks">
    <title>The Locking Hook Scripts</title>
    <para>
      When you create a new repository with Subversion 1.2 or higher,
      four hook templates are created in the repository
      <filename>hooks</filename> directory. These are called before
      and after getting a lock, and before and after releasing a lock.
    </para>
    <para>
      It is a good idea to install a <literal>post-lock</literal>
      and <literal>post-unlock</literal> hook script on the server which
      sends out an email indicating the file which has been locked.
      With such a script in place, all your users can be notified if
      someone locks/unlocks a file. You can find an example hook script
      <filename>hooks/post-lock.tmpl</filename> in your repository folder.
    </para>
    <para>
      You might also use hooks to disallow breaking or stealing of locks,
      or perhaps limit it to a named administrator.
      Or maybe you want to email the owner when one of their locks is
      broken or stolen.
    </para>
    <para>
      Read <xref linkend="tsvn-repository-hooks"/> to find out more.
    </para>
  </sect2>
</sect1>
