<!-- <!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook V4.1//EN"> -->
<chapter id="customization">
  <title>Customizing Bugzilla</title>

  <section id="cust-skins">
    <title>Custom Skins</title>
    
    <para>
      Bugzilla allows you to have multiple skins. These are custom CSS and possibly
      also custom images for Bugzilla. To create a new custom skin, you have two
      choices:
      <itemizedlist>
        <listitem>
          <para>
            Make a single CSS file, and put it in the 
            <filename>skins/contrib</filename> directory.
          </para>
        </listitem>
        <listitem>
          <para>
            Make a directory that contains all the same CSS file
            names as <filename>skins/standard/</filename>, and put
            your directory in <filename>skins/contrib/</filename>.
          </para>
        </listitem>
      </itemizedlist>
    </para>
      
    <para>
      After you put the file or the directory there, make sure to run checksetup.pl
      so that it can reset the file permissions correctly.
    </para>
    <para>
      After you have installed the new skin, it will show up as an option in the
      user's General Preferences. If you would like to force a particular skin on all
      users, just select it in the Default Preferences and then uncheck "Enabled" on
      the preference.
    </para>
  </section>
  
  <section id="cust-templates">
    <title>Template Customization</title>
    
    <para>
      Administrators can configure the look and feel of Bugzilla without
      having to edit Perl files or face the nightmare of massive merge
      conflicts when they upgrade to a newer version in the future.
    </para>
    
    <para>
      Templatization also makes localized versions of Bugzilla possible, 
      for the first time. It's possible to have Bugzilla's UI language 
      determined by the user's browser. More information is available in
      <xref linkend="template-http-accept"/>.
    </para>
    
    <section id="template-directory">
      <title>Template Directory Structure</title>
      <para>
        The template directory structure starts with top level directory 
        named <filename>template</filename>, which contains a directory
        for each installed localization. The next level defines the
        language used in the templates. Bugzilla comes with English
        templates, so the directory name is <filename>en</filename>,
        and we will discuss <filename>template/en</filename> throughout
        the documentation. Below <filename>template/en</filename> is the
        <filename>default</filename> directory, which contains all the
        standard templates shipped with Bugzilla.
      </para>

      <warning>
        <para>
          A directory <filename>data/templates</filename> also exists;
          this is where Template Toolkit puts the compiled versions of
          the templates from either the default or custom directories.
          <emphasis>Do not</emphasis> directly edit the files in this
          directory, or all your changes will be lost the next time
          Template Toolkit recompiles the templates.
        </para>
      </warning>
    </section>

    <section id="template-method">
      <title>Choosing a Customization Method</title>
      <para>
        If you want to edit Bugzilla's templates, the first decision
        you must make is how you want to go about doing so. There are two
        choices, and which you use depends mainly on the scope of your 
        modifications, and the method you plan to use to upgrade Bugzilla.
      </para> 

      <para>
        The first method of making customizations is to directly edit the
        templates found in <filename>template/en/default</filename>.
        This is probably the best way to go about it if you are going to
        be upgrading Bugzilla through CVS, because if you then execute
        a <command>cvs update</command>, any changes you have made will
        be merged automagically with the updated versions.
      </para>

      <note>
        <para>
          If you use this method, and CVS conflicts occur during an
          update, the conflicted templates (and possibly other parts
          of your installation) will not work until they are resolved.
        </para>
      </note>

      <para>
        The second method is to copy the templates to be modified
        into a mirrored directory structure under 
        <filename>template/en/custom</filename>. Templates in this
        directory structure automatically override any identically-named
        and identically-located templates in the 
        <filename>default</filename> directory. 
      </para>

      <note>
        <para>
          The <filename>custom</filename> directory does not exist
          at first and must be created if you want to use it.
        </para>
      </note>

      <para>
        The second method of customization should be used if you 
        use the overwriting method of upgrade, because otherwise 
        your changes will be lost.  This method may also be better if
        you are using the CVS method of upgrading and are going to make major
        changes, because it is guaranteed that the contents of this directory
        will not be touched during an upgrade, and you can then decide whether
        to continue using your own templates, or make the effort to merge your
        changes into the new versions by hand.
      </para>

      <para>
        Using this method, your installation may break if incompatible
        changes are made to the template interface.  Such changes should
        be documented in the release notes, provided you are using a
        stable release of Bugzilla.  If you use using unstable code, you will
        need to deal with this one yourself, although if possible the changes
        will be mentioned before they occur in the deprecations section of the
        previous stable release's release notes.
      </para>

      <note>
        <para>
          Regardless of which method you choose, it is recommended that
          you run <command>./checksetup.pl</command> after 
          editing any templates in the <filename>template/en/default</filename>
          directory, and after creating or editing any templates in the 
          <filename>custom</filename> directory.
        </para>
      </note>

      <warning>
        <para>
          It is <emphasis>required</emphasis> that you run 
          <command>./checksetup.pl</command> after creating a new
          template in the <filename>custom</filename> directory. Failure
          to do so will raise an incomprehensible error message.
        </para>
      </warning>
    </section>
    
    <section id="template-edit">
      <title>How To Edit Templates</title>
      
      <note>
        <para>
          If you are making template changes that you intend on submitting back
          for inclusion in standard Bugzilla, you should read the relevant
          sections of the 
          <ulink url="http://www.bugzilla.org/docs/developer.html">Developers'
          Guide</ulink>.
        </para>
      </note>
      
      <para>
        The syntax of the Template Toolkit language is beyond the scope of
        this guide. It's reasonably easy to pick up by looking at the current 
        templates; or, you can read the manual, available on the
        <ulink url="http://www.template-toolkit.org">Template Toolkit home
        page</ulink>.
      </para>
      
      <para>
        One thing you should take particular care about is the need
        to properly HTML filter data that has been passed into the template.
        This means that if the data can possibly contain special HTML characters
        such as &lt;, and the data was not intended to be HTML, they need to be
        converted to entity form, i.e. &amp;lt;.  You use the 'html' filter in the
        Template Toolkit to do this.  If you forget, you may open up
        your installation to cross-site scripting attacks.
      </para>

      <para>
        Also note that Bugzilla adds a few filters of its own, that are not
        in standard Template Toolkit.  In particular, the 'url_quote' filter
        can convert characters that are illegal or have special meaning in URLs,
        such as &amp;, to the encoded form, i.e. %26.  This actually encodes most
        characters (but not the common ones such as letters and numbers and so
        on), including the HTML-special characters, so there's never a need to
        HTML filter afterwards.
      </para>
 
      <para>
        Editing templates is a good way of doing a <quote>poor man's custom
        fields</quote>.
        For example, if you don't use the Status Whiteboard, but want to have
        a free-form text entry box for <quote>Build Identifier</quote>,
        then you can just
        edit the templates to change the field labels. It's still be called
        status_whiteboard internally, but your users don't need to know that.
      </para>
      
    </section>
            
    
    <section id="template-formats">
      <title>Template Formats and Types</title>
      
      <para>
        Some CGI's have the ability to use more than one template. For example,
        <filename>buglist.cgi</filename> can output itself as RDF, or as two 
        formats of HTML (complex and simple). The mechanism that provides this 
        feature is extensible.
      </para>

      <para>
        Bugzilla can support different types of output, which again can have 
        multiple formats. In order to request a certain type, you can append 
        the &amp;ctype=&lt;contenttype&gt; (such as rdf or html) to the 
        <filename>&lt;cginame&gt;.cgi</filename> URL. If you would like to 
        retrieve a certain format, you can use the &amp;format=&lt;format&gt; 
        (such as simple or complex) in the URL.
      </para>
      
      <para>
        To see if a CGI supports multiple output formats and types, grep the
        CGI for <quote>get_format</quote>. If it's not present, adding
        multiple format/type support isn't too hard - see how it's done in
        other CGIs, e.g. config.cgi.
      </para>
      
      <para>
        To make a new format template for a CGI which supports this, 
        open a current template for
        that CGI and take note of the INTERFACE comment (if present.) This 
        comment defines what variables are passed into this template. If 
        there isn't one, I'm afraid you'll have to read the template and
        the code to find out what information you get. 
      </para>     
  
      <para>
        Write your template in whatever markup or text style is appropriate.
      </para>
      
      <para>
        You now need to decide what content type you want your template
        served as. The content types are defined in the
        <filename>Bugzilla/Constants.pm</filename> file in the 
        <filename>contenttypes</filename>
        constant. If your content type is not there, add it. Remember
        the three- or four-letter tag assigned to your content type. 
        This tag will be part of the template filename.
      </para>

      <note>
        <para>
          After adding or changing a content type, it's suitable to edit
          <filename>Bugzilla/Constants.pm</filename> in order to reflect
          the changes. Also, the file should be kept up to date after an
          upgrade if content types have been customized in the past. 
        </para>
      </note>
      
      <para>
        Save the template as <filename>&lt;stubname&gt;-&lt;formatname&gt;.&lt;contenttypetag&gt;.tmpl</filename>. 
        Try out the template by calling the CGI as 
        <filename>&lt;cginame&gt;.cgi?format=&lt;formatname&gt;&amp;ctype=&lt;type&gt;</filename> .
      </para>       
    </section>
    
    
    <section id="template-specific">
      <title>Particular Templates</title>
      
      <para>
        There are a few templates you may be particularly interested in
        customizing for your installation.
      </para>
      
      <para>
        <command>index.html.tmpl</command>:
        This is the Bugzilla front page.
      </para>      

      <para>
        <command>global/header.html.tmpl</command>:
        This defines the header that goes on all Bugzilla pages.
        The header includes the banner, which is what appears to users
        and is probably what you want to edit instead.  However the
        header also includes the HTML HEAD section, so you could for
        example add a stylesheet or META tag by editing the header.
      </para>

      <para>
        <command>global/banner.html.tmpl</command>:
        This contains the <quote>banner</quote>, the part of the header
        that appears
        at the top of all Bugzilla pages.  The default banner is reasonably
        barren, so you'll probably want to customize this to give your
        installation a distinctive look and feel.  It is recommended you
        preserve the Bugzilla version number in some form so the version 
        you are running can be determined, and users know what docs to read.
      </para>

      <para>
        <command>global/footer.html.tmpl</command>:
        This defines the footer that goes on all Bugzilla pages.  Editing
        this is another way to quickly get a distinctive look and feel for
        your Bugzilla installation.
      </para>

      <para>
        <command>global/variables.none.tmpl</command>:
        This defines a list of terms that may be changed in order to
        <quote>brand</quote> the Bugzilla instance In this way, terms
        like <quote>bugs</quote> can be replaced with <quote>issues</quote>
        across the whole Bugzilla installation. The name
        <quote>Bugzilla</quote> and other words can be customized as well.
      </para>

      <para>
        <command>list/table.html.tmpl</command>:
        This template controls the appearance of the bug lists created
        by Bugzilla. Editing this template allows per-column control of 
        the width and title of a column, the maximum display length of
        each entry, and the wrap behaviour of long entries.
        For long bug lists, Bugzilla inserts a 'break' every 100 bugs by
        default; this behaviour is also controlled by this template, and
        that value can be modified here.
       </para>

      <para>
        <command>bug/create/user-message.html.tmpl</command>:
        This is a message that appears near the top of the bug reporting page.
        By modifying this, you can tell your users how they should report
        bugs.
      </para>

      <para>
        <command>bug/process/midair.html.tmpl</command>:
        This is the page used if two people submit simultaneous changes to the
        same bug.  The second person to submit their changes will get this page
        to tell them what the first person did, and ask if they wish to
        overwrite those changes or go back and revisit the bug.  The default
        title and header on this page read "Mid-air collision detected!"  If
        you work in the aviation industry, or other environment where this
        might be found offensive (yes, we have true stories of this happening)
        you'll want to change this to something more appropriate for your
        environment.
      </para>

      <para>
        <command>bug/create/create.html.tmpl</command> and
        <command>bug/create/comment.txt.tmpl</command>:
        You may not wish to go to the effort of creating custom fields in
        Bugzilla, yet you want to make sure that each bug report contains
        a number of pieces of important information for which there is not
        a special field. The bug entry system has been designed in an
        extensible fashion to enable you to add arbitrary HTML widgets,
        such as drop-down lists or textboxes, to the bug entry page
        and have their values appear formatted in the initial comment.
        A hidden field that indicates the format should be added inside
        the form in order to make the template functional. Its value should
        be the suffix of the template filename. For example, if the file
        is called <filename>create-cust.html.tmpl</filename>, then
        <programlisting>&lt;input type="hidden" name="format" value="cust"&gt;</programlisting>
        should be used inside the form.
      </para>

      <para>  
        An example of this is the mozilla.org 
        <ulink url="http://landfill.bugzilla.org/bugzilla-tip/enter_bug.cgi?product=WorldControl&amp;format=guided">guided 
        bug submission form</ulink>. The code for this comes with the Bugzilla
        distribution as an example for you to copy. It can be found in the
        files 
        <filename>create-guided.html.tmpl</filename> and
        <filename>comment-guided.html.tmpl</filename>.
      </para>  

      <para>
        So to use this feature, create a custom template for 
        <filename>enter_bug.cgi</filename>. The default template, on which you
        could base it, is 
        <filename>custom/bug/create/create.html.tmpl</filename>.
        Call it <filename>create-&lt;formatname&gt;.html.tmpl</filename>, and
        in it, add widgets for each piece of information you'd like
        collected - such as a build number, or set of steps to reproduce.
      </para>

      <para>
        Then, create a template like 
        <filename>custom/bug/create/comment.txt.tmpl</filename>, and call it 
        <filename>comment-&lt;formatname&gt;.txt.tmpl</filename>. This 
        template should reference the form fields you have created using
        the syntax <filename>[% form.&lt;fieldname&gt; %]</filename>. When a 
        bug report is
        submitted, the initial comment attached to the bug report will be
        formatted according to the layout of this template.
      </para> 

      <para>
        For example, if your custom enter_bug template had a field
        <programlisting>&lt;input type="text" name="buildid" size="30"&gt;</programlisting>
        and then your comment.txt.tmpl had
        <programlisting>BuildID: [% form.buildid %]</programlisting>
        then something like
        <programlisting>BuildID: 20020303</programlisting>
        would appear in the initial comment.
      </para>            
    </section>          


    <section id="template-http-accept">
      <title>Configuring Bugzilla to Detect the User's Language</title>

      <para>Bugzilla honours the user's Accept: HTTP header. You can install
      templates in other languages, and Bugzilla will pick the most appropriate
      according to a priority order defined by you. Many
      language templates can be obtained from <ulink 
      url="http://www.bugzilla.org/download.html#localizations"/>. Instructions
      for submitting new languages are also available from that location.
      </para>

      <para>After untarring the localizations (or creating your own) in the 
      <filename class="directory">BUGZILLA_ROOT/template</filename> directory,
      you must update the <option>languages</option> parameter to contain any
      localizations you'd like to permit. You may also wish to set the
      <option>defaultlanguage</option> parameter to something other than
      <quote>en</quote> if you don't want English to be the default language.
      </para>
    </section>
      
  </section>

  <section id="cust-hooks">
    <title>The Bugzilla Extension Mechanism</title>
    
    <warning>
      <para>
        Note that the below paths are inconsistent and confusing. They will
        likely be changed in Bugzilla 4.0.
      </para>
    </warning>
       
    <para>
      Extensions are a way for extensions to Bugzilla to insert code
      into the standard Bugzilla templates and source files
      without modifying these files themselves.  The extension mechanism 
      defines a consistent API for extending the standard templates and source files 
      in a way that cleanly separates standard code from extension code.  
      Hooks reduce merge conflicts and make it easier to write extensions that work 
      across multiple versions of Bugzilla, making upgrading a Bugzilla installation 
      with installed extensions easier. Furthermore, they make it easy to install 
      and remove extensions as each extension is nothing more than a 
      simple directory structure. 
    </para>
    
    <para>
      There are two main types of hooks: code hooks and template hooks. Code 
      hooks allow extensions to invoke code at specific points in various 
      source files, while template hooks allow extensions to add elements to 
      the Bugzilla user interface. 
    </para>

    <para>
      A hook is just a named place in a standard source or template file
      where extension source code or template files for that hook get processed. 
      Each extension has a corresponding directory in the Bugzilla directory 
      tree (<filename>BUGZILLA_ROOT/extensions/extension_name</filename>).  Hooking 
      an extension source file or template to a hook is as simple as putting 
      the extension file into extension's template or code directory. 
      When Bugzilla processes the source file or template and reaches the hook, 
      it will process all extension files in the hook's directory. 
      The hooks themselves can be added into any source file or standard template 
      upon request by extension authors.
    </para>
    
    <para>
      To use hooks to extend Bugzilla, first make sure there is
      a hook at the appropriate place within the source file or template you 
      want to extend. The exact appearance of a hook depends on if the hook 
      is a code hook or a template hook. 
    </para>
    
    <para>
      Code hooks appear in Bugzilla source files as a single method call 
      in the format <literal role="code">Bugzilla::Hook->process("<varname>name</varname>");</literal>.
      For instance, <filename>enter_bug.cgi</filename> may invoke the hook 
      "<varname>enter_bug-entrydefaultvars</varname>". Thus, a source file at 
      <filename>BUGZILLA_ROOT/extensions/EXTENSION_NAME/code/enter_bug-entrydefaultvars.pl</filename>
      will be automatically invoked when the code hook is reached. 
    </para>
   
    <para>  
      Template hooks appear in the standard Bugzilla templates as a 
      single directive in the format
      <literal role="code">[% Hook.process("<varname>name</varname>") %]</literal>,
      where <varname>name</varname> is the unique name of the hook.
    </para>

    <para>
      If you aren't sure what you want to extend or just want to browse the 
      available hooks, either use your favorite multi-file search
      tool (e.g. <command>grep</command>) to search the standard templates
      for occurrences of <methodname>Hook.process</methodname> or the source 
      files for occurrences of <methodname>Bugzilla::Hook::process</methodname>.
    </para>

    <para>
      If there is no hook at the appropriate place within the Bugzilla 
      source file or template you want to extend,
      <ulink url="http://bugzilla.mozilla.org/enter_bug.cgi?product=Bugzilla&amp;component=User%20Interface">file
      a bug requesting one</ulink>, specifying:
    </para>

    <simplelist>
      <member>the source or template file for which you are 
          requesting a hook;</member>
      <member>
        where in the file you would like the hook to be placed
        (line number/position for latest version of the file in CVS
        or description of location);
      </member>
      <member>the purpose of the hook;</member>
      <member>a link to information about your extension, if any.</member>
    </simplelist>

    <para>
      The Bugzilla reviewers will promptly review each hook request,
      name the hook, add it to the template or source file, and check 
      the new version of the template into CVS.
    </para>

    <para>
      You may optionally attach a patch to the bug which implements the hook
      and check it in yourself after receiving approval from a Bugzilla
      reviewer.  The developers may suggest changes to the location of the
      hook based on their analysis of your needs or so the hook can satisfy
      the needs of multiple extensions, but the process of getting hooks
      approved and checked in is not as stringent as the process for general
      changes to Bugzilla, and any extension, whether released or still in
      development, can have hooks added to meet their needs.
    </para>

    <para>
      After making sure the hook you need exists (or getting it added if not),
      add your extension to the directory within the Bugzilla
      extensions tree corresponding to the hook. 
    </para>
    
    <para>
      That's it!  Now, when the source file or template containing the hook
      is processed, your extension file will be processed at the point 
      where the hook appears.
    </para>

    <para>
      For example, let's say you have an extension named Projman that adds
      project management capabilities to Bugzilla.  Projman has an
      administration interface <filename>edit-projects.cgi</filename>, 
      and you want to add a link to it into the navigation bar at the bottom
      of every Bugzilla page for those users who are authorized
      to administer projects.
    </para>

    <para>
      The navigation bar is generated by the template file
      <filename>useful-links.html.tmpl</filename>, which is located in
      the <filename>global/</filename> subdirectory on the standard Bugzilla 
      template path
      <filename>BUGZILLA_ROOT/template/en/default/</filename>.
      Looking in <filename>useful-links.html.tmpl</filename>, you find
      the following hook at the end of the list of standard Bugzilla
      administration links:
    </para>

    <programlisting><![CDATA[...
    [% ', <a href="editkeywords.cgi">keywords</a>' 
                                              IF user.groups.editkeywords %]
    [% Hook.process("edit") %]
...]]></programlisting>

    <para>
      The corresponding extension file for this hook is
      <filename>BUGZILLA_ROOT/extensions/projman/template/en/global/useful-links-edit.html.tmpl</filename>.
      You then create that template file and add the following constant:
    </para>

    <programlisting><![CDATA[...[% ', <a href="edit-projects.cgi">projects</a>' IF user.groups.projman_admins %]]]></programlisting>

    <para>
      Voila!  The link now appears after the other administration links in the
      navigation bar for users in the <literal>projman_admins</literal> group.
    </para>
    
    <para>
      Now, let us say your extension adds a custom "project_manager" field 
      to enter_bug.cgi. You want to modify the CGI script to set the default 
      project manager to be productname@company.com. Looking at 
      <filename>enter_bug.cgi</filename>, you see the enter_bug-entrydefaultvars 
      hook near the bottom of the file before the default form values are set. 
      The corresponding extension source file for this hook is located at 
      <filename>BUGZILLA_ROOT/extensions/projman/code/enter_bug-entrydefaultvars.pl</filename>.
      You then create that file and add the following:
    </para>
    
    <programlisting>$default{'project_manager'} = $product.'@company.com';</programlisting>
    
    <para>
      This code will be invoked whenever enter_bug.cgi is executed. 
      Assuming that the rest of the customization was completed (e.g. the 
      custom field was added to the enter_bug template and the required hooks 
      were used in process_bug.cgi), the new field will now have this 
      default value. 
    </para>
      
    <para>
      Notes:
    </para>

    <itemizedlist>
      <listitem>
        <para>
          If your extension includes entirely new templates in addition to
          extensions of standard templates, it should store those new
          templates in its
          <filename>BUGZILLA_ROOT/extensions/template/en/</filename> 
          directory. Extension template directories, like the 
          <filename>default/</filename> and <filename>custom/</filename>
          directories, are part of the template search path, so putting templates
          there enables them to be found by the template processor.
        </para>

        <para>
          The template processor looks for templates first in the
          <filename>custom/</filename> directory (i.e. templates added by the 
          specific installation), then in the <filename>extensions/</filename>
          directory (i.e. templates added by extensions), and finally in the 
          <filename>default/</filename> directory (i.e. the standard Bugzilla 
          templates). Thus, installation-specific templates override both 
          default and extension templates.
        </para>
      </listitem>

      <listitem>
        <para>
          If you are looking to customize Bugzilla, you can also take advantage 
          of template hooks. To do so, create a directory in
          <filename>BUGZILLA_ROOT/template/en/custom/hook/</filename>
          that corresponds to the hook you wish to use, then place your 
          customization templates into those directories. For example, 
          if you wanted to use the hook "end" in 
          <filename>global/useful-links.html.tmpl</filename>, you would 
          create the directory <filename>BUGZILLA_ROOT/template/en/custom/hook/
          global/useful-links.html.tmpl/end/</filename> and add your customization 
          template to this directory. 
        </para>

        <para>
          Obviously this method of customizing Bugzilla only lets you add code
          to the standard source files and templates; you cannot change the 
          existing code. Nevertheless, for those customizations that only add 
          code, this method can reduce conflicts when merging changes, 
          making upgrading your customized Bugzilla installation easier.
        </para>
      </listitem>
    </itemizedlist>    
  </section>

  <section id="cust-change-permissions">
    <title>Customizing Who Can Change What</title>
    
    <warning>
      <para>
        This feature should be considered experimental; the Bugzilla code you
        will be changing is not stable, and could change or move between 
        versions. Be aware that if you make modifications as outlined here, 
        you may have
        to re-make them or port them if Bugzilla changes internally between
        versions, and you upgrade.
      </para>
    </warning>
      
    <para>
      Companies often have rules about which employees, or classes of employees,
      are allowed to change certain things in the bug system. For example, 
      only the bug's designated QA Contact may be allowed to VERIFY the bug.
      Bugzilla has been
      designed to make it easy for you to write your own custom rules to define
      who is allowed to make what sorts of value transition.
    </para>

    <para>
     By default, assignees, QA owners and users
     with <emphasis>editbugs</emphasis> privileges can edit all fields of bugs, 
     except group restrictions (unless they are members of the groups they 
     are trying to change). Bug reporters also have the ability to edit some 
     fields, but in a more restrictive manner. Other users, without 
     <emphasis>editbugs</emphasis> privileges, can not edit 
     bugs, except to comment and add themselves to the CC list.
    </para> 
    
    <para>
      For maximum flexibility, customizing this means editing Bugzilla's Perl 
      code. This gives the administrator complete control over exactly who is
      allowed to do what. The relevant method is called
      <filename>check_can_change_field()</filename>,
      and is found in <filename>Bug.pm</filename> in your
      Bugzilla/ directory. If you open that file and search for
      <quote>sub check_can_change_field</quote>, you'll find it.
    </para>
    
    <para>
      This function has been carefully commented to allow you to see exactly
      how it works, and give you an idea of how to make changes to it.
      Certain marked sections should not be changed - these are
      the <quote>plumbing</quote> which makes the rest of the function work.
      In between those sections, you'll find snippets of code like:
      <programlisting>    # Allow the assignee to change anything.
    if ($ownerid eq $whoid) {
        return 1;
    }</programlisting>
      It's fairly obvious what this piece of code does.
    </para>      
      
    <para>
      So, how does one go about changing this function? Well, simple changes
      can be made just by removing pieces - for example, if you wanted to 
      prevent any user adding a comment to a bug, just remove the lines marked
      <quote>Allow anyone to change comments.</quote> If you don't want the
      Reporter to have any special rights on bugs they have filed, just
      remove the entire section that deals with the Reporter.
    </para>
    
    <para>
      More complex customizations are not much harder. Basically, you add
      a check in the right place in the function, i.e. after all the variables
      you are using have been set up. So, don't look at $ownerid before 
      $ownerid has been obtained from the database. You can either add a
      positive check, which returns 1 (allow) if certain conditions are true,
      or a negative check, which returns 0 (deny.) E.g.:
      <programlisting>    if ($field eq "qacontact") {
        if (Bugzilla->user->groups("quality_assurance")) {
            return 1;
        } 
        else {
            return 0;
        }
    }</programlisting>
      This says that only users in the group "quality_assurance" can change
      the QA Contact field of a bug.
    </para>

    <para>
      Getting more weird:
      <programlisting><![CDATA[    if (($field eq "priority") &&
        (Bugzilla->user->email =~ /.*\@example\.com$/))
    {
        if ($oldvalue eq "P1") {
            return 1;
        } 
        else {
            return 0;
        }
    }]]></programlisting>
      This says that if the user is trying to change the priority field,
      and their email address is @example.com, they can only do so if the
      old value of the field was "P1". Not very useful, but illustrative.
    </para>

    <warning>
      <para>
        If you are modifying <filename>process_bug.cgi</filename> in any
        way, do not change the code that is bounded by DO_NOT_CHANGE blocks.
        Doing so could compromise security, or cause your installation to
        stop working entirely.
      </para>
    </warning>
    
    <para>
      For a list of possible field names, look at the bugs table in the 
      database. If you need help writing custom rules for your organization,
      ask in the newsgroup.
    </para>    
  </section>   

  <!-- Integrating Bugzilla with Third-Party Tools -->
  &integration;

</chapter>

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