web commit by madmax

[git.ikiwiki.info.git] / doc / plugins / write.mdwn

diff --git a/doc/plugins/write.mdwn b/doc/plugins/write.mdwn
index f808d2e1e089ac412cbabbc2a379e465ca19cbf4..34caf83f68e8aa469febccf1031a1dae95896a7f 100644 (file)
--- a/doc/plugins/write.mdwn
+++ b/doc/plugins/write.mdwn
@@ -1,11 +1,36 @@
-ikiwiki [[plugins]] are written in perl. Each plugin is a perl module, in
-the `IkiWiki::Plugin` namespace. The name of the plugin is typically in
-lowercase, such as `IkiWiki::Plugin::inline`. Ikiwiki includes a
-`IkiWiki::Plugin::skeleton` that can be fleshed out to make a useful
-plugin. `IkiWiki::Plugin::pagecount` is another simple example.
+Ikiwiki's plugin interface allows all kinds of useful [[plugins]] to be
+written to extend ikiwiki in many ways. Despite the length of this page,
+it's not really hard. This page is a complete reference to everything a
+plugin might want to do. There is also a quick [[tutorial]].

 
 [[toc levels=2]]
 
 
 [[toc levels=2]]
 

+## Types of plugins
+
+Most ikiwiki [[plugins]] are written in perl, like ikiwiki. This gives the
+plugin full access to ikiwiki's internals, and is the most efficient.
+However, plugins can actually be written in any language that supports XML
+RPC. These are called [[external]] plugins.
+
+A plugin written in perl is a perl module, in the `IkiWiki::Plugin`
+namespace. The name of the plugin is typically in lowercase, such as
+`IkiWiki::Plugin::inline`. Ikiwiki includes a `IkiWiki::Plugin::skeleton`
+that can be fleshed out to make a useful plugin.
+`IkiWiki::Plugin::pagecount` is another simple example. All perl plugins
+should `use IkiWiki` to import the ikiwiki plugin interface. It's a good
+idea to include the version number of the plugin interface that your plugin
+expects: `use IkiWiki 2.00`.
+
+An external plugin is an executable program. It can be written in any
+language. Its interface to ikiwiki is via XML RPC, which it reads from
+ikiwiki on its standard input, and writes to ikiwiki on its standard
+output. For more details on writing external plugins, see [[external]].
+
+Despite these two types of plugins having such different interfaces,
+they're the same as far as how they hook into ikiwiki. This document will
+explain how to write both sorts of plugins, albeit with an emphasis on perl
+plugins.
+

 ## Considerations
 
 One thing to keep in mind when writing a plugin is that ikiwiki is a wiki
 ## Considerations
 
 One thing to keep in mind when writing a plugin is that ikiwiki is a wiki


@@ -19,22 +44,13 @@ being edited.
 
 ## Registering plugins
 
 
 ## Registering plugins
 

-All plugins should `use IkiWiki` to import the ikiwiki plugin interface.
-

 Plugins should, when imported, call `hook()` to hook into ikiwiki's
 processing. The function uses named parameters, and use varies depending on
 the type of hook being registered -- see below. Note that a plugin can call
 the function more than once to register multiple hooks. All calls to
 `hook()` should be passed a "type" parameter, which gives the type of
 hook, a "id" paramter, which should be a unique string for this plugin, and
 Plugins should, when imported, call `hook()` to hook into ikiwiki's
 processing. The function uses named parameters, and use varies depending on
 the type of hook being registered -- see below. Note that a plugin can call
 the function more than once to register multiple hooks. All calls to
 `hook()` should be passed a "type" parameter, which gives the type of
 hook, a "id" paramter, which should be a unique string for this plugin, and

-a "call" parameter, which is a reference to a function to call for the
-hook.
-
-An optional "scan" parameter, if set to a true value, makes the hook be
-called during the preliminary scan that ikiwiki makes of updated pages,
-before begining to render pages. This parameter should be set to true if
-the hook modifies data in `%links`. Note that doing so will make the hook
-be run twice per page build, so avoid doing it for expensive hooks.
+a "call" parameter, which tells what function to call for the hook.

 
 An optional "last" parameter, if set to a true value, makes the hook run
 after all other hooks of its type. Useful if the hook depends on some other
 
 An optional "last" parameter, if set to a true value, makes the hook run
 after all other hooks of its type. Useful if the hook depends on some other


@@ -66,26 +82,27 @@ configuration. It's called early in the startup process. The
 function is passed no values. It's ok for the function to call
 `error()` if something isn't configured right.
 
 function is passed no values. It's ok for the function to call
 `error()` if something isn't configured right.
 

-### filter
+### needsbuild

 
 

-       hook(type => "filter", id => "foo", call => \&filter);
+       hook(type => "needsbuild", id => "foo", call => \&needsbuild);

 
 

-Runs on the raw source of a page, before anything else touches it, and can
-make arbitrary changes. The function is passed named parameters `page` and
-`content` and should return the filtered content.
+This allows a plugin the manipulate the list of files that need to be
+built when the wiki is refreshed. The function is passed a reference to an
+array of pages that will be rebuilt, and can modify the array, either
+adding or removing files from it.

 
 

-### scan
+### filter

 
 

-       hook(type => "scan", id => "foo", call => \&scan);
+       hook(type => "filter", id => "foo", call => \&filter);

 
 

-This is identical to a preprocess hook (see below), except that it is
-called in the initial pass that scans pages for data that will be used in
-later passes. Scan hooks are the only hook that should modify 
+Runs on the raw source of a page, before anything else touches it, and can
+make arbitrary changes. The function is passed named parameters "page",
+"destpage", and "content". It should return the filtered content.

 
 ### preprocess
 
 
 ### preprocess
 

-Adding a [[PreProcessorDirective]] is probably the most common use of a
-plugin.
+Adding a [[ikiwiki/PreProcessorDirective]] is probably the most common use
+of a plugin.

 
         hook(type => "preprocess", id => "foo", call => \&preprocess);
 
 
         hook(type => "preprocess", id => "foo", call => \&preprocess);
 


@@ -96,16 +113,24 @@ Each time the directive is processed, the referenced function (`preprocess`
 in the example above) is called, and is passed named parameters. A "page"
 parameter gives the name of the page that embedded the preprocessor
 directive, while a "destpage" parameter gives the name of the page the
 in the example above) is called, and is passed named parameters. A "page"
 parameter gives the name of the page that embedded the preprocessor
 directive, while a "destpage" parameter gives the name of the page the

-content is going to (different for inlined pages). All parameters included
-in the directive are included as named parameters as well. Whatever the
-function returns goes onto the page in place of the directive.
+content is going to (different for inlined pages), and a "preview"
+parameter is set to a true value if the page is being previewed. All
+parameters included in the directive are included as named parameters as
+well. Whatever the function returns goes onto the page in place of the
+directive.
+
+An optional "scan" parameter, if set to a true value, makes the hook be
+called during the preliminary scan that ikiwiki makes of updated pages,
+before begining to render pages. This parameter should be set to true if
+the hook modifies data in `%links`. Note that doing so will make the hook
+be run twice per page build, so avoid doing it for expensive hooks.

 
 Note that if the [[htmlscrubber]] is enabled, html in
 
 Note that if the [[htmlscrubber]] is enabled, html in

-[[PreProcessorDirective]] output is sanitised, which may limit what your
-plugin can do. Also, the rest of the page content is not in html format at
-preprocessor time. Text output by a preprocessor directive will be
-linkified and passed through markdown (or whatever engine is used to htmlize
-the page) along with the rest of the page.
+[[ikiwiki/PreProcessorDirective]] output is sanitised, which may limit what
+your plugin can do. Also, the rest of the page content is not in html
+format at preprocessor time. Text output by a preprocessor directive will
+be linkified and passed through markdown (or whatever engine is used to
+htmlize the page) along with the rest of the page.

 
 ### htmlize
 
 
 ### htmlize
 


@@ -123,17 +148,28 @@ return the htmlized content.
 
        hook(type => "pagetemplate", id => "foo", call => \&pagetemplate);
 
 
        hook(type => "pagetemplate", id => "foo", call => \&pagetemplate);
 

-[[Templates]] are filled out for many different things in ikiwiki, like
-generating a page, or part of a blog page, or an rss feed, or a cgi. This
-hook allows modifying those templates. The function is passed named
-parameters. The "page" and "destpage" parameters are the same as for a
-preprocess hook. The "template" parameter is a [[cpan HTML::Template]]
-object that is the template that will be used to generate the page. The
-function can manipulate that template object.
+[[Templates|wikitemplates]] are filled out for many different things in
+ikiwiki, like generating a page, or part of a blog page, or an rss feed, or
+a cgi. This hook allows modifying the variables available on those
+templates. The function is passed named parameters. The "page" and
+"destpage" parameters are the same as for a preprocess hook. The "template"
+parameter is a [[cpan HTML::Template]] object that is the template that
+will be used to generate the page. The function can manipulate that
+template object.

 
 

-The most common thing to do is probably to call $template->param() to add
+The most common thing to do is probably to call `$template->param()` to add

 a new custom parameter to the template.
 
 a new custom parameter to the template.
 

+### templatefile
+
+       hook(type => "templatefile", id => "foo", call => \&templatefile);
+
+This hook allows plugins to change the [[template|wikitemplates]] that is
+used for a page in the wiki. The hook is passed a "page" parameter, and
+should return the name of the template file to use, or undef if it doesn't
+want to change the default ("page.tmpl"). Template files are looked for in
+/usr/share/ikiwiki/templates by default.
+

 ### sanitize
 
        hook(type => "sanitize", id => "foo", call => \&sanitize);
 ### sanitize
 
        hook(type => "sanitize", id => "foo", call => \&sanitize);


@@ -176,7 +212,7 @@ source files that were rendered.
 
 Use this to hook into ikiwiki's cgi script. Each registered cgi hook is
 called in turn, and passed a CGI object. The hook should examine the
 
 Use this to hook into ikiwiki's cgi script. Each registered cgi hook is
 called in turn, and passed a CGI object. The hook should examine the

-parameters, and if it will handle this CGI request, output a page and
+parameters, and if it will handle this CGI request, output a page (including the http headers) and

 terminate the program.
 
 ### auth
 terminate the program.
 
 ### auth


@@ -192,6 +228,46 @@ object's "name" parameter to the authenticated user's name. Note that
 if the name is set to the name of a user who is not registered,
 a basic registration of the user will be automatically performed.
 
 if the name is set to the name of a user who is not registered,
 a basic registration of the user will be automatically performed.
 

+### sessioncgi
+
+       hook(type => "sessioncgi", id => "foo", call => \&sessioncgi);
+
+Unlike the cgi hook, which is run as soon as possible, the sessioncgi hook
+is only run once a session object is available. It is passed both a CGI
+object and a session object. To check if the user is in fact signed in, you
+can check if the session object has a "name" parameter set.
+
+### canedit
+
+       hook(type => "canedit", id => "foo", call => \&pagelocked);
+
+This hook can be used to implement arbitrary access methods to control when
+a page can be edited using the web interface (commits from revision control
+bypass it). When a page is edited, each registered canedit hook is called
+in turn, and passed the page name, a CGI object, and a session object.
+
+If the hook has no opinion about whether the edit can proceed, return
+`undef`, and the next plugin will be asked to decide. If edit can proceed,
+the hook should return "". If the edit is not allowed by this hook, the
+hook should return an error message for the user to see, or a function 
+that can be run to log the user in or perform other action necessary for
+them to be able to edit the page.
+
+This hook should avoid directly redirecting the user to a signin page,
+since it's sometimes used to test to see which pages in a set of pages a
+user can edit.
+
+### editcontent
+
+       hook(type => "editcontent", id => "foo", call => \&editcontent);
+
+This hook is called when a page is saved (or previewed) using the web
+interface. It is passed named parameters: `content`, `page`, `cgi`, and
+`session`. These are, respectively, the new page content as entered by the
+user, the page name, a `CGI` object, and the user's `CGI::Session`. 
+
+It can modify the content as desired, and should return the content.
+

 ### formbuilder
 
        hook(type => "formbuilder_setup", id => "foo", call => \&formbuilder_setup);
 ### formbuilder
 
        hook(type => "formbuilder_setup", id => "foo", call => \&formbuilder_setup);


@@ -199,20 +275,18 @@ a basic registration of the user will be automatically performed.
 
 These hooks allow tapping into the parts of ikiwiki that use [[cpan
 CGI::FormBuilder]] to generate web forms. These hooks are passed named
 
 These hooks allow tapping into the parts of ikiwiki that use [[cpan
 CGI::FormBuilder]] to generate web forms. These hooks are passed named

-parameters: `cgi`, `session`, and `form`. These are, respectively, the
-`CGI` object, the user's `CGI::Session`, and a `CGI::FormBuilder`.
+parameters: `cgi`, `session`, `form`, and `buttons`. These are, respectively,
+the `CGI` object, the user's `CGI::Session`, a `CGI::FormBuilder`, and a
+reference to an array of names of buttons to go on the form.

 
 

-Each time a form is set up, the formbuilder_setup hook is called.
-Typically the formbuilder_setup hook will check the form's title, and if
+Each time a form is set up, the `formbuilder_setup` hook is called.
+Typically the `formbuilder_setup` hook will check the form's title, and if

 it's a form that it needs to modify, will call various methods to
 add/remove/change fields, tweak the validation code for the fields, etc. It
 will not validate or display the form.
 
 it's a form that it needs to modify, will call various methods to
 add/remove/change fields, tweak the validation code for the fields, etc. It
 will not validate or display the form.
 

-Form validation and display can be overridden by the formbuilder hook.
-By default, ikiwiki will do a basic validation and display of the form,
-but if this hook is registered, it will stop that and let the hook take
-over. This hook is passed an additional named parameter: `buttons` is an
-array of the submit buttons for the form.
+Just before a form is displayed to the user, the `formbuilder` hook is
+called. It can be used to validate the form, but should not display it.

 
 ### savestate
 
 
 ### savestate
 


@@ -233,7 +307,7 @@ namespace. These variables and functions are the ones most plugins need,
 and a special effort will be made to avoid changing them in incompatible
 ways, and to document any changes that have to be made in the future.
 
 and a special effort will be made to avoid changing them in incompatible
 ways, and to document any changes that have to be made in the future.
 

-Note that IkiWiki also provides other variables functions that are not
+Note that IkiWiki also provides other variables and functions that are not

 exported by default. No guarantee is made about these in the future, so if
 it's not exported, the wise choice is to not use it.
 
 exported by default. No guarantee is made about these in the future, so if
 it's not exported, the wise choice is to not use it.
 


@@ -243,6 +317,23 @@ A plugin can access the wiki's configuration via the `%config`
 hash. The best way to understand the contents of the hash is to look at
 [[ikiwiki.setup]], which sets the hash content to configure the wiki.
 
 hash. The best way to understand the contents of the hash is to look at
 [[ikiwiki.setup]], which sets the hash content to configure the wiki.
 

+### %pagestate
+
+The `%pagestate` hash can be used by plugins to save state that they will need
+next time ikiwiki is run. The hash holds per-page state, so to set a value,
+use `%pagestate{$page}{$id}{$key}=$value`, and to retrieve the value,
+use `%pagestate{$page}{$id}{$key}`.
+
+`$key` can be any string you like, but `$id` must be the same as the "id"
+parameter passed to `hook()` when registering the plugin. This is so
+ikiwiki can know when to delete pagestate for plugins that are no longer
+used.
+
+When pages are deleted, ikiwiki automatically deletes their pagestate too.
+
+Note that page state does not persist across wiki rebuilds, only across
+wiki updates.
+

 ### Other variables
 
 If your plugin needs to access data about other pages in the wiki. It can
 ### Other variables
 
 If your plugin needs to access data about other pages in the wiki. It can


@@ -250,9 +341,9 @@ use the following hashes, using a page name as the key:
 
 * `%links` lists the names of each page that a page links to, in an array
   reference.
 
 * `%links` lists the names of each page that a page links to, in an array
   reference.

-* `%renderedfiles` lists names of the files rendered by a page, in an array
-  reference.
-* `%pagesources` contains the name of the source file for a page.
+* `%destsources` contains the name of the source file used to create each
+  destination file.
+* `%pagesources` contains the name of the source file for each page.

 
 Also, the %IkiWiki::version variable contains the version number for the
 ikiwiki program.
 
 Also, the %IkiWiki::version variable contains the version number for the
 ikiwiki program.


@@ -264,7 +355,7 @@ ikiwiki program.
 Hook into ikiwiki's processing. See the discussion of hooks above.
 
 Note that in addition to the named parameters described above, a parameter
 Hook into ikiwiki's processing. See the discussion of hooks above.
 
 Note that in addition to the named parameters described above, a parameter

-named no_override is supported, If it's set to a true value, then this hook
+named `no_override` is supported, If it's set to a true value, then this hook

 will not override any existing hook with the same id. This is useful if
 the id can be controled by the user.
 
 will not override any existing hook with the same id. This is useful if
 the id can be controled by the user.
 


@@ -273,21 +364,24 @@ the id can be controled by the user.
 Logs a debugging message. These are supressed unless verbose mode is turned
 on.
 
 Logs a debugging message. These are supressed unless verbose mode is turned
 on.
 

-#### `error($)`
+#### `error($;$)`

 
 

-Aborts with an error message.
+Aborts with an error message. If the second parameter is passed, it is a
+function that is called after the error message is printed, to do any final
+cleanup.

 
 Note that while any plugin can use this for a fatal error, plugins should
 try to avoid dying on bad input, as that will halt the entire wiki build
 
 Note that while any plugin can use this for a fatal error, plugins should
 try to avoid dying on bad input, as that will halt the entire wiki build

-and make the wiki unusable. So for example, if a [[PreProcessorDirective]]
-is passed bad parameters, it's better to return an error message, which can
-appear on the wiki page, rather than calling error().
+and make the wiki unusable. So for example, if a
+[[ikiwiki/PreProcessorDirective]] is passed bad parameters, it's better to
+return an error message, which can appear on the wiki page, rather than
+calling error().

 
 #### `template($;@)`
 
 Creates and returns a [[cpan HTML::Template]] object. The first parameter
 is the name of the file in the template directory. The optional remaining
 
 #### `template($;@)`
 
 Creates and returns a [[cpan HTML::Template]] object. The first parameter
 is the name of the file in the template directory. The optional remaining

-parameters are passed to HTML::Template->new.
+parameters are passed to `HTML::Template->new`.

 
 #### `htmlpage($)`
 
 
 #### `htmlpage($)`
 


@@ -296,12 +390,17 @@ page created from it. (Ie, it appends ".html".)
 
 #### `add_depends($$)`
 
 
 #### `add_depends($$)`
 

-Makes the specified page depend on the specified [[PageSpec]].
+Makes the specified page depend on the specified [[ikiwiki/PageSpec]].

 
 

-#### `pagespec_match($$)`
+#### `pagespec_match($$;@)`

 
 

-Passed a page name, and a [[PageSpec]], returns true if the [[PageSpec]]
-matches the page.
+Passed a page name, and [[ikiwiki/PageSpec]], returns true if the
+[[ikiwiki/PageSpec]] matches the page.
+
+Additional named parameters can be passed, to further limit the match.
+The most often used is "location", which specifies the location the
+PageSpec should match against. If not passed, relative PageSpecs will match
+relative to the top of the wiki.

 
 #### `bestlink($$)`
 
 
 #### `bestlink($$)`
 


@@ -309,9 +408,9 @@ Given a page and the text of a link on the page, determine which
 existing page that link best points to. Prefers pages under a
 subdirectory with the same name as the source page, failing that
 goes down the directory tree to the base looking for matching
 existing page that link best points to. Prefers pages under a
 subdirectory with the same name as the source page, failing that
 goes down the directory tree to the base looking for matching

-pages, as described in [[SubPage/LinkingRules]].
+pages, as described in [[ikiwiki/SubPage/LinkingRules]].

 
 

-#### `htmllink($$$;$$$)`
+#### `htmllink($$$;@)`

 
 Many plugins need to generate html links and add them to a page. This is
 done by using the `htmllink` function. The usual way to call
 
 Many plugins need to generate html links and add them to a page. This is
 done by using the `htmllink` function. The usual way to call


@@ -328,11 +427,15 @@ Here `$destpage` is the inlining page. A `destpage` parameter is passed to
 some of the hook functions above; the ones that are not passed it are not used
 during inlining and don't need to worry about this issue.
 
 some of the hook functions above; the ones that are not passed it are not used
 during inlining and don't need to worry about this issue.
 

-The remaining three optional parameters to `htmllink` are:
+After the three required parameters, named parameters can be used to
+control some options. These are:

 
 

-1. noimageinline - set to true to avoid turning links into inline html images
-1. forcesubpage  - set to force a link to a subpage
-1. linktext - set to force the link text to something
+* noimageinline - set to true to avoid turning links into inline html images
+* forcesubpage  - set to force a link to a subpage
+* linktext - set to force the link text to something
+* anchor - set to make the link include an anchor
+* rel - set to add a rel attribute to the link
+* class - set to add a css class to the link

 
 #### `readfile($;$)`
 
 
 #### `readfile($;$)`
 


@@ -343,25 +446,35 @@ in binary mode.
 
 A failure to read the file will result in it dying with an error.
 
 
 A failure to read the file will result in it dying with an error.
 

-#### `writefile($$$;$)`
+#### `writefile($$$;$$)`

 
 Given a filename, a directory to put it in, and the file's content,
 writes a file. 
 
 
 Given a filename, a directory to put it in, and the file's content,
 writes a file. 
 

-The optional second parameter, if set to a true value, makes the file be
+The optional fourth parameter, if set to a true value, makes the file be

 written in binary mode.
 
 written in binary mode.
 

+The optional fifth parameter can be used to pass a function reference that
+will be called to handle writing to the file. The function will be called
+and passed a file descriptor it should write to, and an error recovery
+function it should call if the writing fails. (You will not normally need to
+use this interface.)
+

 A failure to write the file will result in it dying with an error.
 
 If the destination directory doesn't exist, it will first be created.
 
 A failure to write the file will result in it dying with an error.
 
 If the destination directory doesn't exist, it will first be created.
 

-### `will_render($$)`
+#### `will_render($$)`

 
 Given a page name and a destination file name (not including the base
 destination directory), register that the page will result in that file
 being rendered. It's important to call this before writing to any file in
 the destination directory.
 
 
 Given a page name and a destination file name (not including the base
 destination directory), register that the page will result in that file
 being rendered. It's important to call this before writing to any file in
 the destination directory.
 

+Ikiwiki uses this information to automatically clean up rendered files when
+the page that rendered them goes away or is changes to no longer render
+them. will_render also does a few important security checks.
+

 #### `pagetype($)`
 
 Given the name of a source file, returns the type of page it is, if it's
 #### `pagetype($)`
 
 Given the name of a source file, returns the type of page it is, if it's


@@ -375,21 +488,58 @@ that corresponds to that file.
 #### `srcfile($)`
 
 Given the name of a source file in the wiki, searches for the file in
 #### `srcfile($)`
 
 Given the name of a source file in the wiki, searches for the file in

-the source directory and the underlay directory, and returns the full
-path to the first file found.
+the source directory and the underlay directories (most recently added
+underlays first), and returns the full path to the first file found.
+
+#### `add_underlay($)`

 
 

-#### `displaytime($)`
+Adds a directory to the set of underlay directories that ikiwiki will
+search for files.
+
+If the directory name is not absolute, ikiwiki will assume it is in
+the parent directory of the configured underlaydir.
+
+#### `displaytime($;$)`

 
 Given a time, formats it for display.
 
 
 Given a time, formats it for display.
 

+The optional second parameter is a strftime format to use to format the
+time.
+
+#### `gettext`
+
+This is the standard gettext function, although slightly optimised.
+
+#### `urlto($$)`
+
+Construct a relative url to the first parameter from the page named by the
+second. The first parameter can be either a page name, or some other
+destination file, as registered by `will_render`.
+
+#### `targetpage($$)`
+
+Passed a page and an extension, returns the filename that page will be
+rendered to.
+

 ## RCS plugins
 
 ## RCS plugins
 

-ikiwiki's support for revision control systems also uses pluggable perl
-modules. These are in the `IkiWiki::RCS` namespace, for example
+ikiwiki's support for [[revision_control_systems|rcs]] also uses pluggable
+perl modules. These are in the `IkiWiki::RCS` namespace, for example

 `IkiWiki::RCS::svn`. 
 
 `IkiWiki::RCS::svn`. 
 

-Each RCS plugin must support all the IkiWiki::rcs\_* functions.
+Each RCS plugin must support all the `IkiWiki::rcs_*` functions.

 See IkiWiki::RCS::Stub for the full list of functions. It's ok if
 See IkiWiki::RCS::Stub for the full list of functions. It's ok if

-rcs\_getctime does nothing except for throwing an error.
+`rcs_getctime` does nothing except for throwing an error.
+
+See [[RCS_details|rcs/details]] for some more info.
+
+## PageSpec plugins

 
 

-See [[about_RCS_backends]] for some more info.
+It's also possible to write plugins that add new functions to
+[[PageSpecs|ikiwiki/PageSpec]]. Such a plugin should add a function to the
+IkiWiki::PageSpec package, that is named `match_foo`, where "foo()" is
+how it will be accessed in a [[ikiwiki/PageSpec]]. The function will be passed
+two parameters: The name of the page being matched, and the thing to match
+against. It may also be passed additional, named parameters. It should return
+a IkiWiki::SuccessReason object if the match succeeds, or an
+IkiWiki::FailReason object if the match fails.