more tweaks to urlto documentation

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

diff --git a/doc/plugins/write.mdwn b/doc/plugins/write.mdwn
index f2b96b6d96eb214f24fda961fec826639e2ef17a..3cab7588ec99c7a83fa6755fe3870fc93d6fb4d3 100644 (file)
--- a/doc/plugins/write.mdwn
+++ b/doc/plugins/write.mdwn
@@ -1,4 +1,4 @@
-Ikiwiki's plugin interface allows all kinds of useful [[plugins]] to be
+lkiwiki'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]].
 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]].


@@ -9,16 +9,78 @@ Ikiwiki is a compiler
 One thing to keep in mind when writing a plugin is that ikiwiki is a wiki
 *compiler*. So plugins influence pages when they are built, not when they
 are loaded. A plugin that inserts the current time into a page, for
 One thing to keep in mind when writing a plugin is that ikiwiki is a wiki
 *compiler*. So plugins influence pages when they are built, not when they
 are loaded. A plugin that inserts the current time into a page, for

-example, will insert the build time. Also, as a compiler, ikiwiki avoids
-rebuilding pages unless they have changed, so a plugin that prints some
-random or changing thing on a page will generate a static page that won't
-change until ikiwiki rebuilds the page for some other reason, like the page
-being edited. The [[tutorial]] has some other examples of ways that ikiwiki
-being a compiler may trip up the unwary.
+example, will insert the build time.
+
+Also, as a compiler, ikiwiki avoids rebuilding pages unless they have
+changed, so a plugin that prints some random or changing thing on a page
+will generate a static page that won't change until ikiwiki rebuilds the
+page for some other reason, like the page being edited.
+
+The [[tutorial]] has some other examples of ways that ikiwiki being a
+compiler may trip up the unwary.

 """]]
 
 [[!toc levels=2]]
 
 """]]
 
 [[!toc levels=2]]
 

+## Highlevel view of ikiwiki
+
+Ikiwiki mostly has two modes of operation. It can either be running
+as a compiler, building or updating a wiki; or as a cgi program, providing
+user interface for editing pages, etc. Almost everything ikiwiki does
+is accomplished by calling various hooks provided by plugins.
+
+### compiler
+
+As a compiler, ikiwiki starts by calling the `refresh` hook. Then it checks
+the wiki's source to find new or changed pages. The `needsbuild` hook is
+then called to allow manipulation of the list of pages that need to be
+built. 
+
+Now that it knows what pages it needs to build, ikiwiki runs two
+compile passes. First, it runs `scan` hooks, which collect metadata about
+the pages. Then it runs a page rendering pipeline, by calling in turn these
+hooks: `filter`, `preprocess`, `linkify`, `htmlize`, `indexhtml`,
+`pagetemplate`, `sanitize`, `format`.
+
+After all necessary pages are built, it calls the `change` hook. Finally,
+if a page is was deleted, the `delete` hook is called, and the files that
+page had previously produced are removed.
+
+### cgi
+
+The flow between hooks when ikiwiki is run as a cgi is best illustrated by
+an example.
+
+Alice browses to a page and clicks Edit.
+
+* Ikiwiki is run as a cgi. It assigns Alice a session cookie, and,
+  by calling the `auth` hooks, sees that she is not yet logged in.
+* The `sessioncgi` hooks are then called, and one of them,
+  from the [[editpage]] plugin, notices that the cgi has been told "do=edit".
+* The [[editpage]] plugin calls the `canedit` hook to check if this
+  page edit is allowed. The [[signinedit]] plugin has a hook that says not:
+  Alice is not signed in.
+* The [[signinedit]] plugin then launches the signin process. A signin
+  page is built by calling the `formbuilder_setup` hook.
+
+Alice signs in with her openid.
+
+* The [[openid]] plugin's `formbuilder` hook sees that an openid was
+  entered in the signin form, and redirects to Alice's openid provider.
+* Alice's openid provider calls back to ikiwiki. The [[openid]] plugin
+  has an `auth` hook that finishes the openid signin process.
+* Signin complete, ikiwiki returns to what Alice was doing before; editing
+  a page.
+* Now all the `canedit` hooks are happy. The [[editpage]] plugin calls
+  `formbuilder_setup` to display the page editing form.
+
+Alice saves her change to the page.
+
+* The [[editpage]] plugin's `formbuilder` hook sees that the Save button
+  was pressed, and calls the `checkcontent` and `editcontent` hooks.
+  Then it saves the page to disk, and branches into the compiler part
+  of ikiwiki to refresh the wiki.
+

 ## Types of plugins
 
 Most ikiwiki [[plugins]] are written in perl, like ikiwiki. This gives the
 ## Types of plugins
 
 Most ikiwiki [[plugins]] are written in perl, like ikiwiki. This gives the


@@ -115,10 +177,15 @@ function is passed no values.
 
        hook(type => "needsbuild", id => "foo", call => \&needsbuild);
 
 
        hook(type => "needsbuild", id => "foo", call => \&needsbuild);
 

-This allows a plugin to 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 files that will be rebuilt, and can modify the array, either
-adding or removing files from it.
+This allows a plugin to observe or even manipulate the list of files that
+need to be built when the wiki is refreshed. 
+
+As its first parameter, the function is passed a reference to an array of
+files that will be built. It should return an array reference that is a
+modified version of its input. It can add or remove files from it.
+
+The second parameter passed to the function is a reference to an array of
+files that have been deleted.

 
 ### scan
 
 
 ### scan
 


@@ -136,8 +203,8 @@ value is ignored.
 
        hook(type => "filter", id => "foo", call => \&filter);
 
 
        hook(type => "filter", id => "foo", call => \&filter);
 

-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",
+Runs on the full 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
 "destpage", and "content". It should return the filtered content.
 
 ### preprocess


@@ -220,22 +287,22 @@ like `Makefile` that have no extension.
 If `hook` is passed an optional "longname" parameter, this value is used
 when prompting a user to choose a page type on the edit page form.
 
 If `hook` is passed an optional "longname" parameter, this value is used
 when prompting a user to choose a page type on the edit page form.
 

-### postscan
+### indexhtml

 
 

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

 
 This hook is called once the page has been converted to html (but before
 the generated html is put in a template). The most common use is to
 update search indexes. Added in ikiwiki 2.54.
 
 
 This hook is called once the page has been converted to html (but before
 the generated html is put in a template). The most common use is to
 update search indexes. Added in ikiwiki 2.54.
 

-The function is passed named parameters "page" and "content". Its return
-value is ignored.
+The function is passed named parameters "page", "destpage", and "content".
+Its return value is ignored.

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

-[[Templates|wikitemplates]] are filled out for many different things in
+[[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 the variables available on those
 templates. The function is passed named parameters. The "page" and
 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


@@ -251,11 +318,20 @@ a new custom parameter to the template.
 
        hook(type => "templatefile", id => "foo", call => \&templatefile);
 
 
        hook(type => "templatefile", id => "foo", call => \&templatefile);
 

-This hook allows plugins to change the [[template|wikitemplates]] that is
+This hook allows plugins to change the [[template|templates]] that is

 used for a page in the wiki. The hook is passed a "page" parameter, and
 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.
+should return the name of the template file to use (relative to the
+template directory), or undef if it doesn't want to change the default
+("page.tmpl").
+
+### pageactions
+
+       hook(type => "pageactions", id => "foo", call => \&pageactions);
+
+This hook allows plugins to add arbitrary actions to the action bar on a
+page (next to Edit, RecentChanges, etc). The hook is passed a "page"
+parameter, and can return a list of html fragments to add to the action
+bar.

 
 ### sanitize
 
 
 ### sanitize
 


@@ -474,7 +550,7 @@ The data returned is a list of `%config` options, followed by a hash
 describing the option. There can also be an item named "plugin", which
 describes the plugin as a whole. For example:
 
 describing the option. There can also be an item named "plugin", which
 describes the plugin as a whole. For example:
 

-                return
+               return

                        plugin => {
                                description => "description of this plugin",
                                safe => 1,
                        plugin => {
                                description => "description of this plugin",
                                safe => 1,


@@ -524,19 +600,29 @@ describes the plugin as a whole. For example:
        hook(type => "genwrapper", id => "foo", call => \&genwrapper);
 
 This hook is used to inject C code (which it returns) into the `main`
        hook(type => "genwrapper", id => "foo", call => \&genwrapper);
 
 This hook is used to inject C code (which it returns) into the `main`

-function of the ikiwiki wrapper when it is being generated.
+function of the ikiwiki wrapper when it is being generated. 
+
+The code runs before anything else -- in particular it runs before
+the suid wrapper has sanitized its environment.
+
+### disable
+
+       hook(type => "disable", id => "foo", call => \&disable);
+
+This hook is only run when a previously enabled plugin gets disabled
+during ikiwiki setup. Plugins can use this to perform cleanups.

 
 ## Exported variables
 
 Several variables are exported to your plugin when you `use IkiWiki;`
 
 
 ## Exported variables
 
 Several variables are exported to your plugin when you `use IkiWiki;`
 

-### %config
+### `%config`

 
 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
 your ikiwiki setup file, which sets the hash content to configure the wiki.
 
 
 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
 your ikiwiki setup file, which sets the hash content to configure the wiki.
 

-### %pagestate
+### `%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,
 
 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,


@@ -554,7 +640,7 @@ When pages are deleted, ikiwiki automatically deletes their pagestate too.
 Note that page state does not persist across wiki rebuilds, only across
 wiki updates.
 
 Note that page state does not persist across wiki rebuilds, only across
 wiki updates.
 

-### %wikistate
+### `%wikistate`

 
 The `%wikistate` hash can be used by a plugin to store persistant state
 that is not bound to any one page. To set a value, use
 
 The `%wikistate` hash can be used by a plugin to store persistant state
 that is not bound to any one page. To set a value, use


@@ -563,7 +649,7 @@ serialize, `$key` is any string you like, and `$id` must be the same as the
 "id" parameter passed to `hook()` when registering the plugin, so that the
 state can be dropped if the plugin is no longer used.
 
 "id" parameter passed to `hook()` when registering the plugin, so that the
 state can be dropped if the plugin is no longer used.
 

-### %links
+### `%links`

 
 The `%links` hash can be used to look up the names of each page that
 a page links to. The name of the page is the key; the value is an array
 
 The `%links` hash can be used to look up the names of each page that
 a page links to. The name of the page is the key; the value is an array


@@ -571,17 +657,23 @@ reference. Do not modify this hash directly; call `add_link()`.
 
        $links{"foo"} = ["bar", "baz"];
 
 
        $links{"foo"} = ["bar", "baz"];
 

-### %destsources
+### `%typedlinks`

 
 

-The `%destsources` hash records the name of the source file used to
-create each destination file. The key is the output filename (ie,
-"foo/index.html"), and the value is the source filename that it was built
-from (eg, "foo.mdwn"). Note that a single source file may create multiple
-destination files. Do not modify this hash directly; call `will_render()`.
-       
-       $destsources{"foo/index.html"} = "foo.mdwn";
+The `%typedlinks` hash records links of specific types. Do not modify this
+hash directly; call `add_link()`. The keys are page names, and the values
+are hash references. In each page's hash reference, the keys are link types
+defined by plugins, and the values are hash references with link targets
+as keys, and 1 as a dummy value, something like this:

 
 

-### %pagesources
+       $typedlinks{"foo"} = {
+               tag => { short_word => 1, metasyntactic_variable => 1 },
+               next_page => { bar => 1 },
+       };
+
+Ordinary [[WikiLinks|ikiwiki/WikiLink]] appear in `%links`, but not in
+`%typedlinks`.
+
+### `%pagesources`

 
 The `%pagesources` has can be used to look up the source filename
 of a page. So the key is the page name, and the value is the source
 
 The `%pagesources` has can be used to look up the source filename
 of a page. So the key is the page name, and the value is the source


@@ -589,8 +681,20 @@ filename. Do not modify this hash.
 
        $pagesources{"foo"} = "foo.mdwn";
 
 
        $pagesources{"foo"} = "foo.mdwn";
 

+### `%destsources`
+
+The `%destsources` hash records the name of the source file used to
+create each destination file. The key is the output filename (ie,
+"foo/index.html"), and the value is the source filename that it was built
+from (eg, "foo.mdwn"). Note that a single source file may create multiple
+destination files. Do not modify this hash directly; call `will_render()`.
+       
+       $destsources{"foo/index.html"} = "foo.mdwn";
+

 ## Library functions
 
 ## Library functions
 

+Several functions are exported to your plugin when you `use IkiWiki;`
+

 ### `hook(@)`
 
 Hook into ikiwiki's processing. See the discussion of hooks above.
 ### `hook(@)`
 
 Hook into ikiwiki's processing. See the discussion of hooks above.


@@ -621,10 +725,31 @@ the entire wiki build and make the wiki unusable.
 
 ### `template($;@)`
 
 
 ### `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
+Creates and returns a [[!cpan HTML::Template]] object. (In a list context,
+returns the parameters needed to construct the obhect.) 
+
+The first parameter is the name of the template file. The optional remaining

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

+Normally, the template file is first looked for in the templates/ subdirectory
+of the srcdir. Failing that, it is looked for in the templatedir.
+
+Wiki pages can be used as templates. This should be done only for templates
+which it is safe to let wiki users edit. Enable it by passing a filename
+with no ".tmpl" extension. Template pages are normally looked for in
+the templates/ directory. If the page name starts with "/", a page
+elsewhere in the wiki can be used.
+
+If the template is not found, or contains a syntax error, an error is thrown.
+
+### `template_depends($$;@)`
+
+Use this instead of `template()` if the content of a template is being
+included into a page. This causes the page to depend on the template,
+so it will be updated if the template is modified.
+
+Like `template()`, except the second parameter is the page.
+

 ### `htmlpage($)`
 
 Passed a page name, returns the base name that will be used for a the html
 ### `htmlpage($)`
 
 Passed a page name, returns the base name that will be used for a the html


@@ -654,7 +779,10 @@ Additional named parameters can be specified:
 * `filter` is a reference to a function, that is called and passed a page,
   and returns true if the page should be filtered out of the list.
 * `sort` specifies a sort order for the list. See
 * `filter` is a reference to a function, that is called and passed a page,
   and returns true if the page should be filtered out of the list.
 * `sort` specifies a sort order for the list. See

-  [[ikiwiki/PageSpec/sorting]] for the avilable sort methods.
+  [[ikiwiki/PageSpec/sorting]] for the avilable sort methods. Note that
+  if a sort method is specified that depends on the
+  page content (such as 'meta(foo)'), the deptype needs to be set to
+  a content dependency.

 * `reverse` if true, sorts in reverse.
 * `num` if nonzero, specifies the maximum number of matching pages that
   will be returned.
 * `reverse` if true, sorts in reverse.
 * `num` if nonzero, specifies the maximum number of matching pages that
   will be returned.


@@ -672,7 +800,7 @@ By default, dependencies are full content dependencies, meaning that the
 page will be updated whenever anything matching the PageSpec is modified.
 This can be overridden by passing a `deptype` value as the third parameter.
 
 page will be updated whenever anything matching the PageSpec is modified.
 This can be overridden by passing a `deptype` value as the third parameter.
 

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

 
 Passed a page name, and [[ikiwiki/PageSpec]], returns a true value if the
 [[ikiwiki/PageSpec]] matches the page.
 
 Passed a page name, and [[ikiwiki/PageSpec]], returns a true value if the
 [[ikiwiki/PageSpec]] matches the page.


@@ -702,7 +830,7 @@ dependency type from one or more of these keywords:
 
 If multiple types are specified, they are combined.
 
 
 If multiple types are specified, they are combined.
 

-#### `bestlink($$)`
+### `bestlink($$)`

 
 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
 
 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


@@ -710,7 +838,7 @@ 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 [[ikiwiki/SubPage/LinkingRules]].
 
 goes down the directory tree to the base looking for matching
 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


@@ -836,25 +964,37 @@ search for files.
 If the directory name is not absolute, ikiwiki will assume it is in
 the parent directory of the configured underlaydir.
 
 If the directory name is not absolute, ikiwiki will assume it is in
 the parent directory of the configured underlaydir.
 

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

 
 Given a time, formats it for display.
 
 The optional second parameter is a strftime format to use to format the
 time.
 
 
 Given a time, formats it for display.
 
 The optional second parameter is a strftime format to use to format the
 time.
 

+If the third parameter is true, this is the publication time of a page.
+(Ie, set the html5 pubdate attribute.)
+

 ### `gettext`
 
 This is the standard gettext function, although slightly optimised.
 
 ### `gettext`
 
 This is the standard gettext function, although slightly optimised.
 

-### `urlto($$;$)`
+### `ngettext`
+
+This is the standard ngettext function, although slightly optimised.
+
+### `urlto($;$$)`

 
 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`.
 
 
 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`.
 

-If the third parameter is passed and is true, an absolute url will be
-constructed instead of the default relative url.
+If the second parameter is not specified (or `undef`), the URL will be
+valid from any page on the wiki, or from the CGI; if possible it'll
+be a path starting with `/`, but an absolute URL will be used if
+the wiki and the CGI are on different domains.
+
+If the third parameter is passed and is true, the url will be a fully
+absolute url. This is useful when generating an url to publish elsewhere.

 
 ### `newpagefile($$)`
 
 
 ### `newpagefile($$)`
 


@@ -871,11 +1011,31 @@ Optionally, a third parameter can be passed, to specify the preferred
 filename of the page. For example, `targetpage("foo", "rss", "feed")`
 will yield something like `foo/feed.rss`.
 
 filename of the page. For example, `targetpage("foo", "rss", "feed")`
 will yield something like `foo/feed.rss`.
 

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

 
 This adds a link to `%links`, ensuring that duplicate links are not
 added. Pass it the page that contains the link, and the link text.
 
 
 This adds a link to `%links`, ensuring that duplicate links are not
 added. Pass it the page that contains the link, and the link text.
 

+An optional third parameter sets the link type. If not specified,
+it is an ordinary [[ikiwiki/WikiLink]].
+
+### `add_autofile($$$)`
+
+Sometimes you may want to add a file to the `srcdir` as a result of content
+of other pages. For example, [[plugins/tag]] pages can be automatically
+created as needed. This function can be used to do that. 
+
+The three parameters are the filename to create (relative to the `srcdir`),
+the name of the plugin, and a callback function. The callback will be
+called if it is appropriate to automatically add the file, and should then
+take care of creating it, and doing anything else it needs to (such as
+checking it into revision control). Note that the callback may not always
+be called. For example, if an automatically added file is deleted by the
+user, ikiwiki will avoid re-adding it again.
+
+This function needs to be called during the scan hook, or earlier in the
+build process, in order to add the file early enough for it to be built.
+

 ## Miscellaneous
 
 ### Internal use pages
 ## Miscellaneous
 
 ### Internal use pages


@@ -913,16 +1073,20 @@ token, that will be passed into `rcs_commit` when committing. For example,
 it might return the current revision ID of the file, and use that
 information later when merging changes.
 
 it might return the current revision ID of the file, and use that
 information later when merging changes.
 

-#### `rcs_commit($$$;$$)`
+#### `rcs_commit(@)`
+
+Passed named parameters: `file`, `message`, `token` (from `rcs_prepedit`),
+and `session` (optional).

 
 

-Passed a file, message, token (from `rcs_prepedit`), user, and ip address.

 Should try to commit the file. Returns `undef` on *success* and a version
 of the page with the rcs's conflict markers on failure.
 
 Should try to commit the file. Returns `undef` on *success* and a version
 of the page with the rcs's conflict markers on failure.
 

-#### `rcs_commit_staged($$$)`
+#### `rcs_commit_staged(@)`
+
+Passed named parameters: `message`, and `session` (optional).

 
 

-Passed a message, user, and ip address. Should commit all staged changes.
-Returns undef on success, and an error message on failure.
+Should commit all staged changes. Returns undef on success, and an
+error message on failure.

 
 Changes can be staged by calls to `rcs_add`, `rcs_remove`, and
 `rcs_rename`.
 
 Changes can be staged by calls to `rcs_add`, `rcs_remove`, and
 `rcs_rename`.


@@ -965,7 +1129,9 @@ The data structure returned for each change is:
 
        {
                rev => # the RCSs id for this commit
 
        {
                rev => # the RCSs id for this commit

-               user => # name of user who made the change,
+               user => # user who made the change (may be an openid),
+               nickname => # short name for user (optional; not an openid),
+               

                committype => # either "web" or the name of the rcs,
                when => # time when the change was made,
                message => [
                committype => # either "web" or the name of the rcs,
                when => # time when the change was made,
                message => [


@@ -993,8 +1159,17 @@ context, and the whole diff in scalar context.
 This is used to get the page creation time for a file from the RCS, by looking
 it up in the history.
 
 This is used to get the page creation time for a file from the RCS, by looking
 it up in the history.
 

+If the RCS cannot determine a ctime for the file, return 0.
+
+#### `rcs_getmtime($)`
+
+This is used to get the page modification time for a file from the RCS, by
+looking it up in the history.
+

 It's ok if this is not implemented, and throws an error.
 
 It's ok if this is not implemented, and throws an error.
 

+If the RCS cannot determine a mtime for the file, return 0.
+

 #### `rcs_receive()`
 
 This is called when ikiwiki is running as a pre-receive hook (or
 #### `rcs_receive()`
 
 This is called when ikiwiki is running as a pre-receive hook (or


@@ -1004,9 +1179,9 @@ sense to implement for all RCSs.
 
 It should examine the incoming changes, and do any sanity 
 checks that are appropriate for the RCS to limit changes to safe file adds,
 
 It should examine the incoming changes, and do any sanity 
 checks that are appropriate for the RCS to limit changes to safe file adds,

-removes, and changes. If something bad is found, it should exit
-nonzero, to abort the push. Otherwise, it should return a list of
-files that were changed, in the form:
+removes, and changes. If something bad is found, it should die, to abort
+the push. Otherwise, it should return a list of files that were changed,
+in the form:

 
        {
                file => # name of file that was changed
 
        {
                file => # name of file that was changed


@@ -1019,6 +1194,28 @@ files that were changed, in the form:
 The list will then be checked to make sure that each change is one that
 is allowed to be made via the web interface.
 
 The list will then be checked to make sure that each change is one that
 is allowed to be made via the web interface.
 

+#### `rcs_preprevert($)`
+
+This is called by the revert web interface. It is passed a RCS-specific
+change ID, and should determine what the effects would be of reverting
+that change, and return the same data structure as `rcs_receive`.
+
+Like `rcs_receive`, it should do whatever sanity checks are appropriate
+for the RCS to limit changes to safe changes, and die if a change would
+be unsafe to revert.
+
+#### `rcs_revert($)`
+
+This is called by the revert web interface. It is passed a named
+parameter rev that is the RCS-specific change ID to revert.
+
+It should try to revert the specified rev, and leave the reversion staged
+so `rcs_commit_staged` will complete it. It should return undef on _success_
+and an error message on failure.
+
+This hook and `rcs_preprevert` are optional, if not implemented, no revert
+web interface will be available.
+

 ### PageSpec plugins
 
 It's also possible to write plugins that add new functions to
 ### PageSpec plugins
 
 It's also possible to write plugins that add new functions to


@@ -1042,6 +1239,24 @@ For example, "backlink(foo)" is influenced by the contents of page foo;
 they match; "created_before(foo)" is influenced by the metadata of foo;
 while "glob(*)" is not influenced by the contents of any page.
 
 they match; "created_before(foo)" is influenced by the metadata of foo;
 while "glob(*)" is not influenced by the contents of any page.
 

+### Sorting plugins
+
+Similarly, it's possible to write plugins that add new functions as
+[[ikiwiki/pagespec/sorting]] methods. To achieve this, add a function to
+the IkiWiki::SortSpec package named `cmp_foo`, which will be used when sorting
+by `foo` or `foo(...)` is requested.
+
+The names of pages to be compared are in the global variables `$a` and `$b`
+in the IkiWiki::SortSpec package. The function should return the same thing
+as Perl's `cmp` and `<=>` operators: negative if `$a` is less than `$b`,
+positive if `$a` is greater, or zero if they are considered equal. It may
+also raise an error using `error`, for instance if it needs a parameter but
+one isn't provided.
+
+The function will also be passed one or more parameters. The first is
+`undef` if invoked as `foo`, or the parameter `"bar"` if invoked as `foo(bar)`;
+it may also be passed additional, named parameters.
+

 ### Setup plugins
 
 The ikiwiki setup file is loaded using a pluggable mechanism. If you look
 ### Setup plugins
 
 The ikiwiki setup file is loaded using a pluggable mechanism. If you look