First cut at the revert plugin.

IkiWiki.pm

@@ -1941,6 +1941,18 @@ sub rcs_receive () {
 	$hooks{rcs}{rcs_receive}{call}->();
 }
 
+sub rcs_preprevert ($) {
+	$hooks{rcs}{rcs_preprevert}{call}->(@_);
+}
+
+sub rcs_revert (@) {
+	$hooks{rcs}{rcs_revert}{call}->(@_);
+}
+
+sub rcs_showpatch ($) {
+	$hooks{rcs}{rcs_showpatch}{call}->(@_);
+}
+
 sub add_depends ($$;$) {
 	my $page=shift;
 	my $pagespec=shift;

IkiWiki/Plugin/git.pm

@@ -27,6 +27,10 @@ sub import {
 	hook(type => "rcs", id => "rcs_getctime", call => \&rcs_getctime);
 	hook(type => "rcs", id => "rcs_getmtime", call => \&rcs_getmtime);
 	hook(type => "rcs", id => "rcs_receive", call => \&rcs_receive);
+ 	hook(type => "rcs", id => "rcs_preprevert", call => \&rcs_preprevert);
+ 	hook(type => "rcs", id => "rcs_revert", call => \&rcs_revert);
+ 	hook(type => "rcs", id => "rcs_showpatch", call => \&rcs_showpatch);
+	hook(type => "rcs", id => "rcs_revert", call => \&rcs_revert);
 }
 
 sub checkconfig () {
@@ -811,4 +815,35 @@ sub rcs_receive () {
 	return reverse @rets;
 }
 
+sub rcs_preprevert ($) {
+    # FIXME implement
+}
+
+sub rcs_revert (@) {
+    # Try to revert the given patch; returns undef on _success_.
+    # Same parameters as rcs_commit_staged + 'rev', the patch ID to be
+    # reverted.
+    my %params = @_;
+    my $rev = $params{rev};
+
+    if(run_or_non('git', 'revert', '--no-commit', $rev)) {
+        debug "Committing revert for patch '$rev'.";
+        rcs_commit_staged(message => "This reverts commit $rev", @_);
+    } else {
+        # No idea what is actually getting reverted, so all we can do is say we failed.
+        run_or_die('git', 'reset', '--hard');
+        return "Failed to revert patch $rev.";
+    }
+}
+
+sub rcs_showpatch ($) {
+    # Show the patch with the given revision id.
+    my ($rev) = @_;
+
+    # FIXME check
+    my @r = run_or_die('git', 'show', $rev);
+
+    return join "\n", @r;
+}
+
 1

IkiWiki/Plugin/recentchanges.pm

@@ -60,6 +60,72 @@ sub refresh ($) {
 	}
 }
 
+sub confirmation_form {
+    my ($q, $session, $rev) = @_;
+
+    eval q{use CGI::FormBuilder};
+    error($@) if $@;
+    my $f = CGI::FormBuilder->new(
+		name => "revert",
+		header => 0,
+		charset => "utf-8",
+		method => 'POST',
+		javascript => 0,
+		params => $q,
+		action => $config{cgiurl},
+		stylesheet => 1,
+                template => { template('revert.tmpl') },
+	);
+
+    $f->field(name => "sid", type => "hidden", value => $session->id,
+              force => 1);
+    $f->field(name => "do", type => "hidden", value => "revert", force => 1);
+    $f->field(name => "rev", type => "hidden", value => $rev, force => 1);
+
+    return $f, ["Revert", "Cancel"];
+}
+
+sub sessioncgi ($$) {
+    my ($q, $session) = @_;
+    my $do = $q->param('do');
+    my $rev = $q->param('rev');
+
+    return unless $do eq 'revert' && $rev;
+
+    # FIXME rcs_preprevert ??
+    IkiWiki::check_canedit('FIXME', $q, $session);
+
+    my ($form, $buttons) = confirmation_form($q, $session);
+    IkiWiki::decode_form_utf8($form);
+
+    if($form->submitted eq 'Revert' && $form->validate) {
+        IkiWiki::checksessionexpiry($q, $session, $q->param('sid'));
+
+        IkiWiki::disable_commit_hook();
+        my $r = IkiWiki::rcs_revert(
+                         session => $session,
+                         rev => $rev);
+        IkiWiki::enable_commit_hook();
+
+        if($r) {
+            die "FIXME revert '$rev' failed.";
+        } else {
+            IkiWiki::refresh();
+            IkiWiki::saveindex();
+            # FIXME indicate success.
+        }
+    } else {
+        $form->title(sprintf(gettext("confirm reversion of %s"), $rev));
+        my $patch_contents = IkiWiki::rcs_showpatch($rev);
+        $form->tmpl_param(patch_contents => encode_entities($patch_contents));
+        IkiWiki::showform($form, $buttons, $session, $q);
+        exit 0;
+    }
+
+    IkiWiki::redirect($q, urlto($config{recentchangespage}, ''));
+    exit 0;
+}
+
 # Enable the recentchanges link.
 sub pagetemplate (@) {
 	my %params=@_;

doc/templates.mdwn

@@ -74,7 +74,7 @@ html out of ikiwiki and in the templates.
 * `editpage.tmpl`, `editconflict.tmpl`, `editcreationconflict.tmpl`,
   `editfailedsave.tmpl`, `editpagegone.tmpl`, `pocreatepage.tmpl`,
   `editcomment.tmpl` `commentmoderation.tmpl`, `renamesummary.tmpl`,
-  `passwordmail.tmpl`, `openid-selector.tmpl` - Parts of ikiwiki's user
+  `passwordmail.tmpl`, `openid-selector.tmpl`, `revert.tmpl` - Parts of ikiwiki's user
   interface; do not normally need to be customised.
 
 [[!meta robots="noindex, follow"]]

templates/revert.tmpl

@@ -0,0 +1,13 @@
+<pre>
+<TMPL_VAR patch_contents>
+</pre>
+<TMPL_VAR FORM-START>
+<div>
+ <TMPL_VAR FIELD-DO>
+ <TMPL_VAR FIELD-SID>
+</div>
+<div class="revert buttons">
+ <TMPL_VAR form-submit>
+ <TMPL_VAR form-cancel>
+</div>
+<TMPL_VAR FORM-END>