WC lock idea and patch with stub functions

diff --git a/doc/bugs/locking_fun.mdwn b/doc/bugs/locking_fun.mdwn
index 8c4e0690bf0d0f632ef91bb2d280a74d82afc41c..5ecf9f846f17ea9015c765279d5ff15958aa6018 100644 (file)
--- a/doc/bugs/locking_fun.mdwn
+++ b/doc/bugs/locking_fun.mdwn
@@ -3,19 +3,118 @@ changes at once with its commit message (see r2779). The loser gets a
 message that there were conflicts and gets to see his own edits as the
 conflicting edits he's supposed to resolve.
 
 message that there were conflicts and gets to see his own edits as the
 conflicting edits he's supposed to resolve.
 

-This can happen because CGI.pm writes the change, then drops the lock
-before calling rcs_commit. It can't keep the lock because the commit hook
-needs to be able to lock.
+This can happen because CGI.pm writes the change, then drops the main wiki 
+lock before calling rcs_commit. It can't keep the lock because the commit
+hook needs to be able to lock.

 
 

-Using a shared reader lock plus an exclusive writer lock would seem to
-allow getting around this. The CGI would need the exclusive lock when
-editing the WC, then it could drop/convert that to the reader lock, keep
-the lock open, and lauch the post-commit hook, which would use the reader
-lock.
+We batted this around for an hour or two on irc. The best solution seems to
+be adding a subsidiary second lock, which is only used to lock the working
+copy and is a blocking read/write lock.

 
 

-One problem with the reader/writer idea is that the post-commit hook writes
-wiki state.
+* As before, the CGI will take the main wiki lock when starting up.
+* Before writing to the WC, the CGI takes an exclusive lock on the WC.
+* After writing to the WC, the CGI can downgrade it to a shared lock.
+  (This downgrade has to happen atomically, to prevent other CGIs from
+  stealing the exclusive lock.)
+* Then the CGI, as before, drops the main wiki lock to prevent deadlock. It
+  keeps its shared WC lock.
+* The commit hook takes first the main wiki lock and then the shared WC lock
+  when starting up, and holds them until it's done.
+* Once the commit is done, the CGI, as before, does not attempt to regain
+  the main wiki lock (that could deadlock). It does its final stuff and
+  exits, dropping the shared WC lock.

 
 

-An alternative approach might be setting a flag that prevents the
-post-commit hook from doing anything, and keeping the lock. Then the CGI
-would do the render & etc that the post-commit hook normally does.
+Sample patch, with stub functions for the new lock:
+
+<pre>
+Index: IkiWiki/CGI.pm
+===================================================================
+--- IkiWiki/CGI.pm     (revision 2774)
++++ IkiWiki/CGI.pm     (working copy)
+@@ -494,9 +494,14 @@
+               $content=~s/\r\n/\n/g;
+               $content=~s/\r/\n/g;
+ 
++              lockwc_exclusive();
++
+               $config{cgi}=0; # avoid cgi error message
+               eval { writefile($file, $config{srcdir}, $content) };
+               $config{cgi}=1;
++
++              lockwc_shared();
++
+               if ($@) {
+                       $form->field(name => "rcsinfo", value => rcs_prepedit($file),
+                               force => 1);
+Index: IkiWiki/Plugin/poll.pm
+===================================================================
+--- IkiWiki/Plugin/poll.pm     (revision 2770)
++++ IkiWiki/Plugin/poll.pm     (working copy)
+@@ -120,7 +120,9 @@
+               $content =~ s{(\\?)\[\[poll\s+([^]]+)\s*\]\]}{$edit->($1, $2)}seg;
+ 
+               # Store their vote, update the page, and redirect to it.
++              IkiWiki::lockwc_exclusive();
+               writefile($pagesources{$page}, $config{srcdir}, $content);
++              IkiWiki::lockwc_shared();
+               $session->param($choice_param, $choice);
+               IkiWiki::cgi_savesession($session);
+               $oldchoice=$session->param($choice_param);
+@@ -130,6 +132,10 @@
+                       IkiWiki::rcs_commit($pagesources{$page}, "poll vote ($choice)",
+                               IkiWiki::rcs_prepedit($pagesources{$page}),
+                               $session->param("name"), $ENV{REMOTE_ADDR});
++                      # Make sure that the repo is up-to-date;
++                      # locking prevents the post-commit hook
++                      # from updating it.
++                      rcs_update();
+               }
+               else {
+                       require IkiWiki::Render;
+Index: ikiwiki.in
+===================================================================
+--- ikiwiki.in (revision 2770)
++++ ikiwiki.in (working copy)
+@@ -121,6 +121,7 @@
+               lockwiki();
+               loadindex();
+               require IkiWiki::Render;
++              lockwc_shared();
+               rcs_update();
+               refresh();
+               rcs_notify() if $config{notify};
+Index: IkiWiki.pm
+===================================================================
+--- IkiWiki.pm (revision 2770)
++++ IkiWiki.pm (working copy)
+@@ -617,6 +617,29 @@
+       close WIKILOCK;
+ } #}}}
+ 
++sub lockwc_exclusive () { #{{{
++      # Take an exclusive lock on the working copy.
++      # The lock will be dropped on program exit.
++      # Note: This lock should only be taken _after_ the main wiki
++      # lock.
++      
++      # TODO
++} #}}}
++
++sub lockwc_shared () { #{{{
++      # Take a shared lock on the working copy. If an exclusive lock
++      # already exists, downgrade it to a shared lock.
++      # The lock will be dropped on program exit.
++      # Note: This lock should only be taken _after_ the main wiki
++      # lock.
++      
++      # TODO
++} #}}}
++
++sub unlockwc () { #{{{
++      close WIKIWCLOCK;
++} #}}}
++
+ sub loadindex () { #{{{
+       open (IN, "$config{wikistatedir}/index") || return;
+       while (<IN>) {
+</pre>