diff options
| -rw-r--r-- | doc/bugs/locking_fun.mdwn | 125 |
1 files changed, 112 insertions, 13 deletions
diff --git a/doc/bugs/locking_fun.mdwn b/doc/bugs/locking_fun.mdwn index 8c4e0690b..5ecf9f846 100644 --- 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. -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> |