X-Git-Url: http://git.vanrenterghem.biz/git.ikiwiki.info.git/blobdiff_plain/9586107d138fc2b88e9960e0f7f11d8dce1dc468..53001f901106ec1846eded1de336083537b7f160:/doc/bugs/locking_fun.mdwn?ds=inline

diff --git a/doc/bugs/locking_fun.mdwn b/doc/bugs/locking_fun.mdwn
index 5ecf9f846..46278b028 100644
--- a/doc/bugs/locking_fun.mdwn
+++ b/doc/bugs/locking_fun.mdwn
@@ -7,6 +7,8 @@ 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.
 
+-------
+
 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.
@@ -14,8 +16,8 @@ copy and is a blocking read/write lock.
 * 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.)
+  (If this downgrade does not happen atomically, other CGIs can
+  steal 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
@@ -24,97 +26,80 @@ copy and is a blocking read/write lock.
   the main wiki lock (that could deadlock). It does its final stuff and
   exits, dropping the shared WC lock.
 
-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>
+Locking:
+
+Using fcntl locking from perl is very hard. flock locking has the problem
+that one some OSes (linux?) converting an exclusive to a shared lock is not
+atomic and can be raced. What happens if this race occurs is that,
+since ikiwiki always uses LOCK_NB, the flock fails. Then we're back to the
+original race. It should be possible though to use a separate exclusive lock,
+wrapped around these flock calls, to force them to be "atomic" and avoid that
+race.
+
+------
+
+My alternative idea, which seems simpler than all this tricky locking
+stuff, is to introduce a new lock file (really a flag file implemented
+using a lock), which tells the commit hook that the CGI is running, and
+makes the commit hook a NOOP.
+
+* CGI takes the wikilock
+* CGI writes changes to WC
+* CGI sets wclock to disable the commit hook
+* CGI does *not* drop the main wikilock
+* CGI commit
+* The commit hook tries to set the wclock, fails, and becomes a noop
+  (it may still need to send commit mails)
+* CGI removes wclock, thus re-enabling the commit hook
+* CGI updates the WC (since the commit hook didn't)
+* CGI renders the wiki (always. commits may have came in and not been
+  rendered)
+* CGI checks for conflicts, and if any are found does its normal dance
+
+> It seems like there are two things to be concerned with: RCS commit between
+> disable of hook and CGI commit, or RCS commit between CGI commit and re-enable
+> of hook. The second case isn't a big deal if the CGI is gonna rerender
+> everything anyhow. --[[Ethan]]
+
+I agree, and I think that the second case points to the hooks still being
+responsible for sending out commit mails. Everything else the CGI can do.
+
+I don't believe that the first case is actually a problem: If the RCS
+commit does not introduce a conflict then the CGI commit's changes will be
+merged into the repo cleanly. OTOH, if the RCS commit does introduces a
+conflict then the CGI commit will fail gracefully. This is exactly what
+happens now if RCS commit happens while a CGI commit is in progress! Ie:
+
+* cgi takes the wikilock
+* cgi writes change to wc
+* svn commit -m "conflict" (this makes a change to repo immediately, then
+  runs the post-commit hook, which waits on the wikilock)
+* cgi drops wikilock
+* the post-commit hook from the above manual commit can now run.
+* cgi calls rcs_commit, which fails due to the conflict just introduced
+
+The only difference to this scenario will be that the CGI will not drop the
+wiki lock before its commit, and that the post-commit hook will turn into a
+NOOP:
+
+* cgi takes the wikilock
+* cgi writes change to wc
+* cgi takes the wclock
+* svn commit -m "conflict" (this makes a change to repo immediately, then
+  runs the post-commit hook, which becomes a NOOP)
+* cgi calls rcs_commit, which fails due to the conflict just introduced
+* cgi renders the wiki
+
+Actually, the only thing that scares me about this apprach a little is that
+we have two locks. The CGI takes them in the order (wikilock, wclock).
+The commit hook takes them in the order (wclock, wikilock). This is a
+classic potential deadlock scenario. _However_, the commit hook should
+close the wclock as soon as it successfully opens it, before taking the
+wikilock, so I think that's ok.
+
+-----
+
+I've committed an implementation of my idea just above, and it seems to
+work, although testing for races etc is tricky. Calling this [[bugs/done]]
+unless someone finds a new bug or finds a problem in my thinking above.
+--[[Joey]]