When inserting multiple images at the same time, we could deadlock -- inserting

new rows into images would (due to the foreign key constraint) lock the corresponding
row in event using a shared lock, then the update (due to last_picture) would
upgrade the lock to exclusive, causing a deadlock later. To fix this, we move
last_picture into its own table; we could also lock it explicitly earlier, but
that would stall other transactions (and they can run for 10-15 seconds or so,
or perhaps even longer) so it is not desirable.

diff --git a/perl/Sesse/pr0n/Common.pm b/perl/Sesse/pr0n/Common.pm
index ecc039e15d8144cb9443891c0929d7cbf8e0d5e8..d03f46f1f01a01a10b656bb0efeb53aeb1adfb6e 100644 (file)
--- a/perl/Sesse/pr0n/Common.pm
+++ b/perl/Sesse/pr0n/Common.pm
@@ -192,7 +192,7 @@ sub update_width_height {
 
        # update the last_picture cache as well (this should of course be done
        # via a trigger, but this is less complicated :-) )
-       $dbh->do('UPDATE events SET last_picture=GREATEST(last_picture, ?) FROM images WHERE event=events.id) WHERE id=(SELECT event FROM images WHERE id=?)',
+       $dbh->do('UPDATE last_picture_cache SET last_picture=GREATEST(last_picture, ?) FROM images WHERE event=events.id) WHERE id=(SELECT event FROM images WHERE id=?)',
                undef, $datetime, $id)
                or die "Couldn't update last_picture in SQL: $!";
 }

diff --git a/perl/Sesse/pr0n/Listing.pm b/perl/Sesse/pr0n/Listing.pm
index 8447040a760421e50718afc3a88f71142766f22a..041e9568046e3625f7820d695a9515b9b418326f 100644 (file)
--- a/perl/Sesse/pr0n/Listing.pm
+++ b/perl/Sesse/pr0n/Listing.pm
@@ -18,7 +18,7 @@ sub handler {
 
 #      my $q = $dbh->prepare('SELECT t1.id,t1.date,t1.name FROM events t1 LEFT JOIN images t2 ON t1.id=t2.event WHERE t1.vhost=? GROUP BY t1.id,t1.date,t1.name ORDER BY COALESCE(MAX(t2.date),\'1970-01-01 00:00:00\'),t1.id') or
 #              dberror($r, "Couldn't list events");
-       my $q = $dbh->prepare('SELECT id,date,name FROM events WHERE vhost=? ORDER BY last_picture DESC')
+       my $q = $dbh->prepare('SELECT id,date,name FROM events e JOIN last_picture_cache c ON e.id=c.event WHERE vhost=? ORDER BY last_picture DESC')
                or dberror($r, "Couldn't list events");
        $q->execute($r->get_server_name)
                or dberror($r, "Couldn't get events");

diff --git a/sql/pr0n.sql b/sql/pr0n.sql
index c1349e7e4e5aea9ad58c382870bf70d653c28b54..9d1f9374d3bae4e86a9fbd92dfa288d17cfb8c77 100644 (file)
--- a/sql/pr0n.sql
+++ b/sql/pr0n.sql
@@ -2,8 +2,13 @@ CREATE TABLE events (
     id character varying NOT NULL PRIMARY KEY,
     date character varying NOT NULL,
     name character varying NOT NULL,
-    vhost character varying NOT NULL,
-    last_picture timestamp without time zone DEFAULT '1970-01-01 00:00:00'::timestamp without time zone NOT NULL
+    vhost character varying NOT NULL
+);
+
+-- In a separate table to avoid deadlocks.
+CREATE TABLE last_picture_cache ( 
+   event varchar PRIMARY KEY references events ( id ),
+   last_picture timestamp without time zone
 );
 
 CREATE TABLE images (