imaptracker: use flock(2) around writes

SQLite only issues non-blocking F_SETLK ops (not F_SETLKW) and
retries failures using a configurable busy_timeout.  SQLite's
busy loop sleeps for a millisecond and retries the lock until
the configured busy_timeout is hit.

Trying to set ->sqlite_busy_timeout to larger values (e.g. 30000
milliseconds) still leads to failure when running the new stress
test with 8 processes with TMPDIR on a 7200 RPM HDD.

Inspection of SQLite source reveals there's no built-in way to
use F_SETLKW, so tack on the existing flock(2) support we use to
synchronize git + SQLite + Xapian for inbox writing.  We use
flock(2) instead of POSIX fcntl(2) locks since Perl doesn't
provide a way to manipulate "struct flock" portably.

1 files changed, 10 insertions, 3 deletions

diff --git a/lib/PublicInbox/IMAPTracker.pm b/lib/PublicInbox/IMAPTracker.pm
index 0bbabe07..102a74ce 100644
--- a/lib/PublicInbox/IMAPTracker.pm
+++ b/lib/PublicInbox/IMAPTracker.pm
@@ -2,6 +2,7 @@
 # License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
 package PublicInbox::IMAPTracker;
 use strict;
+use parent qw(PublicInbox::Lock);
 use DBI;
 use DBD::SQLite;
 use PublicInbox::Config;
@@ -48,7 +49,10 @@ sub update_last ($$$) {
 INSERT OR REPLACE INTO imap_last (url, uid_validity, uid)
 VALUES (?, ?, ?)
 
-        $sth->execute($self->{url}, $validity, $last);
+        $self->lock_acquire;
+        my $rv = $sth->execute($self->{url}, $validity, $last);
+        $self->lock_release;
+        $rv;
 }
 
 sub new {
@@ -68,8 +72,11 @@ sub new {
                 require File::Basename;
                 File::Path::mkpath(File::Basename::dirname($dbname));
         }
-
-        bless { url => $url, dbh => dbh_new($dbname) }, $class;
+        my $self = bless { lock_path => "$dbname.lock", url => $url }, $class;
+        $self->lock_acquire;
+        $self->{dbh} = dbh_new($dbname);
+        $self->lock_release;
+        $self;
 }
 
 1;