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.

2 files changed, 39 insertions, 4 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;
diff --git a/t/imap_tracker.t b/t/imap_tracker.t
index 8dc04ed7..01e1d0b1 100644
--- a/t/imap_tracker.t
+++ b/t/imap_tracker.t
@@ -9,8 +9,8 @@ my ($tmpdir, $for_destroy) = tmpdir();
 mkdir "$tmpdir/old" or die "mkdir $tmpdir/old: $!";
 my $old = "$tmpdir/old/imap.sqlite3";
 my $cur = "$tmpdir/data/public-inbox/imap.sqlite3";
+local $ENV{XDG_DATA_HOME} = "$tmpdir/data";
 {
-        local $ENV{XDG_DATA_HOME} = "$tmpdir/data";
         local $ENV{PI_DIR} = "$tmpdir/old";
 
         my $tracker = PublicInbox::IMAPTracker->new;
@@ -22,5 +22,33 @@ my $cur = "$tmpdir/data/public-inbox/imap.sqlite3";
         $tracker = PublicInbox::IMAPTracker->new;
         ok(!-f $cur, '->new does not create new file if old is present');
 }
+SKIP: {
+        my $nproc = $ENV{TEST_STRESS_NPROC};
+        skip 'TEST_STRESS_NPROC= not set', 1 unless $nproc;
+        my $nr = $ENV{TEST_STRESS_NR} // 10000;
+        diag "TEST_STRESS_NPROC=$nproc TEST_STRESS_NR=$nr";
+        require POSIX;
+        for my $n (1..$nproc) {
+                defined(my $pid = fork) or BAIL_OUT "fork: $!";
+                if ($pid == 0) {
+                        my $url = "imap://example.com/INBOX.$$";
+                        my $uidval = time;
+                        eval {
+                                my $itrk = PublicInbox::IMAPTracker->new($url);
+                                for my $uid (1..$nr) {
+                                        $itrk->update_last($uidval, $uid);
+                                        my ($uv, $u) = $itrk->get_last;
+                                }
+                        };
+                        warn "E: $n $$ - $@\n" if $@;
+                        POSIX::_exit($@ ? 1 : 0);
+                }
+        }
+        while (1) {
+                my $pid = waitpid(-1, 0);
+                last if $pid < 0;
+                is($?, 0, "$pid exited");
+        }
+}
 
 done_testing;