[PATCH v2 2/6] lei_mail_sync: don't hold statement handle into callback

[Eric Wong <e@80x24.org>]

This can cause readers and writers to conflict since the
implicit transaction from SELECT in a LeiRefreshMailSync
worker would block the LeiStore process.
---
 lib/PublicInbox/LeiMailSync.pm | 41 ++++++++++++++++++++++++++--------
 1 file changed, 32 insertions(+), 9 deletions(-)

diff --git a/lib/PublicInbox/LeiMailSync.pm b/lib/PublicInbox/LeiMailSync.pm
index 5a10c127..8f584ccb 100644
--- a/lib/PublicInbox/LeiMailSync.pm
+++ b/lib/PublicInbox/LeiMailSync.pm
@@ -169,22 +169,45 @@ INSERT OR IGNORE INTO blob2name (oidbin, fid, name) VALUES (?, ?, ?)
 sub each_src {
 	my ($self, $folder, $cb, @args) = @_;
 	my $dbh = $self->{dbh} //= dbh_new($self);
-	my ($fid, $sth);
+	my $fid;
 	if (ref($folder) eq 'HASH') {
 		$fid = $folder->{fid} // die "BUG: no `fid'";
 	} else {
 		$fid = $self->{fmap}->{$folder} //=
 			fid_for($self, $folder) // return;
 	}
-	$sth = $dbh->prepare('SELECT oidbin,uid FROM blob2num WHERE fid = ?');
-	$sth->execute($fid);
-	while (my ($oidbin, $id) = $sth->fetchrow_array) {
-		$cb->($oidbin, $id, @args);
+
+	# minimize implicit txn time to avoid blocking writers by
+	# batching SELECTs.  This looks wonky but is necessary since
+	# $cb-> may access the DB on its own.
+	my $ary = $dbh->selectall_arrayref(<<'', undef, $fid);
+SELECT _rowid_,oidbin,uid FROM blob2num WHERE fid = ?
+ORDER BY _rowid_ ASC LIMIT 1000
+
+	my $min = @$ary ? $ary->[-1]->[0] : undef;
+	while (defined $min) {
+		for my $row (@$ary) { $cb->($row->[1], $row->[2], @args) }
+
+		$ary = $dbh->selectall_arrayref(<<'', undef, $fid, $min);
+SELECT _rowid_,oidbin,uid FROM blob2num WHERE fid = ? AND _rowid_ > ?
+ORDER BY _rowid_ ASC LIMIT 1000
+
+		$min = @$ary ? $ary->[-1]->[0] : undef;
 	}
-	$sth = $dbh->prepare('SELECT oidbin,name FROM blob2name WHERE fid = ?');
-	$sth->execute($fid);
-	while (my ($oidbin, $id) = $sth->fetchrow_array) {
-		$cb->($oidbin, \$id, @args);
+
+	$ary = $dbh->selectall_arrayref(<<'', undef, $fid);
+SELECT _rowid_,oidbin,name FROM blob2name WHERE fid = ?
+ORDER BY _rowid_ ASC LIMIT 1000
+
+	$min = @$ary ? $ary->[-1]->[0] : undef;
+	while (defined $min) {
+		for my $row (@$ary) { $cb->($row->[1], \($row->[2]), @args) }
+
+		$ary = $dbh->selectall_arrayref(<<'', undef, $fid, $min);
+SELECT _rowid_,oidbin,name FROM blob2name WHERE fid = ? AND _rowid_ > ?
+ORDER BY _rowid_ ASC LIMIT 1000
+
+		$min = @$ary ? $ary->[-1]->[0] : undef;
 	}
 }