Update Qpsmtpd::DB to work with multiple stores

lib/Qpsmtpd/DB.pm

@@ -5,12 +5,26 @@ use Qpsmtpd::DB::File::DBM;
 
 sub new {
     my ( $class, %arg ) = @_;
-    # The only supported class just now
-    return bless { %arg }, 'Qpsmtpd::DB::File::DBM';
+    # Qpsmtpd::DB::File::DBM is the only supported class just now
+    my @child_classes = qw(
+        Qpsmtpd::DB::File::DBM
+    );
+    my $manual_class = delete $arg{class};
+    return $manual_class->new(%arg) if $manual_class;
+    my ( $child, @errors );
+    for my $child_class ( @child_classes ) {
+        eval {
+            $child = $child_class->new(%arg);
+        };
+        last if ! $@;
+        push @errors, "Couldn't load $child_class: $@";
+    }
+    return $child if $child;
+    die join( "\n", "Couldn't load any storage modules", @errors );
 }
 
 # noop default method for plugins that don't require locking
-sub get_lock { 1 }
+sub lock { 1 }
 
 sub name {
     my ( $self, $name ) = @_;

lib/Qpsmtpd/DB/File.pm

@@ -3,6 +3,11 @@ use strict;
 use warnings;
 use parent 'Qpsmtpd::DB';
 
+sub new {
+    my ( $class, %arg ) = @_;
+    return bless { %arg }, $class;
+}
+
 sub dir {
     my ( $self, @candidate_dirs ) = @_;
     return $self->{dir} if $self->{dir} and ! @candidate_dirs;