From: Peter Rabbitson <ribasushi@cpan.org>
Date: Mon, 27 Sep 2010 07:27:39 +0000 (+0200)
Subject: Make Data::Page total count evaluation lazy
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=65245220;p=dbsrgits%2FDBIx-Class-Historic.git

Make Data::Page total count evaluation lazy

The short version is - now you can do $rs->pager->total_entries($satanic_num)
and DBIC will never even try to fire a COUNT query

This may seem to be a *lot* of code to do something that could
be done with a trivial subclass. However keep in mind that this
is a proof-of-concept of what do we do when we can *not* (or do
not want to) subclass.
---

diff --git a/Changes b/Changes
index b91006b..b0e0a2a 100644
--- a/Changes
+++ b/Changes
@@ -14,6 +14,12 @@ Revision history for DBIx::Class
         - FilterColumn now passes data through when transformations
           are not specified rather than throwing an exception.
         - Optimized RowNum based Oracle limit-dialect (RT#61277)
+        - Requesting a pager on a resultset with cached entries now
+          throws an exception, instead of returning a 1-page object
+          since the amount of rows is always equal to the "pagesize"
+        - $rs->pager now uses a lazy count to determine the amount of
+          total entries only when really needed, instead of doing it
+          at instantiation time
 
     * Fixes
         - Fix memory leak during populate() on 5.8.x perls
diff --git a/Makefile.PL b/Makefile.PL
index cac6e77..f10d2d9 100644
--- a/Makefile.PL
+++ b/Makefile.PL
@@ -66,6 +66,7 @@ my $runtime_requires = {
   'Path::Class'              => '0.18',
   'SQL::Abstract'            => '1.68',
   'Sub::Name'                => '0.04',
+  'Variable::Magic'          => '0.44',
   'Data::Dumper::Concise'    => '1.000',
   'Scope::Guard'             => '0.03',
   'Context::Preserve'        => '0.01',
diff --git a/lib/DBIx/Class/ResultSet.pm b/lib/DBIx/Class/ResultSet.pm
index 0e09041..dcc23b8 100644
--- a/lib/DBIx/Class/ResultSet.pm
+++ b/lib/DBIx/Class/ResultSet.pm
@@ -10,7 +10,8 @@ use Storable;
 use DBIx::Class::ResultSetColumn;
 use DBIx::Class::ResultSourceHandle;
 use List::Util ();
-use Scalar::Util 'blessed';
+use Scalar::Util qw/blessed weaken/;
+use Try::Tiny;
 use namespace::clean;
 
 use overload
@@ -949,6 +950,7 @@ sub next {
     return $cache->[$self->{all_cache_position}++];
   }
   if ($self->{attrs}{cache}) {
+    delete $self->{pager};
     $self->{all_cache_position} = 1;
     return ($self->all)[0];
   }
@@ -1804,11 +1806,115 @@ C<total_entries> on the L<Data::Page> object.
 
 =cut
 
+# make a wizard good for both a scalar and a hashref
+my $mk_lazy_count_wizard = sub {
+  require Variable::Magic;
+
+  my $stash = { total_rs => shift };
+  my $slot = shift; # only used by the hashref magic
+
+  my $magic = Variable::Magic::wizard (
+    data => sub { $stash },
+
+    (!$slot)
+    ? (
+      # the scalar magic
+      get => sub {
+        # set value lazily, and dispell for good
+        ${$_[0]} = $_[1]{total_rs}->count;
+        Variable::Magic::dispell (${$_[0]}, $_[1]{magic_selfref});
+        return 1;
+      },
+      set => sub {
+        # an explicit set implies dispell as well
+        # the unless() is to work around "fun and giggles" below
+        Variable::Magic::dispell (${$_[0]}, $_[1]{magic_selfref})
+          unless (caller(2))[3] eq 'DBIx::Class::ResultSet::pager';
+        return 1;
+      },
+    )
+    : (
+      # the uvar magic
+      fetch => sub {
+        if ($_[2] eq $slot and !$_[1]{inactive}) {
+          my $cnt = $_[1]{total_rs}->count;
+          $_[0]->{$slot} = $cnt;
+
+          # attempting to dispell in a fetch handle (works in store), seems
+          # to invariable segfault on 5.10, 5.12, 5.13 :(
+          # so use an inactivator instead
+          #Variable::Magic::dispell (%{$_[0]}, $_[1]{magic_selfref});
+          $_[1]{inactive}++;
+        }
+        return 1;
+      },
+      store => sub {
+        if (! $_[1]{inactive} and $_[2] eq $slot) {
+          #Variable::Magic::dispell (%{$_[0]}, $_[1]{magic_selfref});
+          $_[1]{inactive}++
+            unless (caller(2))[3] eq 'DBIx::Class::ResultSet::pager';
+        }
+        return 1;
+      },
+    ),
+  );
+
+  $stash->{magic_selfref} = $magic;
+  weaken ($stash->{magic_selfref}); # this fails on 5.8.1
+
+  return $magic;
+};
+
+# the tie class for 5.8.1
+{
+  package DBIx::Class::__DBIC_LAZY_RS_COUNT__;
+  use base qw/Tie::Hash/;
+
+  sub FIRSTKEY { my $dummy = scalar keys %{$_[0]{data}}; each %{$_[0]{data}} }
+  sub NEXTKEY  { each %{$_[0]{data}} }
+  sub EXISTS   { exists $_[0]{data}{$_[1]} }
+  sub DELETE   { delete $_[0]{data}{$_[1]} }
+  sub CLEAR    { %{$_[0]{data}} = () }
+  sub SCALAR   { scalar %{$_[0]{data}} }
+
+  sub TIEHASH {
+    $_[1]{data} = {%{$_[1]{selfref}}};
+    %{$_[1]{selfref}} = ();
+    Scalar::Util::weaken ($_[1]{selfref});
+    return bless ($_[1], $_[0]);
+  };
+
+  sub FETCH {
+    if ($_[1] eq $_[0]{slot}) {
+      my $cnt = $_[0]{data}{$_[1]} = $_[0]{total_rs}->count;
+      untie %{$_[0]{selfref}};
+      %{$_[0]{selfref}} = %{$_[0]{data}};
+      return $cnt;
+    }
+    else {
+      $_[0]{data}{$_[1]};
+    }
+  }
+
+  sub STORE {
+    $_[0]{data}{$_[1]} = $_[2];
+    if ($_[1] eq $_[0]{slot}) {
+      untie %{$_[0]{selfref}};
+      %{$_[0]{selfref}} = %{$_[0]{data}};
+    }
+    $_[2];
+  }
+}
+
 sub pager {
   my ($self) = @_;
 
   return $self->{pager} if $self->{pager};
 
+  if ($self->get_cache) {
+    $self->throw_exception ('Pagers on cached resultsets are not supported');
+  }
+
   my $attrs = $self->{attrs};
   $self->throw_exception("Can't create pager for non-paged rs")
     unless $self->{attrs}{page};
@@ -1818,13 +1924,69 @@ sub pager {
   # with a subselect) to get the real total count
   my $count_attrs = { %$attrs };
   delete $count_attrs->{$_} for qw/rows offset page pager/;
-  my $total_count = (ref $self)->new($self->result_source, $count_attrs)->count;
+  my $total_rs = (ref $self)->new($self->result_source, $count_attrs);
+
+
+### the following may seem awkward and dirty, but it's a thought-experiment
+### necessary for future development of DBIx::DS. Do *NOT* change this code
+### before talking to ribasushi/mst
 
-  return $self->{pager} = Data::Page->new(
-    $total_count,
+  my $pager = Data::Page->new(
+    0,  #start with an empty set
     $attrs->{rows},
-    $self->{attrs}{page}
+    $self->{attrs}{page},
   );
+
+  my $data_slot = 'total_entries';
+
+  # Since we are interested in a cached value (once it's set - it's set), every
+  # technique will detach from the magic-host once the time comes to fire the
+  # ->count (or in the segfaulting case of >= 5.10 it will deactivate itself)
+
+  if ($] < 5.008003) {
+    # 5.8.1 throws 'Modification of a read-only value attempted' when one tries
+    # to weakref the magic container :(
+    # tested on 5.8.1
+    tie (%$pager, 'DBIx::Class::__DBIC_LAZY_RS_COUNT__',
+      { slot => $data_slot, total_rs => $total_rs, selfref => $pager }
+    );
+  }
+  elsif ($] < 5.010) {
+    # We can use magic on the hash value slot. It's interesting that the magic is
+    # attached to the hash-slot, and does *not* stop working once I do the dummy
+    # assignments after the cast()
+    # tested on 5.8.3 and 5.8.9
+    my $magic = $mk_lazy_count_wizard->($total_rs);
+    Variable::Magic::cast ( $pager->{$data_slot}, $magic );
+
+    # this is for fun and giggles
+    $pager->{$data_slot} = -1;
+    $pager->{$data_slot} = 0;
+
+    # this does not work for scalars, but works with
+    # uvar magic below
+    #my %vals = %$pager;
+    #%$pager = ();
+    #%{$pager} = %vals;
+  }
+  else {
+    # And the uvar magic
+    # works on 5.10.1, 5.12.1 and 5.13.4 in its current form,
+    # however see the wizard maker for more notes
+    my $magic = $mk_lazy_count_wizard->($total_rs, $data_slot);
+    Variable::Magic::cast ( %$pager, $magic );
+
+    # still works
+    $pager->{$data_slot} = -1;
+    $pager->{$data_slot} = 0;
+
+    # this now works
+    my %vals = %$pager;
+    %$pager = ();
+    %{$pager} = %vals;
+  }
+
+  return $self->{pager} = $pager;
 }
 
 =head2 page
diff --git a/t/67pager.t b/t/67pager.t
index b017afb..1835d6d 100644
--- a/t/67pager.t
+++ b/t/67pager.t
@@ -2,26 +2,49 @@ use strict;
 use warnings;
 
 use Test::More;
+use Test::Exception;
 use lib qw(t/lib);
 use DBICTest;
 
 my $schema = DBICTest->init_schema();
 
+is ($schema->resultset("CD")->count, 5, 'Initial count sanity check');
+
+my $qcnt;
+$schema->storage->debugcb(sub { $qcnt++ });
+$schema->storage->debug (1);
+
+my $rs = $schema->resultset("CD");
+
 # first page
-my $it = $schema->resultset("CD")->search(
+$qcnt = 0;
+my $it = $rs->search(
     {},
     { order_by => 'title',
       rows => 3,
       page => 1 }
 );
+my $pager = $it->pager;
+is ($qcnt, 0, 'No queries on rs/pager creation');
 
-is( $it->pager->entries_on_this_page, 3, "entries_on_this_page ok" );
+is ($pager->entries_per_page, 3, 'Pager created with correct entries_per_page');
+ok ($pager->current_page(-1), 'Set nonexistent page');
+is ($pager->current_page, 1, 'Page set behaves correctly');
+ok ($pager->current_page(2), 'Set 2nd page');
 
-is( $it->pager->next_page, 2, "next_page ok" );
+is ($qcnt, 0, 'No queries on total_count-independent methods');
 
-is( $it->count, 3, "count on paged rs ok" );
+is( $it->pager->entries_on_this_page, 2, "entries_on_this_page ok for page 2" );
+
+is ($qcnt, 1, 'Count fired to get pager page entries');
 
-is( $it->pager->total_entries, 5, "total_entries ok" );
+$qcnt = 0;
+is ($pager->previous_page, 1, 'Correct previous_page');
+is ($pager->next_page, undef, 'No more pages');
+is ($qcnt, 0, 'No more counts - amount of entries cached in pager');
+
+is( $it->count, 3, "count on paged rs ok" );
+is ($qcnt, 1, 'An $rs->count still fires properly');
 
 is( $it->next->title, "Caterwaulin' Blues", "iterator->next ok" );
 
@@ -30,8 +53,9 @@ $it->next;
 
 is( $it->next, undef, "next past end of page ok" );
 
+
 # second page, testing with array
-my @page2 = $schema->resultset("CD")->search( 
+my @page2 = $rs->search(
     {},
     { order_by => 'title',
       rows => 3,
@@ -41,7 +65,7 @@ my @page2 = $schema->resultset("CD")->search(
 is( $page2[0]->title, "Generic Manufactured Singles", "second page first title ok" );
 
 # page a standard resultset
-$it = $schema->resultset("CD")->search(
+$it = $rs->search(
   {},
   { order_by => 'title',
     rows => 3 }
@@ -52,8 +76,9 @@ is( $page->count, 2, "standard resultset paged rs count ok" );
 
 is( $page->next->title, "Generic Manufactured Singles", "second page of standard resultset ok" );
 
+
 # test software-based limit paging
-$it = $schema->resultset("CD")->search(
+$it = $rs->search(
   {},
   { order_by => 'title',
     rows => 3,
@@ -69,7 +94,7 @@ is( $it->count, 2, "software count on paged rs ok" );
 is( $it->next->title, "Generic Manufactured Singles", "software iterator->next ok" );
 
 # test paging with chained searches
-$it = $schema->resultset("CD")->search(
+$it = $rs->search(
     {},
     { rows => 2,
       page => 2 }
@@ -77,23 +102,15 @@ $it = $schema->resultset("CD")->search(
 
 is( $it->count, 2, "chained searches paging ok" );
 
-my $p = sub { $schema->resultset("CD")->page(1)->pager->entries_per_page; };
-
-is($p->(), 10, 'default rows is 10');
-
-$schema->default_resultset_attributes({ rows => 5 });
-
-is($p->(), 5, 'default rows is 5');
-
 # test page with offset
-$it = $schema->resultset('CD')->search({}, {
+$it = $rs->search({}, {
     rows => 2,
     page => 2,
     offset => 1,
     order_by => 'cdid'
 });
 
-my $row = $schema->resultset('CD')->search({}, {
+my $row = $rs->search({}, {
     order_by => 'cdid', 
     offset => 3,
     rows => 1
@@ -101,4 +118,61 @@ my $row = $schema->resultset('CD')->search({}, {
 
 is($row->cdid, $it->first->cdid, 'page with offset');
 
+
+# test pager on non-title page behavior
+$qcnt = 0;
+$it = $rs->search({}, { rows => 3 })->page (2);
+ok ($it->pager);
+is ($qcnt, 0, 'No count on past-first-page pager instantiation');
+
+is ($it->pager->current_page, 2, 'Page set properby by $rs');
+is( $it->pager->total_entries, 5, 'total_entries correct' );
+
+$rs->create ({ artist => 1, title => 'MOAR!', year => 2010 });
+is( $it->count, 3, 'Dynamic count on filling up page' );
+$rs->create ({ artist => 1, title => 'MOAR!!!', year => 2011 });
+is( $it->count, 3, 'Count still correct (does not overflow' );
+
+$qcnt = 0;
+is( $it->pager->total_entries, 5, 'total_entries properly cached at old value' );
+is ($qcnt, 0, 'No queries');
+
+# test fresh pager with explicit total count assignment
+$qcnt = 0;
+$pager = $rs->search({}, { rows => 4 })->page (2)->pager;
+$pager->total_entries (13);
+
+is ($pager->current_page, 2, 'Correct start page');
+is ($pager->next_page, 3, 'One more page');
+is ($pager->last_page, 4, 'And one more page');
+is ($pager->previous_page, 1, 'One page in front');
+
+is ($qcnt, 0, 'No queries with explicitly sey total count');
+
+# test cached resultsets
+my $init_cnt = $rs->count;
+
+$it = $rs->search({}, { rows => 3, cache => 1 })->page(3);
+is ($it->count, 1, 'One row');
+is (scalar $it->all, 1, 'One object');
+
+$it->delete;
+is ($rs->count, $init_cnt - 1, 'One row deleted as expected');
+
+is ($it->count, 1, 'One row (cached)');
+is (scalar $it->all, 1, 'One object (cached)');
+
+throws_ok { $it->pager }
+  qr/Pagers on cached resultsets are not supported/, 'No pagers on cached resultsets';
+
+# test fresh rs creation with modified defaults
+my $p = sub { $schema->resultset('CD')->page(1)->pager->entries_per_page; };
+
+is($p->(), 10, 'default rows is 10');
+
+$schema->default_resultset_attributes({ rows => 5 });
+
+is($p->(), 5, 'default rows is 5');
+
+
 done_testing;