fglock · fglock · Apr 29, 2026
diff --git a/dev/modules/moose_support.md b/dev/modules/moose_support.md
@@ -1991,6 +1991,103 @@ Tests fixed:
   - A handful of cmop/method introspection edge cases (constants,
     forward declarations, eval-defined subs).
 
+## Phase D-W6.6: precise failure-stack diagnosis (2026-04-29)
+
+Installing a `$SIG{__DIE__}` probe before `use Class::MOP` (with the
+gate disabled) finally pinpoints WHERE the failure originates:
+
+```
+FAILURE at:
+  main::__ANON__         at .../Try/Tiny.pm:139
+  (eval)                 at .../Try/Tiny.pm:140
+  Try::Tiny::try         at .../Class/MOP/Class.pm:897
+  Class::MOP::Class::_post_add_attribute
+                         at .../Class/MOP/Mixin/HasAttributes.pm:41
+  Class::MOP::Mixin::HasAttributes::add_attribute
+                         at .../Class/MOP.pm:188
+```
+
+The chain inside the bootstrap:
+
+```perl
+# CMOP.pm:188 — bootstrap
+Class::MOP::Mixin::HasMethods->meta->add_attribute(
+    Class::MOP::Attribute->new('wrapped_method_metaclass' => (
+        reader => { 'wrapped_method_metaclass' => \&...wrapped_method_metaclass },
+        ...
+    ))
+);
+
+# HasAttributes.pm:41 — add_attribute body
+$self->_attach_attribute($attribute);   # weaken back-ref
+$self->{_attribute_map}{...} = $attr;
+$self->_post_add_attribute($attribute);
+
+# Class.pm:897 — _post_add_attribute body
+try {
+    local $SIG{__DIE__};
+    $attribute->install_accessors;     # ← dies inside here somewhere
+} catch {
+    $self->remove_attribute($attribute->name);   # ← then THIS dies because
+    die $_;                                       #   $attr->associated_class
+};                                                #   reads as undef
+```
+
+So `install_accessors` dies first — when `$self->associated_class`
+reads as undef inside `_process_accessors`. Then the catch block
+calls `remove_attribute` which calls `remove_accessors` which calls
+`_remove_accessor($accessor, $self->associated_class())` — and the
+weak ref is undef, so `$class->get_method($accessor)` dies.
+
+The visible "Can't call method 'get_method' on an undefined value at
+Attribute.pm line 475" is the SECOND failure; the first is hidden by
+Try::Tiny's `local $SIG{__DIE__}` and only manifests via the catch.
+
+### Reproducer
+
+`src/test/resources/unit/refcount/drift/cmop_add_attr_loop.t` — 11
+iterations of the exact `_attach_attribute` →
+`_attribute_map` insert → `_post_add_attribute` → `try {
+install_accessors } catch { remove_attribute }` shape, with
+`reader => { name => CV }` HASH-form readers (which is what triggers
+`install_accessors` to actually do `$self->associated_class->name`).
+
+**Both Perl 5 and PerlOnJava (gate disabled) pass this reproducer.**
+So the simple shape isn't enough to surface the drift.
+
+### What's still missing from the reproducer
+
+The real `Class::MOP` bootstrap differs from the reproducer in that
+`Class::MOP::Mixin::HasMethods->meta` returns a metaclass that is:
+
+- An instance of `Class::MOP::Class` (not bare-Perl Meta).
+- That metaclass itself has a multi-level `@ISA` chain (Module,
+  HasAttributes, HasMethods, HasOverloads).
+- Has been built up by *itself* — `meta` initialises a metaclass for
+  the package using the same `_construct_class_instance` →
+  `add_attribute` chain.
+- Has Class::MOP's own `add_method`, `attribute_metaclass`,
+  `method_metaclass`, etc. — each a CMOP-machine method.
+
+The bug is somewhere in **the recursive bootstrap**, where the
+metaclass's own add_attribute uses methods that themselves traverse
+the metaclass-method-map (which is being built simultaneously). The
+specific transient drift happens when `install_accessors` calls a
+method that walks `$class->linearized_isa` or `$class->_method_map`
+during the partial-build state.
+
+### Suggested next probe
+
+- Run with `PJ_DESTROY_TRACE=1 PJ_WEAKCLEAR_TRACE=1` and add a
+  `$SIG{__DIE__}` Perl-side probe that prints
+  `$self->associated_class // 'UNDEF'` immediately before each `die`.
+  The first die where `associated_class` is `'UNDEF'` reveals the
+  exact LAST PRIOR refCount-decrement step that took the metaclass
+  to 0.
+- That decrement's stack will identify the specific `linearized_isa`
+  / `_method_map` traversal inside `install_accessors` that loses
+  the metaclass strong hold.
+
 ## Related Documents
 
 - [xs_fallback.md](xs_fallback.md) — XS fallback mechanism

diff --git a/src/test/resources/unit/refcount/drift/cmop_add_attr_loop.t b/src/test/resources/unit/refcount/drift/cmop_add_attr_loop.t
@@ -0,0 +1,91 @@
+# D-W6.6 — Class::MOP add_attribute loop drift reproducer.
+#
+# Recreates the exact shape of the Class::MOP bootstrap that fails
+# during `use Class::MOP` with the gate disabled. The bootstrap calls
+# `Class::MOP::Mixin::HasMethods->meta->add_attribute(...)` repeatedly
+# for each MOP attribute. Each call:
+#
+#   1. _attach_attribute — sets `weaken($attr->{associated_class} = $self)`.
+#   2. Stores attribute in `$self->{_attribute_map}{...}` (strong).
+#   3. _post_add_attribute — `try { install_accessors } catch { ... }`.
+#
+# Both Perl 5 and PerlOnJava handle this correctly. The bare-Perl
+# shape passes. The actual failure during real Class::MOP load must
+# involve something more specific (multi-level @ISA, role
+# composition, or a specific code path inside install_accessors).
+use strict;
+use warnings;
+use Test::More;
+use Try::Tiny;
+
+package Meta;
+use Try::Tiny;
+use Scalar::Util qw(weaken);
+
+sub new { bless { name => $_[1], _attrs => {} }, $_[0] }
+sub name { $_[0]->{name} }
+sub _attach_attribute {
+    my ($self, $attr) = @_;
+    $attr->{associated_class} = $self;
+    weaken($attr->{associated_class});
+}
+sub _post_add_attribute {
+    my ($self, $attr) = @_;
+    try {
+        local $SIG{__DIE__};
+        $attr->install_accessors;
+    } catch {
+        $self->remove_attribute($attr->{name});
+    };
+}
+sub add_attribute {
+    my ($self, $attribute) = @_;
+    $self->_attach_attribute($attribute);
+    $self->{_attrs}{$attribute->{name}} = $attribute;
+    $self->_post_add_attribute($attribute);
+}
+sub remove_attribute {
+    my ($self, $name) = @_;
+    my $attr = delete $self->{_attrs}{$name} or return;
+    $attr->remove_accessors;
+}
+
+package Attr;
+sub new { bless { %{$_[1]} }, $_[0] }
+sub install_accessors {
+    my $self = shift;
+    my $reader = $self->{reader} or return;
+    if (ref $reader eq 'HASH') {
+        my $cls = $self->{associated_class};
+        return if defined $cls;
+        die "install: associated_class UNDEF for $self->{name}!";
+    }
+}
+sub remove_accessors {
+    my $self = shift;
+    return if defined $self->{associated_class};
+    die "*** remove: UNDEF associated_class for $self->{name}";
+}
+
+package main;
+our %METAS;
+
+my $cv = sub { 'reader' };
+my $meta = Meta->new('TestPkg');
+$METAS{TestPkg} = $meta;
+
+my $err = 0;
+for my $n (qw(_method_map method_metaclass wrapped_method_metaclass
+              attribute_metaclass attribute_map list_methods
+              foo bar baz quux corge)) {
+    eval {
+        $meta->add_attribute(Attr->new({
+            name => $n,
+            reader => { "${n}_reader" => $cv },
+        }));
+    };
+    if ($@) { $err++; diag("attr $n: $@") }
+}
+is $err, 0, '11 add_attribute iterations all succeed';
+is scalar(keys %{$meta->{_attrs}}), 11, '11 attributes registered';
+done_testing;
diff --git a/src/test/resources/unit/refcount/drift/try_tiny_weak.t b/src/test/resources/unit/refcount/drift/try_tiny_weak.t
@@ -0,0 +1,142 @@
+# D-W6.6 — Try::Tiny + weak back-ref drift reproducer.
+#
+# The actual failure during `use Class::MOP` (with the gate disabled)
+# happens at Class/MOP/Class.pm:897 inside `_post_add_attribute`:
+#
+#   sub _post_add_attribute {
+#       my ($self, $attribute) = @_;
+#       $self->invalidate_meta_instances;
+#       try {
+#           local $SIG{__DIE__};
+#           $attribute->install_accessors;
+#       } catch {
+#           $self->remove_attribute($attribute->name);
+#           die $_;
+#       };
+#   }
+#
+# In the catch path, `$attribute->associated_class()` (a WEAK ref to
+# `$self`) reads as undef — even though `$self` is alive on the stack.
+#
+# Hypothesis: Try::Tiny's `try { ... } catch { ... }` builds a closure
+# that captures `$self` and `$attr`. The captures drop refCount
+# transiently. The captured weak-ref-target's refCount hits 0 and
+# `clearWeakRefsTo` fires, wiping the back-ref before the catch
+# reads it.
+use strict;
+use warnings;
+use Test::More;
+use Try::Tiny;
+use Scalar::Util qw(weaken);
+
+# Bare-Perl recreation of Class::MOP's _post_add_attribute pattern.
+
+package Meta;
+sub new {
+    my ($class, $name) = @_;
+    bless { name => $name }, $class;
+}
+sub name { $_[0]->{name} }
+sub attach_attr {
+    my ($self, $attr) = @_;
+    $attr->{associated_class} = $self;
+    Scalar::Util::weaken($attr->{associated_class});
+}
+sub remove_attr {
+    my ($self, $attr) = @_;
+    my $cls = $attr->associated_class;
+    die "associated_class is undef!" unless defined $cls;
+    return $cls->name;
+}
+
+package Attr;
+sub new { bless { name => $_[1] }, $_[0] }
+sub name { $_[0]->{name} }
+sub associated_class { $_[0]->{associated_class} }
+sub install_accessors_with_die {
+    my $self = shift;
+    die "install_accessors deliberately dying\n";
+}
+
+package main;
+
+# ---- Pattern A: bare Try::Tiny + weak back-ref ---------------------------
+{
+    my $meta = Meta->new('FooA');
+    my $attr = Attr->new('attr1');
+    $meta->attach_attr($attr);
+
+    my $caught_class;
+    try {
+        local $SIG{__DIE__};
+        $attr->install_accessors_with_die;
+    } catch {
+        # In Class::MOP this is `$self->remove_attribute($attr->name)`
+        # which dispatches into _remove_accessor that calls
+        # `$attr->associated_class()`. The weak ref must still resolve.
+        $caught_class = $meta->remove_attr($attr);
+    };
+
+    is $caught_class, 'FooA',
+        'A: weak ref to $meta survived Try::Tiny try/catch';
+}
+
+# ---- Pattern B: $self captured by try-block, no outer my-var -------------
+# Mimics _post_add_attribute where $self is a parameter, not a global.
+sub do_post_add_attribute {
+    my ($self, $attr) = @_;
+    my $caught_class;
+    try {
+        local $SIG{__DIE__};
+        $attr->install_accessors_with_die;
+    } catch {
+        $caught_class = $self->remove_attr($attr);
+    };
+    return $caught_class;
+}
+
+{
+    my $meta = Meta->new('FooB');
+    my $attr = Attr->new('attr1');
+    $meta->attach_attr($attr);
+    my $r = do_post_add_attribute($meta, $attr);
+    is $r, 'FooB',
+        'B: weak ref survives Try::Tiny inside a sub';
+}
+
+# ---- Pattern C: 20-iteration loop ----------------------------------------
+# Mimics Class::MOP bootstrap building many attributes.
+{
+    my @failures;
+    for my $i (1 .. 20) {
+        my $meta = Meta->new("PkgC$i");
+        my $attr = Attr->new("attr$i");
+        $meta->attach_attr($attr);
+        my $r = do_post_add_attribute($meta, $attr);
+        push @failures, $i unless defined $r && $r eq "PkgC$i";
+    }
+    is scalar(@failures), 0,
+        'C: 20 iterations of try/catch + weak ref all succeed';
+}
+
+# ---- Pattern D: meta in a global hash --------------------------------------
+# Mimics Class::MOP's %METAS pattern.
+{
+    package Reg;
+    our %METAS;
+    package main;
+    %Reg::METAS = ();
+    my @failures;
+    for my $i (1 .. 5) {
+        my $meta = Meta->new("PkgD$i");
+        $Reg::METAS{"PkgD$i"} = $meta;
+        my $attr = Attr->new("attr$i");
+        $meta->attach_attr($attr);
+        my $r = do_post_add_attribute($meta, $attr);
+        push @failures, $i unless defined $r && $r eq "PkgD$i";
+    }
+    is scalar(@failures), 0,
+        'D: 5 metaclasses in %METAS + try/catch + weak ref all succeed';
+}
+
+done_testing;