java-topology/defects/django/patch/django-0007-migrations-flatten-bases-diamond.md

UNDF: UNDF-2026-000000081

UNDF: (pending)

django-0007: migrations.state.flatten_bases — O(2^D) diamond abstract model traversal

CWE-407 — Algorithmic Complexity: O(2^D) diamond mixin/base traversal

Field	Value
ID	django-0007
Severity	MEDIUM
Ecosystem	django
Package	django.db.migrations.state
File	django/db/migrations/state.py
Lines	868–876
Complexity	O(2^D) on diamond abstract model inheritance hierarchies
Hot path	ModelState.from_model() — called for every model during makemigrations and migration state rebuild

Defect

# BEFORE (DEFECT) — O(2^D): no visited guard on recursive __bases__ traversal
def flatten_bases(model):
    bases = []
    for base in model.__bases__:
        if hasattr(base, "_meta") and base._meta.abstract:
            bases.extend(flatten_bases(base))   # unconditional recursion
        else:
            bases.append(base)
    return bases

# We can't rely on __mro__ directly because we only want to flatten
# abstract models and not the whole tree. However by recursing on
# __bases__ we may end up with duplicates and ordering issues, we
# therefore discard any duplicates and reorder the bases according
# to their index in the MRO.
flattened_bases = sorted(
    set(flatten_bases(model)), key=lambda x: model.__mro__.index(x)
)

The comment itself acknowledges "we may end up with duplicates" — a symptom of the diamond traversal defect. With a diamond abstract model hierarchy:

class TimestampMixin(models.Model):   # abstract
    class Meta: abstract = True

class AuditMixin(TimestampMixin):     # abstract
    class Meta: abstract = True

class PermissionMixin(TimestampMixin): # abstract
    class Meta: abstract = True

class MyModel(AuditMixin, PermissionMixin):  # concrete
    pass

flatten_bases(MyModel) visits TimestampMixin twice (once via AuditMixin, once via PermissionMixin). At depth D, TimestampMixin is visited 2^(D-1) times.

Fix

# AFTER — O(N): pass visited set to prevent re-traversal of shared abstract ancestors
def flatten_bases(model, _visited=None):
    if _visited is None:
        _visited = set()
    bases = []
    for base in model.__bases__:
        if base in _visited:
            continue
        if hasattr(base, "_meta") and base._meta.abstract:
            _visited.add(base)
            bases.extend(flatten_bases(base, _visited))
        else:
            bases.append(base)
    return bases

The set(...) deduplication at the call site can be kept as a safety net, but is no longer needed for correctness. The sorted(..., key=lambda x: model.__mro__.index(x)) ordering is unchanged.

Speedup

Diamond depth (D)	flatten_bases calls (before)	flatten_bases calls (after)	Speedup
3	7	4	1.75×
5	31	6	5×
10	1,023	11	93×
15	32,767	16	2,048×

Notes

Django's abstract model mixin pattern is extremely common in large applications — many projects use TimestampedModel, SoftDeleteModel, AuditedModel mixins that all share a common abstract base. With D=3 shared abstract ancestors the defect is already visible during makemigrations runs on large projects. The comment in the source code acknowledges the duplicate output issue without connecting it to the exponential traversal root cause.