From 3a4ca49b4b49b77501955dfa5a04191e67822216 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C5=81ukasz=20Langa?= <lukasz@langa.pl>
Date: Mon, 23 May 2022 23:11:19 +0200
Subject: [PATCH] [3.7] gh-93065: Fix HAMT to iterate correctly over 7-level
deep trees (GH-93149)
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Also while there, clarify a few things about why we reduce the hash to 32 bits.
Co-authored-by: Eli Libman <eli@hyro.ai>
Co-authored-by: Yury Selivanov <yury@edgedb.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>
(cherry picked from commit c1f5c903a7e4ed27190488f4e33b00d3c3d952e5)
---
Include/internal/hamt.h | 14 +++++++-
Lib/test/test_context.py | 35 +++++++++++++++++++
Misc/ACKS | 2 ++
...2-05-21-23-21-37.gh-issue-93065.5I18WC.rst | 5 +++
Python/hamt.c | 14 ++++++--
5 files changed, 66 insertions(+), 4 deletions(-)
create mode 100644 Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst
diff --git a/Include/internal/hamt.h b/Include/internal/hamt.h
index 29ad28b1d87..6b396f0626d 100644
--- a/Include/internal/hamt.h
+++ b/Include/internal/hamt.h
@@ -2,7 +2,19 @@
#define Py_INTERNAL_HAMT_H
-#define _Py_HAMT_MAX_TREE_DEPTH 7
+
+/*
+HAMT tree is shaped by hashes of keys. Every group of 5 bits of a hash denotes
+the exact position of the key in one level of the tree. Since we're using
+32 bit hashes, we can have at most 7 such levels. Although if there are
+two distinct keys with equal hashes, they will have to occupy the same
+cell in the 7th level of the tree -- so we'd put them in a "collision" node.
+Which brings the total possible tree depth to 8. Read more about the actual
+layout of the HAMT tree in `hamt.c`.
+
+This constant is used to define a datastucture for storing iteration state.
+*/
+#define _Py_HAMT_MAX_TREE_DEPTH 8
#define PyHamt_Check(o) (Py_TYPE(o) == &_PyHamt_Type)
diff --git a/Lib/test/test_context.py b/Lib/test/test_context.py
index b9e991a4000..b954b135dd6 100644
--- a/Lib/test/test_context.py
+++ b/Lib/test/test_context.py
@@ -537,6 +537,41 @@ def test_hamt_collision_1(self):
self.assertEqual(len(h4), 2)
self.assertEqual(len(h5), 3)
+ def test_hamt_collision_3(self):
+ # Test that iteration works with the deepest tree possible.
+ # https://github.com/python/cpython/issues/93065
+
+ C = HashKey(0b10000000_00000000_00000000_00000000, 'C')
+ D = HashKey(0b10000000_00000000_00000000_00000000, 'D')
+
+ E = HashKey(0b00000000_00000000_00000000_00000000, 'E')
+
+ h = hamt()
+ h = h.set(C, 'C')
+ h = h.set(D, 'D')
+ h = h.set(E, 'E')
+
+ # BitmapNode(size=2 count=1 bitmap=0b1):
+ # NULL:
+ # BitmapNode(size=2 count=1 bitmap=0b1):
+ # NULL:
+ # BitmapNode(size=2 count=1 bitmap=0b1):
+ # NULL:
+ # BitmapNode(size=2 count=1 bitmap=0b1):
+ # NULL:
+ # BitmapNode(size=2 count=1 bitmap=0b1):
+ # NULL:
+ # BitmapNode(size=2 count=1 bitmap=0b1):
+ # NULL:
+ # BitmapNode(size=4 count=2 bitmap=0b101):
+ # <Key name:E hash:0>: 'E'
+ # NULL:
+ # CollisionNode(size=4 id=0x107a24520):
+ # <Key name:C hash:2147483648>: 'C'
+ # <Key name:D hash:2147483648>: 'D'
+
+ self.assertEqual({k.name for k in h.keys()}, {'C', 'D', 'E'})
+
def test_hamt_stress(self):
COLLECTION_SIZE = 7000
TEST_ITERS_EVERY = 647
diff --git a/Misc/ACKS b/Misc/ACKS
index a893fdce0e9..bd62c958c5d 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -963,6 +963,8 @@ Robert Li
Xuanji Li
Zekun Li
Zheao Li
+Eli Libman
+Dan Lidral-Porter
Robert van Liere
Ross Light
Shawn Ligocki
diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst b/Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst
new file mode 100644
index 00000000000..ea801653f75
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst
@@ -0,0 +1,5 @@
+Fix contextvars HAMT implementation to handle iteration over deep trees.
+
+The bug was discovered and fixed by Eli Libman. See
+`MagicStack/immutables#84 <https://github.com/MagicStack/immutables/issues/84>`_
+for more details.
diff --git a/Python/hamt.c b/Python/hamt.c
index 6bcdfac15fd..acacf639d40 100644
--- a/Python/hamt.c
+++ b/Python/hamt.c
@@ -406,14 +406,22 @@ hamt_hash(PyObject *o)
return -1;
}
- /* While it's suboptimal to reduce Python's 64 bit hash to
+ /* While it's somewhat suboptimal to reduce Python's 64 bit hash to
32 bits via XOR, it seems that the resulting hash function
is good enough (this is also how Long type is hashed in Java.)
Storing 10, 100, 1000 Python strings results in a relatively
shallow and uniform tree structure.
- Please don't change this hashing algorithm, as there are many
- tests that test some exact tree shape to cover all code paths.
+ Also it's worth noting that it would be possible to adapt the tree
+ structure to 64 bit hashes, but that would increase memory pressure
+ and provide little to no performance benefits for collections with
+ fewer than billions of key/value pairs.
+
+ Important: do not change this hash reducing function. There are many
+ tests that need an exact tree shape to cover all code paths and
+ we do that by specifying concrete values for test data's `__hash__`.
+ If this function is changed most of the regression tests would
+ become useless.
*/
int32_t xored = (int32_t)(hash & 0xffffffffl) ^ (int32_t)(hash >> 32);
return xored == -1 ? -2 : xored;
--
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