[Python-checkins] gh-93065: Fix HAMT to iterate correctly over 7-level deep trees (GH-93066) (#93147)

[ambv]

https://github.com/python/cpython/commit/95c9c2b9cb2d3c1d29c8ce77f154de8bd5313dae
commit: 95c9c2b9cb2d3c1d29c8ce77f154de8bd5313dae
branch: 3.9
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: ambv <lukasz at langa.pl>
date: 2022-05-24T10:52:49+02:00
summary:

gh-93065: Fix HAMT to iterate correctly over 7-level deep trees (GH-93066) (#93147)

Also while there, clarify a few things about why we reduce the hash to 32 bits.

Co-authored-by: Eli Libman <eli at hyro.ai>
Co-authored-by: Yury Selivanov <yury at edgedb.com>
Co-authored-by: Łukasz Langa <lukasz at langa.pl>

(cherry picked from commit c1f5c903a7e4ed27190488f4e33b00d3c3d952e5)

files:
A Misc/NEWS.d/next/Core and Builtins/2022-05-21-23-21-37.gh-issue-93065.5I18WC.rst
M Include/internal/pycore_hamt.h
M Lib/test/test_context.py
M Misc/ACKS
M Python/hamt.c

diff --git a/Include/internal/pycore_hamt.h b/Include/internal/pycore_hamt.h
index aaf655909551a..357d96616272a 100644
--- a/Include/internal/pycore_hamt.h
+++ b/Include/internal/pycore_hamt.h
@@ -5,7 +5,19 @@
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
-#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_IS_TYPE(o, &_PyHamt_Type)
diff --git a/Lib/test/test_context.py b/Lib/test/test_context.py
index 2d8b63a1f5958..689e3d4dc4591 100644
--- a/Lib/test/test_context.py
+++ b/Lib/test/test_context.py
@@ -533,6 +533,41 @@ 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 a9f15b4f96726..c6e7c3a0ddf95 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1033,6 +1033,7 @@ Robert Li
 Xuanji Li
 Zekun Li
 Zheao Li
+Eli Libman
 Dan Lidral-Porter
 Robert van Liere
 Ross Light
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 0000000000000..ea801653f7502
--- /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 8801c5ea418c7..3296109f7265b 100644
--- a/Python/hamt.c
+++ b/Python/hamt.c
@@ -407,14 +407,22 @@ 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;