gh-122854: Add Py_HashBuffer() function (#122855)

Author: vstinner

Doc/c-api/hash.rst

@@ -89,6 +89,25 @@ See also the :c:member:`PyTypeObject.tp_hash` member and :ref:`numeric-hash`.
 
    .. versionadded:: 3.13
 
+
+.. c:function:: Py_hash_t Py_HashBuffer(const void *ptr, Py_ssize_t len)
+
+   Compute and return the hash value of a buffer of *len* bytes
+   starting at address *ptr*. The hash is guaranteed to match that of
+   :class:`bytes`, :class:`memoryview`, and other built-in objects
+   that implement the :ref:`buffer protocol <bufferobjects>`.
+
+   Use this function to implement hashing for immutable objects whose
+   :c:member:`~PyTypeObject.tp_richcompare` function compares to another
+   object's buffer.
+
+   *len* must be greater than or equal to ``0``.
+
+   This function always succeeds.
+
+   .. versionadded:: 3.14
+
+
 .. c:function:: Py_hash_t PyObject_GenericHash(PyObject *obj)
 
    Generic hashing function that is meant to be put into a type

Doc/whatsnew/3.14.rst

@@ -489,6 +489,9 @@ New Features
   similar to ``sep.join(iterable)`` in Python.
   (Contributed by Victor Stinner in :gh:`121645`.)
 
+* Add :c:func:`Py_HashBuffer` to compute and return the hash value of a buffer.
+  (Contributed by Antoine Pitrou and Victor Stinner in :gh:`122854`.)
+
 
 Porting to Python 3.14
 ----------------------

Include/cpython/pyhash.h

@@ -45,3 +45,5 @@ PyAPI_FUNC(PyHash_FuncDef*) PyHash_GetFuncDef(void);
 
 PyAPI_FUNC(Py_hash_t) Py_HashPointer(const void *ptr);
 PyAPI_FUNC(Py_hash_t) PyObject_GenericHash(PyObject *);
+
+PyAPI_FUNC(Py_hash_t) Py_HashBuffer(const void *ptr, Py_ssize_t len);

Include/internal/pycore_pyhash.h

@@ -20,9 +20,6 @@ _Py_HashPointerRaw(const void *ptr)
     return (Py_hash_t)x;
 }
 
-// Export for '_datetime' shared extension
-PyAPI_FUNC(Py_hash_t) _Py_HashBytes(const void*, Py_ssize_t);
-
 /* Hash secret
  *
  * memory layout on 64 bit systems

Lib/test/test_capi/test_hash.py

@@ -78,6 +78,16 @@ def python_hash_pointer(x):
         VOID_P_MAX = -1 & (2 ** (8 * SIZEOF_VOID_P) - 1)
         self.assertEqual(hash_pointer(VOID_P_MAX), -2)
 
+    def test_hash_buffer(self):
+        hash_buffer = _testcapi.hash_buffer
+
+        def check(data):
+            self.assertEqual(hash_buffer(data), hash(data))
+
+        check(b'')
+        check(b'abc')
+        check(b'x' * 1024)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/C_API/2024-08-09-13-12-20.gh-issue-122854.-1OgvU.rst

@@ -0,0 +1,2 @@
+Add :c:func:`Py_HashBuffer` to compute and return the hash value of a buffer.
+Patch by Antoine Pitrou and Victor Stinner.

Modules/_datetimemodule.c

@@ -3842,7 +3842,7 @@ datetime_date_replace_impl(PyDateTime_Date *self, int year, int month,
 static Py_hash_t
 generic_hash(unsigned char *data, int len)
 {
-    return _Py_HashBytes(data, len);
+    return Py_HashBuffer(data, len);
 }
 
 

Modules/_hashopenssl.c

@@ -25,7 +25,6 @@
 #include <stdbool.h>
 #include "Python.h"
 #include "pycore_hashtable.h"
-#include "pycore_pyhash.h"        // _Py_HashBytes()
 #include "pycore_strhex.h"        // _Py_strhex()
 #include "hashlib.h"
 
@@ -186,7 +185,7 @@ static const py_hashentry_t py_hashes[] = {
 
 static Py_uhash_t
 py_hashentry_t_hash_name(const void *key) {
-    return _Py_HashBytes(key, strlen((const char *)key));
+    return Py_HashBuffer(key, strlen((const char *)key));
 }
 
 static int

Modules/_sre/sre.c

@@ -2944,7 +2944,7 @@ pattern_hash(PatternObject *self)
         return -1;
     }
 
-    hash2 = _Py_HashBytes(self->code, sizeof(self->code[0]) * self->codesize);
+    hash2 = Py_HashBuffer(self->code, sizeof(self->code[0]) * self->codesize);
     hash ^= hash2;
 
     hash ^= self->flags;

Modules/_testcapi/hash.c

@@ -45,6 +45,14 @@ hash_getfuncdef(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
 }
 
 
+static PyObject *
+long_from_hash(Py_hash_t hash)
+{
+    Py_BUILD_ASSERT(sizeof(long long) >= sizeof(hash));
+    return PyLong_FromLongLong(hash);
+}
+
+
 static PyObject *
 hash_pointer(PyObject *Py_UNUSED(module), PyObject *arg)
 {
@@ -54,8 +62,21 @@ hash_pointer(PyObject *Py_UNUSED(module), PyObject *arg)
     }
 
     Py_hash_t hash = Py_HashPointer(ptr);
-    Py_BUILD_ASSERT(sizeof(long long) >= sizeof(hash));
-    return PyLong_FromLongLong(hash);
+    return long_from_hash(hash);
+}
+
+
+static PyObject *
+hash_buffer(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    char *ptr;
+    Py_ssize_t len;
+    if (!PyArg_ParseTuple(args, "y#", &ptr, &len)) {
+        return NULL;
+    }
+
+    Py_hash_t hash = Py_HashBuffer(ptr, len);
+    return long_from_hash(hash);
 }
 
 
@@ -64,14 +85,14 @@ object_generichash(PyObject *Py_UNUSED(module), PyObject *arg)
 {
     NULLABLE(arg);
     Py_hash_t hash = PyObject_GenericHash(arg);
-    Py_BUILD_ASSERT(sizeof(long long) >= sizeof(hash));
-    return PyLong_FromLongLong(hash);
+    return long_from_hash(hash);
 }
 
 
 static PyMethodDef test_methods[] = {
     {"hash_getfuncdef", hash_getfuncdef, METH_NOARGS},
     {"hash_pointer", hash_pointer, METH_O},
+    {"hash_buffer", hash_buffer, METH_VARARGS},
     {"object_generichash", object_generichash, METH_O},
     {NULL},
 };

Modules/_xxtestfuzz/fuzzer.c

@@ -15,7 +15,6 @@
 #endif
 
 #include <Python.h>
-#include "pycore_pyhash.h"        // _Py_HashBytes()
 #include <stdlib.h>
 #include <inttypes.h>
 
@@ -45,7 +44,7 @@ static int fuzz_builtin_int(const char* data, size_t size) {
     /* Pick a random valid base. (When the fuzzed function takes extra
        parameters, it's somewhat normal to hash the input to generate those
        parameters. We want to exercise all code paths, so we do so here.) */
-    int base = _Py_HashBytes(data, size) % 37;
+    int base = Py_HashBuffer(data, size) % 37;
     if (base == 1) {
         // 1 is the only number between 0 and 36 that is not a valid base.
         base = 0;

Objects/bytesobject.c

@@ -1598,7 +1598,7 @@ _Py_COMP_DIAG_PUSH
 _Py_COMP_DIAG_IGNORE_DEPR_DECLS
     if (a->ob_shash == -1) {
         /* Can't fail */
-        a->ob_shash = _Py_HashBytes(a->ob_sval, Py_SIZE(a));
+        a->ob_shash = Py_HashBuffer(a->ob_sval, Py_SIZE(a));
     }
     return a->ob_shash;
 _Py_COMP_DIAG_POP

Objects/codeobject.c

@@ -2561,12 +2561,12 @@ hash_const(const void *key)
     if (PySlice_Check(op)) {
         PySliceObject *s = (PySliceObject *)op;
         PyObject *data[3] = { s->start, s->stop, s->step };
-        return _Py_HashBytes(&data, sizeof(data));
+        return Py_HashBuffer(&data, sizeof(data));
     }
     else if (PyTuple_CheckExact(op)) {
         Py_ssize_t size = PyTuple_GET_SIZE(op);
         PyObject **data = _PyTuple_ITEMS(op);
-        return _Py_HashBytes(data, sizeof(PyObject *) * size);
+        return Py_HashBuffer(data, sizeof(PyObject *) * size);
     }
     Py_hash_t h = PyObject_Hash(op);
     if (h == -1) {

Objects/memoryobject.c

@@ -3087,7 +3087,7 @@ memory_hash(PyObject *_self)
         }
 
         /* Can't fail */
-        self->hash = _Py_HashBytes(mem, view->len);
+        self->hash = Py_HashBuffer(mem, view->len);
 
         if (mem != view->buf)
             PyMem_Free(mem);

Objects/unicodeobject.c

@@ -11688,7 +11688,7 @@ unicode_hash(PyObject *self)
     if (hash != -1) {
         return hash;
     }
-    x = _Py_HashBytes(PyUnicode_DATA(self),
+    x = Py_HashBuffer(PyUnicode_DATA(self),
                       PyUnicode_GET_LENGTH(self) * PyUnicode_KIND(self));
 
     FT_ATOMIC_STORE_SSIZE_RELAXED(_PyUnicode_HASH(self), x);

Python/import.c

@@ -1174,7 +1174,7 @@ hashtable_key_from_2_strings(PyObject *str1, PyObject *str2, const char sep)
 static Py_uhash_t
 hashtable_hash_str(const void *key)
 {
-    return _Py_HashBytes(key, strlen((const char *)key));
+    return Py_HashBuffer(key, strlen((const char *)key));
 }
 
 static int

Python/pyhash.c

@@ -22,7 +22,7 @@ extern PyHash_FuncDef PyHash_Func;
 static PyHash_FuncDef PyHash_Func;
 #endif
 
-/* Count _Py_HashBytes() calls */
+/* Count Py_HashBuffer() calls */
 #ifdef Py_HASH_STATS
 #define Py_HASH_STATS_MAX 32
 static Py_ssize_t hashstats[Py_HASH_STATS_MAX + 1] = {0};
@@ -146,9 +146,8 @@ PyObject_GenericHash(PyObject *obj)
 }
 
 Py_hash_t
-_Py_HashBytes(const void *src, Py_ssize_t len)
+Py_HashBuffer(const void *ptr, Py_ssize_t len)
 {
-    Py_hash_t x;
     /*
       We make the hash of the empty string be 0, rather than using
       (prefix ^ suffix), since this slightly obfuscates the hash secret
@@ -161,11 +160,12 @@ _Py_HashBytes(const void *src, Py_ssize_t len)
     hashstats[(len <= Py_HASH_STATS_MAX) ? len : 0]++;
 #endif
 
+    Py_hash_t x;
 #if Py_HASH_CUTOFF > 0
     if (len < Py_HASH_CUTOFF) {
         /* Optimize hashing of very small strings with inline DJBX33A. */
         Py_uhash_t hash;
-        const unsigned char *p = src;
+        const unsigned char *p = ptr;
         hash = 5381; /* DJBX33A starts with 5381 */
 
         switch(len) {
@@ -186,10 +186,13 @@ _Py_HashBytes(const void *src, Py_ssize_t len)
     }
     else
 #endif /* Py_HASH_CUTOFF */
-        x = PyHash_Func.hash(src, len);
+    {
+        x = PyHash_Func.hash(ptr, len);
+    }
 
-    if (x == -1)
+    if (x == -1) {
         return -2;
+    }
     return x;
 }