Add support for free-threaded build

.github/workflows/cibuildwheel.yml

@@ -14,7 +14,6 @@ jobs:
       contents: write
       actions: read
     with:
-      pure-python: false
+      wheel-name-pattern: 'cymem-*.whl'
     secrets:
       gh-token: ${{ secrets.GITHUB_TOKEN }}
-

.github/workflows/tests.yml

@@ -24,7 +24,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        python_version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
+        python_version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14", "3.14t"]
         exclude:
           # The 3.9/3.10 arm64 Python binaries that `setup-python` tries to use
           # don't work on macOS 15 anymore, so skip those.

README.md

@@ -195,3 +195,28 @@ objects, but we'd still like the laziness of the `Pool`.
 from cymem.cymem cimport Pool, WrapMalloc, WrapFree
 cdef Pool mem = Pool(WrapMalloc(priv_malloc), WrapFree(priv_free))
 ```
+
+## Thread Safety
+
+As of version 2.0.12, `cymem.Pool` is thread-safe when used with CPython 3.13+
+free-threaded builds (PEP 703). All operations on the Pool, including `alloc()`,
+`free()`, and `realloc()`, can be safely called from multiple threads concurrently.
+
+**Key guarantees:**
+- Multiple threads can safely call `alloc()`, `free()`, and `realloc()` on the
+    same `Pool` instance.
+- The Pool's internal bookkeeping (`addresses` dict and `size` accounting) is
+    protected from race conditions. Reading the internal state without
+    holding a lock on the `Pool` instance via a critical section is not
+    thread-safe.
+
+**Important notes:**
+- Individual Pool instances are thread-safe, but you are still responsible for
+    proper synchronization when accessing the memory contents themselves. Note
+    that holding a lock on the Pool itself is typically not the right approach:
+    since malloc is thread-safe, memory allocated by separate calls to `alloc`
+    can be safely accessed concurrently without locking. You should only
+    synchronize access to specific memory regions that are being shared across
+    threads, using fine-grained locks appropriate to your use case rather than a
+    coarse-grained lock on the entire `Pool`.
+- Custom memory allocators need to be thread-safe themselves.

cymem/cymem.pyx

@@ -1,7 +1,7 @@
-# cython: embedsignature=True
+# cython: embedsignature=True, freethreading_compatible=True
 
+cimport cython
 from cpython.mem cimport PyMem_Malloc, PyMem_Free
-from cpython.ref cimport Py_INCREF, Py_DECREF
 from libc.string cimport memset
 from libc.string cimport memcpy
 import warnings
@@ -44,17 +44,29 @@ cdef class Pool:
         addresses (dict): The currently allocated addresses and their sizes. Read-only.
         pymalloc (PyMalloc): The allocator to use (default uses PyMem_Malloc).
         pyfree (PyFree): The free to use (default uses PyMem_Free).
+
+    Thread-safety:
+        All public methods in this class can be safely called from multiple threads.
+        Testing the thread-safety of this class can be done by checking out the repository
+        at https://github.com/lysnikolaou/test-cymem-threadsafety and following the
+        instructions on the README.
     """
 
     def __cinit__(self, PyMalloc pymalloc=Default_Malloc,
                   PyFree pyfree=Default_Free):
+        # size, addresses and refs are mutable. note that operations on
+        # dicts and lists are atomic.
         self.size = 0
         self.addresses = {}
         self.refs = []
+
+        # pymalloc and pyfree are immutable.
         self.pymalloc = pymalloc
         self.pyfree = pyfree
 
     def __dealloc__(self):
+        # No need for locking here since the methods will be unreachable
+        # by the time __dealloc__ is called
         cdef size_t addr
         if self.addresses is not None:
             for addr in self.addresses:
@@ -73,28 +85,54 @@ cdef class Pool:
         if p == NULL:
             raise MemoryError("Error assigning %d bytes" % (number * elem_size))
         memset(p, 0, number * elem_size)
-        self.addresses[<size_t>p] = number * elem_size
-        self.size += number * elem_size
+
+        # We need a critical section here so that addresses and size get
+        # updated atomically. If we were to acquire a critical section on self,
+        # mutating the dictionary would try to acquire a critical section on
+        # the dictionary and therefore release the critical section on self.
+        # Acquiring a critical section on self.addresses works because that's
+        # the only C API that gets called inside the block and acquiring the
+        # critical section on the top-most held lock does not release it.
+        with cython.critical_section(self.addresses):
+            self.addresses[<size_t>p] = number * elem_size
+            self.size += number * elem_size
         return p
 
     cdef void* realloc(self, void* p, size_t new_size) except NULL:
         """Resizes the memory block pointed to by p to new_size bytes, returning
         a non-NULL pointer to the new block. new_size must be larger than the
         original.
 
-        If p is not in the Pool or new_size is 0, a MemoryError is raised.
+        If p is not in the Pool or new_size isn't larger than the previous size,
+        a ValueError is raised.
         """
-        if <size_t>p not in self.addresses:
-            raise ValueError("Pointer %d not found in Pool %s" % (<size_t>p, self.addresses))
-        if new_size == 0:
-            raise ValueError("Realloc requires new_size > 0")
-        assert new_size > self.addresses[<size_t>p]
-        cdef void* new_ptr = self.alloc(1, new_size)
+        cdef size_t old_size
+        cdef void* new_ptr
+
+        new_ptr = self.pymalloc.malloc(new_size)
         if new_ptr == NULL:
             raise MemoryError("Error reallocating to %d bytes" % new_size)
-        memcpy(new_ptr, p, self.addresses[<size_t>p])
-        self.free(p)
-        self.addresses[<size_t>new_ptr] = new_size
+
+        # See comment in alloc on why we're acquiring a critical section on
+        # self.addresses instead of self.
+        with cython.critical_section(self.addresses):
+            try:
+                old_size = self.addresses.pop(<size_t>p)
+            except KeyError:
+                self.pyfree.free(new_ptr)
+                raise ValueError("Pointer %d not found in Pool %s" % (<size_t>p, self.addresses))
+
+            if old_size >= new_size:
+                self.addresses[<size_t>p] = old_size
+                self.pyfree.free(new_ptr)
+                raise ValueError("Realloc requires new_size > previous size")
+
+            memcpy(new_ptr, p, old_size)
+            memset(<char*> new_ptr + old_size, 0, new_size - old_size)
+            self.size += new_size - old_size
+            self.addresses[<size_t>new_ptr] = new_size
+
+        self.pyfree.free(p)
         return new_ptr
 
     cdef void free(self, void* p) except *:
@@ -105,10 +143,14 @@ cdef class Pool:
 
         If p is not in Pool.addresses, a KeyError is raised.
         """
-        self.size -= self.addresses.pop(<size_t>p)
+        # See comment in alloc on why we're acquiring a critical section on
+        # self.addresses instead of self.
+        with cython.critical_section(self.addresses):
+            self.size -= <size_t>self.addresses.pop(<size_t>p)
         self.pyfree.free(p)
 
     def own_pyref(self, object py_ref):
+        # Calling append here is atomic, no critical section needed.
         self.refs.append(py_ref)
 
 
@@ -142,9 +184,9 @@ cdef class Address:
             raise MemoryError("Error assigning %d bytes" % number * elem_size)
         memset(self.ptr, 0, number * elem_size)
 
-    property addr:
-        def __get__(self):
-            return <size_t>self.ptr
+    @property
+    def addr(self):
+        return <size_t>self.ptr
 
     def __dealloc__(self):
         if self.ptr != NULL:

pyproject.toml

@@ -1,15 +1,15 @@
 [build-system]
 requires = [
     "setuptools",
-    "cython>=0.25",
+    "cython>=3.1",
 ]
 build-backend = "setuptools.build_meta"
 
 [tool.cibuildwheel]
 build = "*"
 skip = "pp* cp36* cp37* cp38*"
 test-skip = ""
-free-threaded-support = false
+enable = ["cpython-freethreading"]
 
 archs = ["native"]
 

requirements.txt

@@ -1,2 +1,2 @@
-cython>=0.25
+cython>=3.1
 pytest

setup.py

@@ -100,8 +100,8 @@ def setup_package():
             version=about["__version__"],
             url=about["__uri__"],
             license=about["__license__"],
-            ext_modules=cythonize(ext_modules, language_level=2),
-            setup_requires=["cython>=0.25"],
+            ext_modules=cythonize(ext_modules),
+            setup_requires=["cython>=3.1"],
             classifiers=[
                 "Environment :: Console",
                 "Intended Audience :: Developers",
@@ -116,6 +116,8 @@ def setup_package():
                 "Programming Language :: Python :: 3.11",
                 "Programming Language :: Python :: 3.12",
                 "Programming Language :: Python :: 3.13",
+                "Programming Language :: Python :: 3.14",
+                "Programming Language :: Python :: Free Threading :: 2 - Beta",
                 "Topic :: Scientific/Engineering",
             ],
             cmdclass={"build_ext": build_ext_subclass},