allow selecting backend with PYZMQ_BACKEND at build time

Author: minrk

.github/workflows/test.yml

@@ -82,6 +82,10 @@ jobs:
           - os: ubuntu-24.04
             python: "3.12"
 
+          - os: ubuntu-24.04
+            python: "3.13"
+            backend: cffi
+
           - os: ubuntu-24.04
             python: "3.13"
 
@@ -193,6 +197,11 @@ jobs:
         run: |
           echo "ZMQ_PREFIX=${{ matrix.zmq }}" >> "$GITHUB_ENV"
 
+      - name: set $PYZMQ_BACKEND
+        if: matrix.backend
+        run: |
+          echo "PYZMQ_BACKEND=${{ matrix.backend }}" >> "$GITHUB_ENV"
+
       - name: install libzmq-dev
         if: matrix.zmq == 'head'
         run: |

CMakeLists.txt

@@ -29,6 +29,7 @@ set(PYZMQ_LIBSODIUM_URL "" CACHE STRING "full URL to download bundled libsodium"
 set(PYZMQ_LIBSODIUM_CONFIGURE_ARGS "" CACHE STRING "semicolon-separated list of arguments to pass to ./configure for bundled libsodium")
 set(PYZMQ_LIBSODIUM_MSBUILD_ARGS "" CACHE STRING "semicolon-separated list of arguments to pass to msbuild for bundled libsodium")
 set(PYZMQ_LIBSODIUM_VS_VERSION "" CACHE STRING "Visual studio solution version for bundled libsodium (default: detect from MSVC_VERSION)")
+set(PYZMQ_BACKEND "" CACHE STRING "pyzmq backend to build ('cffi' or 'cython'). Default: cffi on PyPy, else Cython.")
 
 if (NOT CMAKE_BUILD_TYPE)
   # default to Release
@@ -57,6 +58,7 @@ foreach(_optname
   PYZMQ_LIBSODIUM_CONFIGURE_ARGS
   PYZMQ_LIBSODIUM_MSBUILD_ARGS
   PYZMQ_LIBSODIUM_VS_VERSION
+  PYZMQ_BACKEND
 )
   if (DEFINED ENV{${_optname}})
     if (_optname MATCHES ".*_ARGS")
@@ -367,8 +369,19 @@ set(EXT_SRC_DIR "${CMAKE_CURRENT_BINARY_DIR}/_src")
 set(ZMQ_BUILDUTILS "${CMAKE_CURRENT_SOURCE_DIR}/buildutils")
 file(MAKE_DIRECTORY "${EXT_SRC_DIR}")
 
-if(Python_INTERPRETER_ID STREQUAL "PyPy")
+if (NOT PYZMQ_BACKEND)
+  if(Python_INTERPRETER_ID STREQUAL "PyPy")
+    set(PYZMQ_BACKEND "cffi")
+  else()
+    set(PYZMQ_BACKEND "cython")
+  endif()
+endif()
+
+if(PYZMQ_BACKEND STREQUAL "cffi")
   message(STATUS "Building CFFI backend")
+  if(NOT Python_INTERPRETER_ID STREQUAL "PyPy")
+    message(WARNING "Building CFFI backend on ${Python_INTERPRETER_ID}, not PyPy. This is not supported and may not work.")
+  endif()
   set(ZMQ_EXT_NAME "_cffi")
 
   set(ZMQ_BACKEND_DEST "zmq/backend/cffi")
@@ -381,8 +394,11 @@ if(Python_INTERPRETER_ID STREQUAL "PyPy")
             "${ZMQ_BUILDUTILS}/build_cffi.py"
             "${ZMQ_C}"
   )
-else()
+elseif(PYZMQ_BACKEND STREQUAL "cython")
   message(STATUS "Building Cython backend")
+  if(NOT Python_INTERPRETER_ID STREQUAL "Python")
+    message(WARNING "Building Cython backend on ${Python_INTERPRETER_ID}, not CPython. This is not supported and may not work.")
+  endif()
   find_program(CYTHON "cython")
 
   set(ZMQ_BACKEND_DEST "zmq/backend/cython")
@@ -400,6 +416,8 @@ else()
             --module-name "zmq.backend.cython._zmq"
             ${ZMQ_PYX}
   )
+else()
+  message(FATAL_ERROR "Unsupported PYZMQ_BACKEND=${PYZMQ_BACKEND}. Must be 'cffi' or 'cython'.")
 endif()
 
 file(MAKE_DIRECTORY ${ZMQ_BACKEND_DEST})

pyproject.toml

@@ -4,7 +4,7 @@ requires = [
   "cffi; implementation_name == 'pypy'",
   "cython>=3.0.0; implementation_name != 'pypy'",
   "packaging",
-  "scikit-build-core",
+  "scikit-build-core>=0.10",
 ]
 build-backend = "scikit_build_core.build"
 
@@ -55,9 +55,13 @@ wheel.license-files = ["licenses/LICENSE*"]
 # 3.15 is required by scikit-build-core
 cmake.version = ">=3.15"
 # only build/install the pyzmq component
-cmake.targets = ["pyzmq"]
+build.targets = ["pyzmq"]
 install.components = ["pyzmq"]
 
+[[tool.scikit-build.overrides]]
+if.env.PYZMQ_BACKEND = "cffi"
+build.requires = ["cffi"]
+
 [tool.ruff]
 
 [tool.ruff.format]