This is a test of meson-python attempting to build a project with a Cython extension module that depends on GMP. It tries to use a system GMP if installed with a new enough version or otherwise uses a meson subproject to download and build GMP.

The intention is to satisfy these uses:

• It should be possible to setup a development environment from a VCS checkout ideally in editable mode.
• It should be possible to build/install from an sdist
• It should be possible to build wheels in CI and those should be relocatable.

When installing from VCS/sdist there are two further cases:

• The system already has GMP installed and the extension module should link to that.
• The system does not have GMP or does not have a new enough version of GMP and then ideally meson would download and build GMP.

In the case of the VCS checkout and editable mode we want to have incremental rebuilds so e.g. each rebuild of the extension module should be able to reuse the existing build of GMP.

Since GMP is has an autotools build system we use meson's unstable-external_project feature which handles a configure then make setup. We need to provide a subprojects/packagefiles/meson.build which tells meson how to do this. We use a wrap-file stub to tell meson to download the code for GMP.

So far this does not succeed in buliding a wheel.

The extension module can be built with:

meson setup build
ninja -C build

That will produce the extension module linking against system GMP if it is provided. Otherwise it downloads, configures and builds GMP and then makes an extension module linking against the locally built GMP. The built files for GMP look like:

 tree build/subprojects/gmp-6.2.1/dist/
build/subprojects/gmp-6.2.1/dist/
└── usr
    └── local
        ├── include
        │   └── gmp.h
        ├── lib
        │   └── x86_64-linux-gnu
        │       ├── libgmp.la
        │       ├── libgmp.so -> libgmp.so.10.4.1
        │       ├── libgmp.so.10 -> libgmp.so.10.4.1
        │       ├── libgmp.so.10.4.1
        │       └── pkgconfig
        │           └── gmp.pc
        └── share
            └── info
                ├── dir
                ├── gmp.info
                ├── gmp.info-1
                └── gmp.info-2

8 directories, 10 files

If we cd into the build directory we can import and use the extension module and call gmp functions:

$ cd build/
$ python
Python 3.11.3 (main, Apr  5 2023, 23:03:48) [GCC 11.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import meson_test
>>> import meson_test._meson_test
>>> meson_test._meson_test.pow1000(2)
"b'10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376'"
>>> 2**1000
10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376

So everything built and seems to work correctly.

We can also build a wheel when the system has GMP installed:

$ sudo apt-get install libgmp3-dev
...
The following additional packages will be installed:
  libgmp-dev libgmpxx4ldbl
...

Now building a wheel succeeds:

$ python -m build
...
Run-time dependency gmp found: YES 6.2.1
...
Copying files to wheel...
[0/0] meson_test.cpython-311-x86_64-linux-gnu.soo
Successfully built meson_test-0.0.1.tar.gz and meson_test-0.0.1-cp311-cp311-linux_x86_64.whl

The wheel can be installed and the extension module works with the system GMP at runtime.

What fails is building a wheel with mesonpy when there is no system GMP installed:

$ sudo apt-get remove libgmp3-dev libgmp-dev libgmpxx4ldbl
$ python -m build
...

...
meson-python: error: Could not map installation path to an equivalent wheel
directory: '{prefix}'

Everything builds correctly but mesonpy does not understand how to package the artifacts into a wheel.

The same is seen if using mesonpy without build isolation:

$ python -c 'import mesonpy; mesonpy.build_wheel(".")'
...
meson-python: error: Could not map installation path to an equivalent wheel
directory: '{prefix}'

Without build isolation though I can patch mesonpy with this diff:

--- __init__.py.backup	2023-04-23 11:50:55.660441459 +0100
+++ __init__.py	2023-04-23 11:51:03.836722723 +0100
@@ -160,7 +160,7 @@ def _map_to_wheel(sources: Dict[str, Dic

             path = _INSTALLATION_PATH_MAP.get(anchor)
             if path is None:
-                raise BuildError(f'Could not map installation path to an equivalent wheel directory: {str(destination)!r}')
+                continue

             if path == 'purelib' or path == 'platlib':
                 package = destination.parts[1]

Now it builds with

$ python -c 'import mesonpy; mesonpy.build_wheel("dist")'
...
Copying files to wheel...
 [0/1] meson_test/_meson_test.cpython-311-x86_64-linux-gnu.so
 [1/1] meson_test/__init__.pyy

The resulting wheel does not contain libgmp.so:

$ cd dist
$ unzip meson_test-0.0.1-cp311-cp311-linux_x86_64.whl
Archive:  meson_test-0.0.1-cp311-cp311-linux_x86_64.whl
 extracting: meson_test-0.0.1.dist-info/METADATA
 extracting: meson_test-0.0.1.dist-info/WHEEL
 extracting: meson_test/_meson_test.cpython-311-x86_64-linux-gnu.so
 extracting: meson_test/__init__.py
 extracting: meson_test-0.0.1.dist-info/RECORD
$ tree
.
├── meson_test
│   ├── __init__.py
│   └── _meson_test.cpython-311-x86_64-linux-gnu.so
├── meson_test-0.0.1-cp311-cp311-linux_x86_64.whl
└── meson_test-0.0.1.dist-info
    ├── METADATA
    ├── RECORD
    └── WHEEL

2 directories, 6 files

After installing it seems to work though:

$ python
Python 3.11.3 (main, Apr  5 2023, 23:03:48) [GCC 11.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import meson_test
>>> meson_test.pow1000
<built-in function pow1000>
>>> meson_test.pow1000(2)
"b'10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376'"

I'm not sure why that works. Maybe it linked GMP statically?