BLD: special: fix dependencies for _ellip_harm_2

[mathstuf]

Generating the source for _ellip_harm_2 requires the cython_special module to already be compiled. Add a dependency for this. Note that it now must be a custom target because generators do not support extra dependencies.

Fixes: #19167

Reference issue

Closes gh-19167

What does this implement/fix?

Fix dependencies for _ellip_harm_2 Cython generation.

Additional information

[mathstuf]

Cc: @rgommers

[mathstuf]

Well, one from-scratch build succeeded here, so any other missing dependencies seem to be masked by available parallelism on the test machine (the only one that found the problem).

[rgommers]

Thanks @mathstuf. This does fix the problem, but wouldn't this more minimal fix do?

diff --git a/scipy/special/meson.build b/scipy/special/meson.build
index e7e25aa54..900064410 100644
--- a/scipy/special/meson.build
+++ b/scipy/special/meson.build
@@ -318,12 +318,12 @@ cython_special_pxd = custom_target('_dummy_cython_special.pxd',
 uf_cython_gen = generator(cython,
   arguments : cython_args,
   output : '@BASENAME@.c',
-  depends : [_cython_tree, _ufuncs_pxi_pxd_sources])
+  depends : [_cython_tree, _ufuncs_pxi_pxd_sources, cython_special_pxd])
 
 uf_cython_gen_cpp = generator(cython,
   arguments : cython_cplus_args,
   output : '@BASENAME@.cpp',
-  depends : [_cython_tree, _ufuncs_pxi_pxd_sources])
+  depends : [_cython_tree, _ufuncs_pxi_pxd_sources, cython_special_pxd])
 
 py3.extension_module('_ufuncs',
   [

That's analogous to how it's already done in linalg/meson.build and optimize/meson.build. I just tested two builds, with -j64 and -j1, as well as only building _ellip_harm_2.so, and that is all happy.

[mathstuf]

That does look better. I think I had first tried that with cython_special and got told "no" because of it being a CustomTarget or something. I didn't go back and try it with the _pxd object.

[rgommers]

In it goes. Thanks @mathstuf!