scalapack: Fix exception when MKLROOT is unset

Fix `scalapack.MKLPkgConfigDependency` not to crash when `MKLROOT`
is unset:

```
  File "/meson/mesonbuild/dependencies/scalapack.py", line 65, in __init__
    super().__init__(name, env, kwargs, language=language)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/meson/mesonbuild/dependencies/pkgconfig.py", line 322, in __init__
    self._set_cargs()
    ~~~~~~~~~~~~~~~^^
  File "/meson/mesonbuild/dependencies/scalapack.py", line 141, in _set_cargs
    cflags = self.pkgconfig.cflags(self.name, allow_system, define_variable=(('prefix', self.__mklroot.as_posix()),))
                                                                                        ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'as_posix'
```

The code is crashing because the `_set_cargs()` method assumes that
`self.__mklroot` will always be set, presumably because it assumes
it will only be called only when `MKLPkgConfigDependency.__init__()`
finishes with `is_found = True`.  However, both `_set_cargs()`
and `_set_libs()` are called during `PkgConfigDependency.__init__()`,
and therefore they will be called if pkg-config lookup succeeds even
without `MKL_ROOT` set.

To avoid the issue, check for `self.__mklroot is None` in both
functions, and raise a `DependencyException` — effectively causing
the pkg-config lookup to fail in a natural way.

Fixes #11172

Signed-off-by: Michał Górny <mgorny@quansight.com>

2 files changed, 9 insertions, 3 deletions

diff --git a/ci/ciimage/arch/install.sh b/ci/ciimage/arch/install.sh
index de43cb2ea..4602d8f32 100755
--- a/ci/ciimage/arch/install.sh
+++ b/ci/ciimage/arch/install.sh
@@ -15,6 +15,7 @@ pkgs=(
   doxygen vulkan-headers vulkan-icd-loader vulkan-validation-layers openssh mercurial gtk-sharp-2 qt5-tools
   libwmf cmake netcdf-fortran openmpi nasm gnustep-base gettext
   python-lxml hotdoc rust-bindgen qt6-base qt6-tools qt6-declarative wayland wayland-protocols
+  intel-oneapi-mkl
   # cuda
 )
 
diff --git a/mesonbuild/dependencies/scalapack.py b/mesonbuild/dependencies/scalapack.py
index c04d1f51f..f34692c25 100644
--- a/mesonbuild/dependencies/scalapack.py
+++ b/mesonbuild/dependencies/scalapack.py
@@ -9,7 +9,7 @@ import os
 import typing as T
 
 from ..options import OptionKey
-from .base import DependencyMethods
+from .base import DependencyException, DependencyMethods
 from .cmake import CMakeDependency
 from .detect import packages
 from .pkgconfig import PkgConfigDependency
@@ -65,8 +65,7 @@ class MKLPkgConfigDependency(PkgConfigDependency):
         super().__init__(name, env, kwargs, language=language)
 
         # Doesn't work with gcc on windows, but does on Linux
-        if (not self.__mklroot or (env.machines[self.for_machine].is_windows()
-                                   and self.clib_compiler.id == 'gcc')):
+        if env.machines[self.for_machine].is_windows() and self.clib_compiler.id == 'gcc':
             self.is_found = False
 
         # This can happen either because we're using GCC, we couldn't find the
@@ -96,6 +95,9 @@ class MKLPkgConfigDependency(PkgConfigDependency):
                 self.version = v
 
     def _set_libs(self) -> None:
+        if self.__mklroot is None:
+            raise DependencyException('MKLROOT not set')
+
         super()._set_libs()
 
         if self.env.machines[self.for_machine].is_windows():
@@ -133,6 +135,9 @@ class MKLPkgConfigDependency(PkgConfigDependency):
             self.link_args.insert(i + 1, '-lmkl_blacs_intelmpi_lp64')
 
     def _set_cargs(self) -> None:
+        if self.__mklroot is None:
+            raise DependencyException('MKLROOT not set')
+
         allow_system = False
         if self.language == 'fortran':
             # gfortran doesn't appear to look in system paths for INCLUDE files,