octave: mkoctfile doesn't use compiler wrappers (#14726)

* Octave: moved the short description in its own paragraph

* Octave: patch mkoctfile.in.cc to avoid using compiler wrappers

* Added a check to ensure mkoctfile works correctly

var/spack/repos/builtin/packages/octave/helloworld.cc

@@ -0,0 +1,11 @@
+#include <octave/oct.h>
+
+DEFUN_DLD (helloworld, args, nargout,
+           "Hello World Help String")
+{
+  octave_stdout << "Hello World has "
+                << args.length () << " input arguments and "
+                << nargout << " output arguments.\n";
+
+  return octave_value_list ();
+}

var/spack/repos/builtin/packages/octave/package.py

@@ -2,17 +2,23 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-from spack import *
+import os.path
+import shutil
 import sys
+import tempfile
+
+import spack.util.environment
 
 
 class Octave(AutotoolsPackage, GNUMirrorPackage):
     """GNU Octave is a high-level language, primarily intended for numerical
-    computations. It provides a convenient command line interface for solving
-    linear and nonlinear problems numerically, and for performing other
-    numerical experiments using a language that is mostly compatible with
-    Matlab. It may also be used as a batch-oriented language."""
+    computations.
+
+    It provides a convenient command line interface for solving linear and
+    nonlinear problems numerically, and for performing other numerical
+    experiments using a language that is mostly compatible with Matlab.
+    It may also be used as a batch-oriented language.
+    """
 
     homepage = "https://www.gnu.org/software/octave/"
     gnu_mirror_path = "octave/octave-4.0.0.tar.gz"
@@ -89,6 +95,57 @@ class Octave(AutotoolsPackage, GNUMirrorPackage):
     depends_on('suite-sparse', when='+suitesparse')
     depends_on('zlib',         when='+zlib')
 
+    def patch(self):
+        # Filter mkoctfile.in.cc to use underlying compilers and not
+        # Spack compiler wrappers. We are patching the template file
+        # and not mkoctfile.cc since the latter is generated as part
+        # of the build.
+        mkoctfile_in = os.path.join(
+            self.stage.source_path, 'src', 'mkoctfile.in.cc'
+        )
+        quote = lambda s: '"' + s + '"'
+        entries_to_patch = {
+            r'%OCTAVE_CONF_MKOCTFILE_CC%': quote(self.compiler.cc),
+            r'%OCTAVE_CONF_MKOCTFILE_CXX%': quote(self.compiler.cxx),
+            r'%OCTAVE_CONF_MKOCTFILE_F77%': quote(self.compiler.f77),
+            r'%OCTAVE_CONF_MKOCTFILE_DL_LD%': quote(self.compiler.cxx),
+            r'%OCTAVE_CONF_MKOCTFILE_LD_CXX%': quote(self.compiler.cxx)
+        }
+
+        for pattern, subst in entries_to_patch.items():
+            filter_file(pattern, subst, mkoctfile_in)
+
+    @run_after('install')
+    @on_package_attributes(run_tests=True)
+    def check_mkoctfile_works_outside_of_build_env(self):
+        # Check that mkoctfile is properly configured and can compile
+        # Octave extensions outside of the build env
+        mkoctfile = Executable(os.path.join(self.prefix, 'bin', 'mkoctfile'))
+        helloworld_cc = os.path.join(
+            os.path.dirname(__file__), 'helloworld.cc'
+        )
+        tmp_dir = tempfile.mkdtemp()
+        shutil.copy(helloworld_cc, tmp_dir)
+
+        # We need to unset these variables since we are still within
+        # Spack's build environment when running tests
+        vars_to_unset = ['CC', 'CXX', 'F77', 'FC']
+
+        with spack.util.environment.preserve_environment(*vars_to_unset):
+            # Delete temporarily the environment variables that point
+            # to Spack compiler wrappers
+            for v in vars_to_unset:
+                del os.environ[v]
+            # Check that mkoctfile outputs the expected value for CC
+            cc = mkoctfile('-p', 'CC', output=str)
+            msg = "mkoctfile didn't output the expected CC compiler"
+            assert self.compiler.cc in cc, msg
+
+            # Try to compile an Octave extension
+            shutil.copy(helloworld_cc, tmp_dir)
+            with working_dir(tmp_dir):
+                mkoctfile('helloworld.cc')
+
     def configure_args(self):
         # See
         # https://github.com/macports/macports-ports/blob/master/math/octave/