UPC++ 2020.3.0 update (#15623)

## Summary

This PR updates and improves the Spack package for [UPC++](https://upcxx.lbl.gov).
I'm an LBL employee and developer on the UPC++ team, as well as the maintainer of this Spack package.

### Key Improvements:
* Adding new 2020.3.0 release and support for use of develop/master branches
    - Our build infrastructure underwent a major change in this release, switching from a hand-rolled Python2 script to a bash-based autoconf work-alike. 
    - The new build system is NOT using autotools (nor does it support some of the more esoteric autoconf options), but the user interface for common builds is similar.
* Add explicit support for an MPI optional dependency
    - New `mpi` variant enables use of the MPI-based spawner (most relevant on loosely coupled clusters), and the (unofficial) mpi-conduit backend
    - This variant is OFF by default, since UPC++ works fine without MPI on many systems, increasing the likelihood first-time Spack users get a working build without needing to correctly setup MPI
* Add support for post-install testing using the test support deployed in the new build infrastructure
* Fix or workaround a few bugs observed during testing 

### Status

The new package has been validated with a variety of specs across over seven different systems, including: NERSC cori, ALCF Theta, OLCF Summit, an in-house Linux cluster, and macOS laptops (Mojave and Catalina).

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

@@ -10,7 +10,7 @@ def cross_detect():
     if spack.architecture.platform().name == 'cray':
         if which('srun'):
             return 'cray-aries-slurm'
-        if which('alps'):
+        if which('aprun'):
             return 'cray-aries-alps'
     return 'none'
 
@@ -25,9 +25,17 @@ class Upcxx(Package):
     homepage = "https://upcxx.lbl.gov"
     maintainers = ['bonachea']
 
+    git = 'https://bonachea@bitbucket.org/berkeleylab/upcxx.git'
+    version('develop', branch='develop')
+    version('master',  branch='master')
+
+    version('2020.3.0', sha256='01be35bef4c0cfd24e9b3d50c88866521b9cac3ad4cbb5b1fc97aea55078810f')
     version('2019.9.0', sha256='7d67ccbeeefb59de9f403acc719f52127a30801a2c2b9774a1df03f850f8f1d4')
     version('2019.3.2', sha256='dcb0b337c05a0feb2ed5386f5da6c60342412b49cab10f282f461e74411018ad')
 
+    variant('mpi', default=False,
+            description='Enables MPI-based spawners and mpi-conduit')
+
     variant('cuda', default=False,
             description='Builds a CUDA-enabled version of UPC++')
 
@@ -35,10 +43,19 @@ class Upcxx(Package):
             description="UPC++ cross-compile target (autodetect by default)")
 
     conflicts('cross=none', when='platform=cray',
-              msg='None is unacceptable on Cray.')
+              msg='cross=none is unacceptable on Cray.' +
+                  'Please specify an appropriate "cross" value')
 
+    depends_on('mpi', when='+mpi')
     depends_on('cuda', when='+cuda')
-    depends_on('python@2.7.5:2.999', type=("build", "run"))
+    # Require Python2 2.7.5+ up to v2019.9.0
+    depends_on('python@2.7.5:2.999',
+               type=("build", "run"), when='@:2019.9.0')
+    # v2020.3.0 and later also permit Python3
+    depends_on('python@2.7.5:', type=("build", "run"), when='@2020.3.0:')
+
+    # All flags should be passed to the build-env in autoconf-like vars
+    flag_handler = env_flags
 
     def url_for_version(self, version):
         if version > Version('2019.3.2'):
@@ -48,8 +65,14 @@ def url_for_version(self, version):
         return url.format(version)
 
     def setup_build_environment(self, env):
-        if 'platform=cray' in self.spec:
+        # ensure we use the correct python
-            env.set('GASNET_CONFIGURE_ARGS', '--enable-mpi=probe')
+        env.set('UPCXX_PYTHON', self.spec['python'].command.path)
+
+        if '+mpi' in self.spec:
+            env.set('GASNET_CONFIGURE_ARGS',
+                    '--enable-mpi --enable-mpi-compat')
+        else:
+            env.set('GASNET_CONFIGURE_ARGS', '--without-mpicc')
 
         if 'cross=none' not in self.spec:
             env.set('CROSS', self.spec.variants['cross'].value)
@@ -59,6 +82,9 @@ def setup_build_environment(self, env):
             env.set('UPCXX_CUDA_NVCC', self.spec['cuda'].prefix.bin.nvcc)
 
     def setup_run_environment(self, env):
+        # ensure we use the correct python
+        env.set('UPCXX_PYTHON', self.spec['python'].command.path)
+
         env.set('UPCXX_INSTALL', self.prefix)
         env.set('UPCXX', self.prefix.bin.upcxx)
         if 'platform=cray' in self.spec:
@@ -76,7 +102,72 @@ def setup_dependent_build_environment(self, env, dependent_spec):
             env.set('UPCXX_NETWORK', 'aries')
 
     def install(self, spec, prefix):
+        # UPC++ follows autoconf naming convention for LDLIBS, which is 'LIBS'
+        if (env.get('LDLIBS')):
+            env['LIBS'] = env['LDLIBS']
+
+        if spec.version <= Version('2019.9.0'):
             env['CC'] = self.compiler.cc
+            if '+mpi' in self.spec:
+                if 'platform=cray' in self.spec:
+                    env['GASNET_CONFIGURE_ARGS'] += \
+                        " --with-mpicc=" + self.compiler.cc
+                else:
+                    env['CXX'] = spec['mpi'].mpicxx
+            else:
                 env['CXX'] = self.compiler.cxx
             installsh = Executable("./install")
             installsh(prefix)
+        else:
+            if 'platform=cray' in self.spec:
+                # Spack loads the cray-libsci module incorrectly on ALCF theta,
+                # breaking the Cray compiler wrappers
+                # cray-libsci is irrelevant to our build, so disable it
+                for var in ['PE_PKGCONFIG_PRODUCTS', 'PE_PKGCONFIG_LIBS']:
+                    env[var] = ":".join(
+                        filter(lambda x: "libsci" not in x.lower(),
+                               env[var].split(":")))
+                # Undo spack compiler wrappers:
+                # the C/C++ compilers must work post-install
+                # hack above no longer works after the fix to UPC++ issue #287
+                real_cc = join_path(env['CRAYPE_DIR'], 'bin', 'cc')
+                real_cxx = join_path(env['CRAYPE_DIR'], 'bin', 'CC')
+                # workaround a bug in the UPC++ installer: (issue #346)
+                env['GASNET_CONFIGURE_ARGS'] += \
+                    " --with-cc=" + real_cc + " --with-cxx=" + real_cxx
+                if '+mpi' in self.spec:
+                    env['GASNET_CONFIGURE_ARGS'] += " --with-mpicc=" + real_cc
+            else:
+                real_cc = self.compiler.cc
+                real_cxx = self.compiler.cxx
+                if '+mpi' in self.spec:
+                    real_cxx = spec['mpi'].mpicxx
+
+            env['CC'] = real_cc
+            env['CXX'] = real_cxx
+
+            installsh = Executable("./configure")
+            installsh('--prefix=' + prefix)
+
+            make()
+
+            make('install')
+
+    @run_after('install')
+    @on_package_attributes(run_tests=True)
+    def test_install(self):
+        if self.spec.version <= Version('2019.9.0'):
+            spack.main.send_warning_to_tty(
+                "run_tests not supported in UPC++ version " +
+                self.spec.version.string + " -- SKIPPED")
+        else:
+            # enable testing of unofficial conduits (mpi)
+            test_networks = 'NETWORKS=$(CONDUITS)'
+            # build hello world against installed tree in all configurations
+            make('test_install', test_networks)
+            make('tests-clean')  # cleanup
+            # build all tests for all networks in debug mode
+            make('tests', test_networks)
+            if 'cross=none' in self.spec:
+                make('run-tests', 'NETWORKS=smp')  # runs tests for smp backend
+            make('tests-clean')  # cleanup