Replace os.system with subprocess.run (HPCSys-Lab#65)

* Replace `os.system` with `subprocess.run`

Besides being recommended by the Python documentation of `os.system()`
itself (https://docs.python.org/3/library/os.html#os.system), the
previous implementation with `os.system()` has the issue of not handling
paths with spaces, since the arguments like `program_path` and
`object_path` were concatenated as strings and then incorrectly parsed
by GCC in the presence of whitespace.

With `subprocess.run()`, we provide a list of arguments and the paths
are handled correctly. A small commented piece of code has been
introduced to build up an arguments list correctly.

* Fix handling of compiler flags with whitespace at the ends

This is relative to the previous commit, and fixes the code in some
instances where the `args` list ended up with empty arguments or
arguments with trailing spaces.

* Fix flake8 lints

Author: gdelazzari

simwave/kernel/backend/compiler.py

@@ -1,4 +1,5 @@
 import os
+import subprocess
 from hashlib import sha1
 
 
@@ -200,25 +201,29 @@ def compile(self, dimension, density, float_precision, operator):
         if os.path.exists(object_path):
             print("Shared object already compiled in:", object_path)
         else:
-            cmd = (
-                self.cc
-                + " "
-                + program_path
-                + " "
-                + self.cflags
-                + " {}".format(float_precision)
-                + language_c
-                + " -o "
-                + object_path
-            )
-
-            print("Compilation command:", cmd)
+            # create arguments list for `subprocess.run`: pay attention to not
+            # providing empty arguments, which the compiler will try to
+            # interpret as source filenames and consequenty fail; moreover
+            # split the compilation flags string to separate arguments to
+            # ensure proper parsing
+            args = [self.cc, program_path]
+            args += [flag.strip()
+                     for flag in self.cflags.split(' ')
+                     if flag.strip() != '']
+            if float_precision.strip() != '':
+                args.append("{}".format(float_precision).strip())
+            if language_c.strip() != '':
+                args.append(language_c.strip())
+            args += ["-o", object_path]
+
+            print("Compilation command:", ' '.join(args))
 
             # create a dir to save the compiled shared object
             os.makedirs(object_dir, exist_ok=True)
 
             # execute the command
-            if os.system(cmd) != 0:
+            result = subprocess.run(args)
+            if result.returncode != 0:
                 raise Exception("Compilation failed")
 
         return object_path