Refactor and generalize run_c

Generalize the very ad hoc run_c function into a function to generate
a C program to print the value of a list of expressions. Refactor the
code into several functions to make it more manageable.

No intended behavior change.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/scripts/test_psa_constant_names.py

@@ -313,40 +313,97 @@ def remove_file_if_exists(filename):
     except OSError:
         pass
 
-def run_c(type_word, expressions, include_path=None, keep_c=False):
-    """Generate and run a program to print out numerical values for expressions."""
-    if include_path is None:
-        include_path = []
-    if type_word == 'status':
-        cast_to = 'long'
-        printf_format = '%ld'
-    else:
-        cast_to = 'unsigned long'
-        printf_format = '0x%08lx'
-    c_name = None
-    exe_name = None
-    try:
-        c_fd, c_name = tempfile.mkstemp(prefix='tmp-{}-'.format(type_word),
-                                        suffix='.c',
-                                        dir='programs/psa')
-        exe_suffix = '.exe' if platform.system() == 'Windows' else ''
-        exe_name = c_name[:-2] + exe_suffix
-        remove_file_if_exists(exe_name)
-        c_file = os.fdopen(c_fd, 'w', encoding='ascii')
-        c_file.write('/* Generated by test_psa_constant_names.py for {} values */'
-                     .format(type_word))
-        c_file.write('''
+def create_c_file(file_label):
+    """Create a temporary C file.
+
+    * ``file_label``: a string that will be included in the file name.
+
+    Return ```(c_file, c_name, exe_name)``` where ``c_file`` is a Python
+    stream open for writing to the file, ``c_name`` is the name of the file
+    and ``exe_name`` is the name of the executable that will be produced
+    by compiling the file.
+    """
+    c_fd, c_name = tempfile.mkstemp(prefix='tmp-{}-'.format(file_label),
+                                    suffix='.c')
+    exe_suffix = '.exe' if platform.system() == 'Windows' else ''
+    exe_name = c_name[:-2] + exe_suffix
+    remove_file_if_exists(exe_name)
+    c_file = os.fdopen(c_fd, 'w', encoding='ascii')
+    return c_file, c_name, exe_name
+
+def generate_c_printf_expressions(c_file, cast_to, printf_format, expressions):
+    """Generate C instructions to print the value of ``expressions``.
+
+    Write the code with ``c_file``'s ``write`` method.
+
+    Each expression is cast to the type ``cast_to`` and printed with the
+    printf format ``printf_format``.
+    """
+    for expr in expressions:
+        c_file.write('    printf("{}\\n", ({}) {});\n'
+                     .format(printf_format, cast_to, expr))
+
+def generate_c_file(c_file,
+                    caller, header,
+                    main_generator):
+    """Generate a temporary C source file.
+
+    * ``c_file`` is an open stream on the C source file.
+    * ``caller``: an informational string written in a comment at the top
+      of the file.
+    * ``header``: extra code to insert before any function in the generated
+      C file.
+    * ``main_generator``: a function called with ``c_file`` as its sole argument
+      to generate the body of the ``main()`` function.
+    """
+    c_file.write('/* Generated by {} */'
+                 .format(caller))
+    c_file.write('''
 #include <stdio.h>
-#include <psa/crypto.h>
+''')
+    c_file.write(header)
+    c_file.write('''
 int main(void)
 {
 ''')
-        for expr in expressions:
-            c_file.write('    printf("{}\\n", ({}) {});\n'
-                         .format(printf_format, cast_to, expr))
-        c_file.write('''    return 0;
+    main_generator(c_file)
+    c_file.write('''    return 0;
 }
 ''')
+
+def get_c_expression_values(
+        cast_to, printf_format,
+        expressions,
+        caller=__name__, file_label='',
+        header='', include_path=None,
+        keep_c=False,
+): # pylint: disable=too-many-arguments
+    """Generate and run a program to print out numerical values for expressions.
+
+    * ``cast_to``: a C type.
+    * ``printf_format``: a printf format suitable for the type ``cast_to``.
+    * ``header``: extra code to insert before any function in the generated
+      C file.
+    * ``expressions``: a list of C language expressions that have the type
+      ``type``.
+    * ``include_path``: a list of directories containing header files.
+    * ``keep_c``: if true, keep the temporary C file (presumably for debugging
+      purposes).
+
+    Return the list of values of the ``expressions``.
+    """
+    if include_path is None:
+        include_path = []
+    c_name = None
+    exe_name = None
+    try:
+        c_file, c_name, exe_name = create_c_file(file_label)
+        generate_c_file(
+            c_file, caller, header,
+            lambda c_file: generate_c_printf_expressions(c_file,
+                                                         cast_to, printf_format,
+                                                         expressions)
+        )
         c_file.close()
         cc = os.getenv('CC', 'cc')
         subprocess.check_call([cc] +
@@ -354,7 +411,7 @@ int main(void)
                               ['-o', exe_name, c_name])
         if keep_c:
             sys.stderr.write('List of {} tests kept at {}\n'
-                             .format(type_word, c_name))
+                             .format(caller, c_name))
         else:
             os.remove(c_name)
         output = subprocess.check_output([exe_name])
@@ -362,6 +419,24 @@ int main(void)
     finally:
         remove_file_if_exists(exe_name)
 
+def run_c(type_word, expressions, include_path=None, keep_c=False):
+    """Generate and run a program to print out numerical values for expressions."""
+    if type_word == 'status':
+        cast_to = 'long'
+        printf_format = '%ld'
+    else:
+        cast_to = 'unsigned long'
+        printf_format = '0x%08lx'
+    return get_c_expression_values(
+        cast_to, printf_format,
+        expressions,
+        caller='test_psa_constant_names.py for {} values'.format(type_word),
+        file_label=type_word,
+        header='#include <psa/crypto.h>',
+        include_path=include_path,
+        keep_c=keep_c
+    )
+
 NORMALIZE_STRIP_RE = re.compile(r'\s+')
 def normalize(expr):
     """Normalize the C expression so as not to care about trivial differences.