qapi-gen: Separate arg-parsing from generation

This is a minor re-work of the entrypoint script. It isolates a
generate() method from the actual command-line mechanism.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20201009161558.107041-4-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[invalid_char() renamed to invalid_prefix_char()]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi-gen.py

 #!/usr/bin/env python3
-# QAPI generator
-#
+
 # This work is licensed under the terms of the GNU GPL, version 2 or later.
 # See the COPYING file in the top-level directory.
 
+"""
+QAPI Generator
+
+This is the main entry point for generating C code from the QAPI schema.
+"""
 
 import argparse
 import re
 import sys
+from typing import Optional
 
 from qapi.commands import gen_commands
+from qapi.error import QAPIError
 from qapi.events import gen_events
 from qapi.introspect import gen_introspect
-from qapi.schema import QAPIError, QAPISchema
+from qapi.schema import QAPISchema
 from qapi.types import gen_types
 from qapi.visit import gen_visit
 
 
-def main(argv):
+def invalid_prefix_char(prefix: str) -> Optional[str]:
+    match = re.match(r'([A-Za-z_.-][A-Za-z0-9_.-]*)?', prefix)
+    if match.end() != len(prefix):
+        return prefix[match.end()]
+    return None
+
+
+def generate(schema_file: str,
+             output_dir: str,
+             prefix: str,
+             unmask: bool = False,
+             builtins: bool = False) -> None:
+    """
+    Generate C code for the given schema into the target directory.
+
+    :param schema_file: The primary QAPI schema file.
+    :param output_dir: The output directory to store generated code.
+    :param prefix: Optional C-code prefix for symbol names.
+    :param unmask: Expose non-ABI names through introspection?
+    :param builtins: Generate code for built-in types?
+
+    :raise QAPIError: On failures.
+    """
+    assert invalid_prefix_char(prefix) is None
+
+    schema = QAPISchema(schema_file)
+    gen_types(schema, output_dir, prefix, builtins)
+    gen_visit(schema, output_dir, prefix, builtins)
+    gen_commands(schema, output_dir, prefix)
+    gen_events(schema, output_dir, prefix)
+    gen_introspect(schema, output_dir, prefix, unmask)
+
+
+def main() -> int:
+    """
+    gapi-gen executable entry point.
+    Expects arguments via sys.argv, see --help for details.
+
+    :return: int, 0 on success, 1 on failure.
+    """
     parser = argparse.ArgumentParser(
         description='Generate code from a QAPI schema')
     parser.add_argument('-b', '--builtins', action='store_true',
                         help="generate code for built-in types")
-    parser.add_argument('-o', '--output-dir', action='store', default='',
+    parser.add_argument('-o', '--output-dir', action='store',
+                        default='',
                         help="write output to directory OUTPUT_DIR")
-    parser.add_argument('-p', '--prefix', action='store', default='',
+    parser.add_argument('-p', '--prefix', action='store',
+                        default='',
                         help="prefix for symbols")
     parser.add_argument('-u', '--unmask-non-abi-names', action='store_true',
                         dest='unmask',
@@ -32,25 +79,23 @@ def main(argv):
     parser.add_argument('schema', action='store')
     args = parser.parse_args()
 
-    match = re.match(r'([A-Za-z_.-][A-Za-z0-9_.-]*)?', args.prefix)
-    if match.end() != len(args.prefix):
-        print("%s: 'funny character '%s' in argument of --prefix"
-              % (sys.argv[0], args.prefix[match.end()]),
-              file=sys.stderr)
-        sys.exit(1)
+    funny_char = invalid_prefix_char(args.prefix)
+    if funny_char:
+        msg = f"funny character '{funny_char}' in argument of --prefix"
+        print(f"{sys.argv[0]}: {msg}", file=sys.stderr)
+        return 1
 
     try:
-        schema = QAPISchema(args.schema)
+        generate(args.schema,
+                 output_dir=args.output_dir,
+                 prefix=args.prefix,
+                 unmask=args.unmask,
+                 builtins=args.builtins)
     except QAPIError as err:
-        print(err, file=sys.stderr)
-        exit(1)
-
-    gen_types(schema, args.output_dir, args.prefix, args.builtins)
-    gen_visit(schema, args.output_dir, args.prefix, args.builtins)
-    gen_commands(schema, args.output_dir, args.prefix)
-    gen_events(schema, args.output_dir, args.prefix)
-    gen_introspect(schema, args.output_dir, args.prefix, args.unmask)
+        print(f"{sys.argv[0]}: {str(err)}", file=sys.stderr)
+        return 1
+    return 0
 
 
 if __name__ == '__main__':
-    main(sys.argv)
+    sys.exit(main())