qapi: pluggable backend code generators

The 'qapi.backend.QAPIBackend' class defines an API contract for code
generators. The current generator is put into a new class
'qapi.backend.QAPICBackend' and made to be the default impl.

A custom generator can be requested using the '-k' arg which takes a
fully qualified python class name

   qapi-gen.py -B the.python.module.QAPIMyBackend

This allows out of tree code to use the QAPI generator infrastructure
to create new language bindings for QAPI schemas. This has the caveat
that the QAPI generator APIs are not guaranteed stable, so consumers
of this feature may have to update their code to be compatible with
future QEMU releases.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250224182030.2089959-1-berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Error checking and messages tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/main.py

@@ -8,18 +8,14 @@ This is the main entry point for generating C code from the QAPI schema.
 """
 
 import argparse
+from importlib import import_module
 import sys
 from typing import Optional
 
-from .commands import gen_commands
+from .backend import QAPIBackend, QAPICBackend
 from .common import must_match
 from .error import QAPIError
-from .events import gen_events
-from .features import gen_features
-from .introspect import gen_introspect
 from .schema import QAPISchema
-from .types import gen_types
-from .visit import gen_visit
 
 
 def invalid_prefix_char(prefix: str) -> Optional[str]:
@@ -29,32 +25,42 @@ def invalid_prefix_char(prefix: str) -> Optional[str]:
     return None
 
 
-def generate(schema_file: str,
-             output_dir: str,
-             prefix: str,
-             unmask: bool = False,
-             builtins: bool = False,
-             gen_tracing: bool = False) -> None:
-    """
-    Generate C code for the given schema into the target directory.
+def create_backend(path: str) -> QAPIBackend:
+    if path is None:
+        return QAPICBackend()
 
-    :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?
+    module_path, dot, class_name = path.rpartition('.')
+    if not dot:
+        print("argument of -B must be of the form MODULE.CLASS",
+              file=sys.stderr)
+        sys.exit(1)
 
-    :raise QAPIError: On failures.
-    """
-    assert invalid_prefix_char(prefix) is None
+    try:
+        mod = import_module(module_path)
+    except Exception as ex:
+        print(f"unable to import '{module_path}': {ex}", file=sys.stderr)
+        sys.exit(1)
 
-    schema = QAPISchema(schema_file)
-    gen_types(schema, output_dir, prefix, builtins)
-    gen_features(schema, output_dir, prefix)
-    gen_visit(schema, output_dir, prefix, builtins)
-    gen_commands(schema, output_dir, prefix, gen_tracing)
-    gen_events(schema, output_dir, prefix)
-    gen_introspect(schema, output_dir, prefix, unmask)
+    try:
+        klass = getattr(mod, class_name)
+    except AttributeError:
+        print(f"module '{module_path}' has no class '{class_name}'",
+              file=sys.stderr)
+        sys.exit(1)
+
+    try:
+        backend = klass()
+    except Exception as ex:
+        print(f"backend '{path}' cannot be instantiated: {ex}",
+              file=sys.stderr)
+        sys.exit(1)
+
+    if not isinstance(backend, QAPIBackend):
+        print(f"backend '{path}' must be an instance of QAPIBackend",
+              file=sys.stderr)
+        sys.exit(1)
+
+    return backend
 
 
 def main() -> int:
@@ -77,6 +83,8 @@ def main() -> int:
     parser.add_argument('-u', '--unmask-non-abi-names', action='store_true',
                         dest='unmask',
                         help="expose non-ABI names in introspection")
+    parser.add_argument('-B', '--backend', default=None,
+                        help="Python module name for code generator")
 
     # Option --suppress-tracing exists so we can avoid solving build system
     # problems.  TODO Drop it when we no longer need it.
@@ -93,12 +101,14 @@ def main() -> int:
         return 1
 
     try:
-        generate(args.schema,
-                 output_dir=args.output_dir,
-                 prefix=args.prefix,
-                 unmask=args.unmask,
-                 builtins=args.builtins,
-                 gen_tracing=not args.suppress_tracing)
+        schema = QAPISchema(args.schema)
+        backend = create_backend(args.backend)
+        backend.generate(schema,
+                         output_dir=args.output_dir,
+                         prefix=args.prefix,
+                         unmask=args.unmask,
+                         builtins=args.builtins,
+                         gen_tracing=not args.suppress_tracing)
     except QAPIError as err:
         print(err, file=sys.stderr)
         return 1