We are going to generate trace events for QMP commands. We should
generate both trace_*() function calls and trace-events files listing
events for trace generator.

So, add an output module FOO.trace-events for each FOO schema module.

Since we're going to add trace events only to command marshallers,
make the trace-events output optional, so we don't generate so many
useless empty files.

Currently nobody set add_trace_events to True, so new functionality is
disabled. It will be enabled for QAPISchemaGenCommandVisitor
in a further commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 scripts/qapi/gen.py | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/scripts/qapi/gen.py b/scripts/qapi/gen.py
index 995a97d2b8..a41a2c1d55 100644
--- a/scripts/qapi/gen.py
+++ b/scripts/qapi/gen.py
@@ -192,6 +192,11 @@ def _bottom(self) -> str:
         return guardend(self.fname)
 
 
+class QAPIGenTrace(QAPIGen):
+    def _top(self) -> str:
+        return super()._top() + '# AUTOMATICALLY GENERATED, DO NOT MODIFY\n\n'
+
+
 @contextmanager
 def ifcontext(ifcond: QAPISchemaIfCond, *args: QAPIGenCCode) -> Iterator[None]:
     """
@@ -244,15 +249,18 @@ def __init__(self,
                  what: str,
                  user_blurb: str,
                  builtin_blurb: Optional[str],
-                 pydoc: str):
+                 pydoc: str,
+                 gen_trace_events: bool = False):
         self._prefix = prefix
         self._what = what
         self._user_blurb = user_blurb
         self._builtin_blurb = builtin_blurb
         self._pydoc = pydoc
         self._current_module: Optional[str] = None
-        self._module: Dict[str, Tuple[QAPIGenC, QAPIGenH]] = {}
+        self._module: Dict[str, Tuple[QAPIGenC, QAPIGenH,
+                                      Optional[QAPIGenTrace]]] = {}
         self._main_module: Optional[str] = None
+        self._gen_trace_events = gen_trace_events
 
     @property
     def _genc(self) -> QAPIGenC:
@@ -264,6 +272,14 @@ def _genh(self) -> QAPIGenH:
         assert self._current_module is not None
         return self._module[self._current_module][1]
 
+    @property
+    def _gent(self) -> QAPIGenTrace:
+        assert self._gen_trace_events
+        assert self._current_module is not None
+        gent = self._module[self._current_module][2]
+        assert gent is not None
+        return gent
+
     @staticmethod
     def _module_dirname(name: str) -> str:
         if QAPISchemaModule.is_user_module(name):
@@ -293,7 +309,14 @@ def _add_module(self, name: str, blurb: str) -> None:
         basename = self._module_filename(self._what, name)
         genc = QAPIGenC(basename + '.c', blurb, self._pydoc)
         genh = QAPIGenH(basename + '.h', blurb, self._pydoc)
-        self._module[name] = (genc, genh)
+
+        gent: Optional[QAPIGenTrace]
+        if self._gen_trace_events:
+            gent = QAPIGenTrace(basename + '.trace-events')
+        else:
+            gent = None
+
+        self._module[name] = (genc, genh, gent)
         self._current_module = name
 
     @contextmanager
@@ -304,11 +327,13 @@ def _temp_module(self, name: str) -> Iterator[None]:
         self._current_module = old_module
 
     def write(self, output_dir: str, opt_builtins: bool = False) -> None:
-        for name, (genc, genh) in self._module.items():
+        for name, (genc, genh, gent) in self._module.items():
             if QAPISchemaModule.is_builtin_module(name) and not opt_builtins:
                 continue
             genc.write(output_dir)
             genh.write(output_dir)
+            if gent is not None:
+                gent.write(output_dir)
 
     def _begin_builtin_module(self) -> None:
         pass
-- 
2.31.1

Let's tweak the title to more closely match existing commits:

    qapi/gen: Add FOO.trace-events output module

Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes:

> We are going to generate trace events for QMP commands. We should
> generate both trace_*() function calls and trace-events files listing
> events for trace generator.
>
> So, add an output module FOO.trace-events for each FOO schema module.
>
> Since we're going to add trace events only to command marshallers,
> make the trace-events output optional, so we don't generate so many
> useless empty files.
>
> Currently nobody set add_trace_events to True, so new functionality is
> disabled. It will be enabled for QAPISchemaGenCommandVisitor
> in a further commit.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  scripts/qapi/gen.py | 33 +++++++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/scripts/qapi/gen.py b/scripts/qapi/gen.py
> index 995a97d2b8..a41a2c1d55 100644
> --- a/scripts/qapi/gen.py
> +++ b/scripts/qapi/gen.py
> @@ -192,6 +192,11 @@ def _bottom(self) -> str:
>          return guardend(self.fname)
>  
>  
> +class QAPIGenTrace(QAPIGen):
> +    def _top(self) -> str:
> +        return super()._top() + '# AUTOMATICALLY GENERATED, DO NOT MODIFY\n\n'
> +
> +
>  @contextmanager
>  def ifcontext(ifcond: QAPISchemaIfCond, *args: QAPIGenCCode) -> Iterator[None]:
>      """
> @@ -244,15 +249,18 @@ def __init__(self,
>                   what: str,
>                   user_blurb: str,
>                   builtin_blurb: Optional[str],
> -                 pydoc: str):
> +                 pydoc: str,
> +                 gen_trace_events: bool = False):

Let's rename to @gen_trace for consistency with PATCH 3's --gen-trace.

Hmm, PATCH 7 replaces it by --no-trace-events.  I'm going to suggest
--suppress-tracing there.  @gen_tracing?

>          self._prefix = prefix
>          self._what = what
>          self._user_blurb = user_blurb
>          self._builtin_blurb = builtin_blurb
>          self._pydoc = pydoc
>          self._current_module: Optional[str] = None
> -        self._module: Dict[str, Tuple[QAPIGenC, QAPIGenH]] = {}
> +        self._module: Dict[str, Tuple[QAPIGenC, QAPIGenH,
> +                                      Optional[QAPIGenTrace]]] = {}
>          self._main_module: Optional[str] = None
> +        self._gen_trace_events = gen_trace_events
>  
>      @property
>      def _genc(self) -> QAPIGenC:
> @@ -264,6 +272,14 @@ def _genh(self) -> QAPIGenH:
>          assert self._current_module is not None
>          return self._module[self._current_module][1]
>  
> +    @property
> +    def _gent(self) -> QAPIGenTrace:
> +        assert self._gen_trace_events
> +        assert self._current_module is not None
> +        gent = self._module[self._current_module][2]
> +        assert gent is not None
> +        return gent
> +
>      @staticmethod
>      def _module_dirname(name: str) -> str:
>          if QAPISchemaModule.is_user_module(name):
> @@ -293,7 +309,14 @@ def _add_module(self, name: str, blurb: str) -> None:
>          basename = self._module_filename(self._what, name)
>          genc = QAPIGenC(basename + '.c', blurb, self._pydoc)
>          genh = QAPIGenH(basename + '.h', blurb, self._pydoc)
> -        self._module[name] = (genc, genh)
> +
> +        gent: Optional[QAPIGenTrace]
> +        if self._gen_trace_events:
> +            gent = QAPIGenTrace(basename + '.trace-events')
> +        else:
> +            gent = None

A bit more compact:

           gent: Optional[QAPIGenTrace] = None
           if self._gen_trace_events:
               gent = QAPIGenTrace(basename + '.trace-events')

> +
> +        self._module[name] = (genc, genh, gent)
>          self._current_module = name
>  
>      @contextmanager
> @@ -304,11 +327,13 @@ def _temp_module(self, name: str) -> Iterator[None]:
>          self._current_module = old_module
>  
>      def write(self, output_dir: str, opt_builtins: bool = False) -> None:
> -        for name, (genc, genh) in self._module.items():
> +        for name, (genc, genh, gent) in self._module.items():
>              if QAPISchemaModule.is_builtin_module(name) and not opt_builtins:
>                  continue
>              genc.write(output_dir)
>              genh.write(output_dir)
> +            if gent is not None:
> +                gent.write(output_dir)
>  
>      def _begin_builtin_module(self) -> None:
>          pass