Subject: Re: [PATCH v2 4/6] trace2: emit cmd_ancestry data for Windows

From: Junio C Hamano <[email-masked]>
Date: Fri, 13 Feb 2026 12:52:22 -0800

"Matthew John Cheetham via GitGitGadget" <[email-masked]>
writes:

> From: Matthew John Cheetham <[email-masked]>
>
> Since 2f732bf15e (tr2: log parent process name, 2021-07-21) it is now
> now possible to emit a specific process ancestry event in TRACE2. We
> should emit the Windows process ancestry data with the correct event
> type.
>
> To not break existing consumers of the data_json "windows/ancestry"
> event, we continue to emit the ancestry data as a JSON event.
>
> Signed-off-by: Matthew John Cheetham <[email-masked]>
> ---
>  compat/win32/trace2_win32_process_info.c | 8 ++++++++
>  1 file changed, 8 insertions(+)

Quite straight-forward.  Given the way json event readers are
organized, I suppose that existing readers will not choke on seeing
the same data twice?  Are we officially deprecating the "old way",
or keeping both to serve two different audiences (i.e., those who
have been in the ecosystem long before this change and want to keep
reading in the format they are used to, and those who are writing
cross platform data analyzers that want to stick to a single
format)?  I do not think we need to decide it now, and if I were
asked, I would probably vote for the latter, but just thinking
aloud.

Thanks.

> diff --git a/compat/win32/trace2_win32_process_info.c b/compat/win32/trace2_win32_process_info.c
> index aceea05430..6a6a396078 100644
> --- a/compat/win32/trace2_win32_process_info.c
> +++ b/compat/win32/trace2_win32_process_info.c
> @@ -172,6 +172,11 @@ void trace2_collect_process_info(enum trace2_process_info_reason reason)
>  		get_is_being_debugged();
>  		get_ancestry(&names);
>  		if (names.nr) {
> +			/*
> +			  Emit the ancestry data as a data_json event to
> +			  maintain compatibility for consumers of the older
> +			  "windows/ancestry" event.
> +			 */
>  			struct json_writer jw = JSON_WRITER_INIT;
>  			jw_array_begin(&jw, 0);
>  			for (size_t i = 0; i < names.nr; i++)
> @@ -180,6 +185,9 @@ void trace2_collect_process_info(enum trace2_process_info_reason reason)
>  			trace2_data_json("process", the_repository,
>  					 "windows/ancestry", &jw);
>  			jw_release(&jw);
> +
> +			/* Emit the ancestry data with the new event. */
> +			trace2_cmd_ancestry(names.v);
>  		}
>  
>  		strvec_clear(&names);