Subject: Re: [PATCH v2 3/6] trace2: refactor Windows process ancestry trace2 event

From: Junio C Hamano <[email-masked]>
Date: Fri, 13 Feb 2026 12:36:41 -0800

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

> From: Matthew John Cheetham <[email-masked]>
>
> In 353d3d77f4 (trace2: collect Windows-specific process information,
> 2019-02-22) we added process ancestry information for Windows to TRACE2
> via a data_json event. It was only later in 2f732bf15e (tr2: log parent
> process name, 2021-07-21) that the specific cmd_ancestry event was
> added to TRACE2.
>
> In a future commit we will emit the ancestry information with the newer
> cmd_ancestry TRACE2 event. Right now, we rework this implementation of
> trace2_collect_process_info to separate the calculation of ancestors
> from building and emiting the JSON array via a data_json event.
>
> Signed-off-by: Matthew John Cheetham <[email-masked]>
> ---
>  compat/win32/trace2_win32_process_info.c | 50 ++++++++++++------------
>  1 file changed, 25 insertions(+), 25 deletions(-)

Looks quite straight-forward refactoring that even I (without Win32
specific knowledge) can follow with ease.  Nicely done.

>
> diff --git a/compat/win32/trace2_win32_process_info.c b/compat/win32/trace2_win32_process_info.c
> index f147da706a..aceea05430 100644
> --- a/compat/win32/trace2_win32_process_info.c
> +++ b/compat/win32/trace2_win32_process_info.c
> @@ -3,6 +3,7 @@
>  #include "../../git-compat-util.h"
>  #include "../../json-writer.h"
>  #include "../../repository.h"
> +#include "../../strvec.h"
>  #include "../../trace2.h"
>  #include "lazyload.h"
>  #include <psapi.h>
> @@ -32,12 +33,7 @@ static int find_pid(DWORD pid, HANDLE hSnapshot, PROCESSENTRY32 *pe32)
>  }
>  
>  /*
> - * Accumulate JSON array of our parent processes:
> - *     [
> - *         exe-name-parent,
> - *         exe-name-grand-parent,
> - *         ...
> - *     ]
> + * Accumulate array of our parent process names.
>   *
>   * Note: we only report the filename of the process executable; the
>   *       only way to get its full pathname is to use OpenProcess()
> @@ -73,7 +69,7 @@ static int find_pid(DWORD pid, HANDLE hSnapshot, PROCESSENTRY32 *pe32)
>   * simple and avoid the alloc/realloc overhead.  It is OK if we
>   * truncate the search and return a partial answer.
>   */
> -static void get_processes(struct json_writer *jw, HANDLE hSnapshot)
> +static void get_processes(struct strvec *names, HANDLE hSnapshot)
>  {
>  	PROCESSENTRY32 pe32;
>  	DWORD pid;
> @@ -82,19 +78,19 @@ static void get_processes(struct json_writer *jw, HANDLE hSnapshot)
>  
>  	pid = GetCurrentProcessId();
>  	while (find_pid(pid, hSnapshot, &pe32)) {
> -		/* Only report parents. Omit self from the JSON output. */
> +		/* Only report parents. Omit self from the output. */
>  		if (nr_pids)
> -			jw_array_string(jw, pe32.szExeFile);
> +			strvec_push(names, pe32.szExeFile);
>  
>  		/* Check for cycle in snapshot. (Yes, it happened.) */
>  		for (k = 0; k < nr_pids; k++)
>  			if (pid == pid_list[k]) {
> -				jw_array_string(jw, "(cycle)");
> +				strvec_push(names, "(cycle)");
>  				return;
>  			}
>  
>  		if (nr_pids == NR_PIDS_LIMIT) {
> -			jw_array_string(jw, "(truncated)");
> +			strvec_push(names, "(truncated)");
>  			return;
>  		}
>  
> @@ -105,24 +101,14 @@ static void get_processes(struct json_writer *jw, HANDLE hSnapshot)
>  }
>  
>  /*
> - * Emit JSON data for the current and parent processes.  Individual
> - * trace2 targets can decide how to actually print it.
> + * Collect the list of parent process names.
>   */
> -static void get_ancestry(void)
> +static void get_ancestry(struct strvec *names)
>  {
>  	HANDLE hSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
>  
>  	if (hSnapshot != INVALID_HANDLE_VALUE) {
> -		struct json_writer jw = JSON_WRITER_INIT;
> -
> -		jw_array_begin(&jw, 0);
> -		get_processes(&jw, hSnapshot);
> -		jw_end(&jw);
> -
> -		trace2_data_json("process", the_repository, "windows/ancestry",
> -				 &jw);
> -
> -		jw_release(&jw);
> +		get_processes(names, hSnapshot);
>  		CloseHandle(hSnapshot);
>  	}
>  }
> @@ -176,13 +162,27 @@ static void get_peak_memory_info(void)
>  
>  void trace2_collect_process_info(enum trace2_process_info_reason reason)
>  {
> +	struct strvec names = STRVEC_INIT;
> +
>  	if (!trace2_is_enabled())
>  		return;
>  
>  	switch (reason) {
>  	case TRACE2_PROCESS_INFO_STARTUP:
>  		get_is_being_debugged();
> -		get_ancestry();
> +		get_ancestry(&names);
> +		if (names.nr) {
> +			struct json_writer jw = JSON_WRITER_INIT;
> +			jw_array_begin(&jw, 0);
> +			for (size_t i = 0; i < names.nr; i++)
> +				jw_array_string(&jw, names.v[i]);
> +			jw_end(&jw);
> +			trace2_data_json("process", the_repository,
> +					 "windows/ancestry", &jw);
> +			jw_release(&jw);
> +		}
> +
> +		strvec_clear(&names);
>  		return;
>  
>  	case TRACE2_PROCESS_INFO_EXIT: