Re: [PATCH 1/4] trace2: add macOS process ancestry tracing

From: Derrick Stolee <stolee@gmail.com>
Date: Feb 9, 2026, 14:36 UTC
Message-ID: <7390e189-16ac-43b3-a63c-a8b942d5934b@gmail.com>
In-Reply-To: <d99a30a1a77f0f23468dba987da08b32dd9a92fa.1770307510.git.gitgitgadget@gmail.com>

On 2/5/2026 11:05 AM, Matthew John Cheetham via GitGitGadget wrote:

Show 5 quoted lines

> Teach Git to also log process ancestry on macOS using the sysctl with
> KERN_PROC to get process information (PPID and process name).
> Like the Linux implementation, we use the cmd_ancestry TRACE2 event
> rather than using a data_json event and creating another custom data
> point.

> +#define USE_THE_REPOSITORY_VARIABLE

If we are creating a new file, then it would be best if we avoid this macro, which is intended for older code to still work until it can be fixed.

But also it seems that you don't use the_repository anywhere, so this can be deleted without consequence!

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> +/*
> + * Recursively push process names onto the ancestry array.
> + * We guard against cycles by limiting the depth to NR_PIDS_LIMIT.
> + */
> +static void push_ancestry_name(struct strvec *names, pid_t pid, int depth)
> +{
> +	struct strbuf name = STRBUF_INIT;
> +	pid_t ppid;
> +
> +	if (depth >= NR_PIDS_LIMIT)
> +		return;

Here is the recursion limit check.

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> +	if (pid <= 0)
> +		return;
> +
> +	if (get_proc_info(pid, &name, &ppid) < 0)
> +		goto cleanup;
> +
> +	strvec_push(names, name.buf);

This is copying the buffer, which is why you release it later.

Question: could we stop copying here and use strbuf_detach() at this
point? That would be a very minor improvement, so feel free to ignore!

I took a look and rediscovered that strvecs do not have an option to not copy. I'm thinking about string_list. I'm not sure if there is any value in converting your code just to avoid some string duplication at this scale.

Show 6 quoted lines

> +	/*
> +	 * Recurse to the parent process. Stop if ppid is 0 or 1
> +	 * (init/launchd) or if we've reached ourselves (cycle).
> +	 */
> +	if (ppid > 1 && ppid != pid)
> +		push_ancestry_name(names, ppid, depth + 1);

This kind of tail recursion could be easily converted into a loop. I usually prefer loops to recursion when possible, in case we want to allow an unlimited number of parents in the future.

> +cleanup:
> +	strbuf_release(&name);
> +}

I got a little confused by the lack of a .h file, but that's probably due to the extra magic being done at compile time to pick this file on a per- platform basis.

Indeed, trace2_collect_process_info() is defined in trace2.h.

Thanks, -Stolee