_perforce_call_p4 foo … will call _call_program foo … which will look up the command style under the foo tag. Thus, instead of

zstyle ':completion:*:p4:*' clients $xargs

one ought to be able to do

zstyle ':completion:*:p4:*:clients' command p4 "${_perforce_global_options[@]}" "${(q)xargs[@]}"

for equivalent functionality. Should we document that rather than add a new (and rather specific) style?

(The extra (q) is because _call_program uses eval-like as opposed to exec-like semantics for its positional arguments.)

To be clear, the second code snippet should work without this patch.

EDIT: It won't work at all; see my next comment.

Got it.
With my patch I'm passing the following zstyle customizations:

zstyle ':completion:*:p4-*:changes' changes -c `p4 -Ztag -F "%clientName%" info`
zstyle ':completion:*:p4-*:clients' clients -u $USER -m 20 
zstyle ':completion:*:p4-*:labels' labels -u $USER -m 20 

I think documenting how to do this without adding new code is ideal. However I'm not clear how to achieve say the -u $USER -m 20 with the snippet you provided. Can you provide an example? Sorry I'm new to this and was just following the _perforce_changes way of handling xargs.

However I'm not clear how to achieve say the -u $USER -m 20 with the snippet you provided. Can you provide an example?

Sorry, you're right. It was rather a bad thinko on my part. The global variable won't have been created at the time the zstyle gets set, so a detour through zstyle -e is needed:

zstyle -e ':completion:*:p4-*:clients' command \
        'reply=( p4 "${_perforce_global_options[@]}" -u ${(q)USER} -m 20 )'

The principal change here is adding -e and assigning to $reply; adding the (q) is an independent change.

Note that _perforce_call_p4 clients … is also invoked by _perforce_client_dirs, so the value of the command style under the clients tag would be picked up by that callsite as well. If the two calls of _perforce_call_p4 clients … shouldn't always use the same command as each other, then they shouldn't both be using the same value (in this case, clients) as the "tag" argument of _call_program.

Sorry I'm new to this and was just following the _perforce_changes way of handling xargs.

It's always a good idea to follow prior art elsewhere in the same file. As to _perforce_changes, there are a number of moving parts there: two styles, one of which may be overridden by $NUMERIC under certain conditions. I'm not immediately sure what to suggest there.