Details of new proposed annotation for specifying which action in a table-with-action-selector's actions list is the one for groups

[jafingerhut]

I was just thinking about the options for the new proposed annotation, which I will call @groupaction in this issue for now.

Aside: There is another PR that is being reviewed that might change the name to that, or perhaps to something else depending on how review comments proceed: #604

Before that annotation was added, the standard annotations for actions were @tableonly and @defaultonly. Here are all supported combinations of those annotations on a single action in a table's action list:

(1) no annotation: the action can be used as (a) the action of any number of table entries, or (b) as the table's default_action, or both (a) and (b) simultaneously.

(2) annotation @tableonly: the action can be used as (a), but not (b)

(3) annotation @defaultonly: the action can be used as (b), but not (a)

(4) both annotations @tableonly and @defaultonly on the same action: this does not make sense semantically, and thankfully p4c gives an error if you attempt to do so. Here is an example of the error message:

[--Werror=invalid] error: Table 'ingressImpl.ipv4_da_lpm' has an action reference ('ingressImpl.set_l2ptr') which is annotated with both '@tableonly' and '@defaultonly'

When we have the new annotation @groupaction, how should the list of possibilities be expanded? Is the proposal that things will behave as follows?

(5) no annotation: same as (1), and in particular the action cannot be configured as (c) the action of any number of the action selector's groups.

(6) same as (2), and action cannot be used for (c)

(7) same as (3), and action cannot be used for (c)

(8) same as (4)

(9) annotation @groupaction: the action can be used for (c), but not (a) or (b)

(10) annotation @groupaction and also one or both of @tableonly or @defaultonly: disallowed with an error at compile time.

If that is the proposal, note that it is then impossible to have the same named action be used as a group action of the table, and also a default action. This is easy to work around by creating a new action with a different name, of course, but I wanted to point out that this would be necessary, and perhaps worth pointing out as a consequence of the design of these annotations.

The names @defaultonly and @tableonly unfortunately do not lend themselves well to supporting all 2^3-1 combinations of enabling/disabling an action in each of the 3 kinds of uses independently.

[smolkaj]

@matthewtlam @jonathan-dilorenzo @kheradmandG

[jafingerhut]

Note: I am OK with the proposal above in the original comment, if that is what is intended. While it might be nice if we had created annotations such that for N possible "places" an action should be allowed to be used on a table, we had a way to specify all 2^N-1 subsets of those places via annotations, I don't think N is likely to grow above 3 any time soon (it has been 10 years for it to increase from 2 to 3 with this recent group action proposal). Also, the workaround of creating multiple named actions seems like a very reasonable one, since with the 3 "places", at most one additional action with the @groupaction annotation is sufficient to cover all possibilities.

[kheradmandG]

Is the proposal that things will behave as follows?
(5) ... (10)

Yes, that is my understanding. Please correct if wrong @jonathan-dilorenzo @smolkaj

If that is the proposal, note that it is then impossible to have the same named action be used as a group action of the table, and also a default action.

Seems to be an okay limitation. This group action seems to be a niche use case anyway, and I'm not expecting such action to be need to be used as anything beside the group action.

and perhaps worth pointing out as a consequence of the design of these annotations.

+1

it might be nice if we had created annotations such that for N possible "places" an action should be allowed to be used on a table, we had a way to specify all 2^N-1 subsets of those places via annotations,

+1, that would have been ideal
Given the way @defaultonly and @tableonly have been defined, It seems hard to change this now if we want "no annotation" to have the old meaning: default and table (but not group action).