Add command "Client Capa subv2" to change behavior for SUBSCRIBE and SSUBSCRIBE commands: one call with only one response

[hwware]

Now for the command SUBSCRIBE and SSUBSCRIBE, if any client call with multiply channels, multiply responses come back.

It does not match with the Valkey command response logic, it causes client side to parse the response with extra effort on them. The problem exists long time.

Thus, to fix this design error, I add a new command parameter: client capa subv2
The command "client capa" is imported in Valkey 8, and I add a new parameter "subv2" for it
Note: subv2 is just a temporarily name, I have no idea which name is better, welcome any suggestion.

The update behavior as below: one call with one response

Note: for existing users, I think this is a break change. If any client chooses the behavior that one call with one response, the command "client capa subv2" can be called first.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.52%. Comparing base (20d583f) to head (da1d8d0).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #962      +/-   ##
============================================
- Coverage     70.54%   70.52%   -0.02%     
============================================
  Files           114      114              
  Lines         61644    61652       +8     
============================================
- Hits          43488    43483       -5     
- Misses        18156    18169      +13     

Files with missing lines	Coverage Δ
src/pubsub.c	93.69% <100.00%> (-2.66%)	⬇️
src/valkey-cli.c	55.50% <100.00%> (-0.02%)	⬇️

... and 11 files with indirect coverage changes

[zuiderkwast]

This was a design error from the beginning, and another design error when RESP3 was introduced, but it is too late to change it now.

This is a serious breaking change that will break all clients that support pubsub. This is very dangerous to do, considering there are many redis-compatible services out there and clients try to be compatible with not only valkey.

[zuiderkwast]

So with RESP3, this is still no normal in-band replies and one push message (instead of one per channel). This is still the main problem of [UN][P|S]SUBSCRIBE IMO.

To fix this behaviour, SUBSCRIBE should return an in-band reply. Either the list of channels as an array or map-reply, or just +OK and send the channels as push.

[hwware]

I do not want to import this change in Valkey 8.0. I wish we could do this change on Valkey 8.X or 9 version. Error need to be correct, never late ^_^

[zuiderkwast]

I think we cannot fix it ever. Only maybe with a CLIENT CAPA or RESP4.

[madolson]

Yeah, I agree with this being a potentially significant breaking change. I think maybe CLIENT CAPA could fix it (or RESP4).

[hwware]

I update the design, add a new parameter for client capa command, but the parameter name is just temporarily. If you have better name, please let me know or we can discuss it in the meeting