This is using a non-intrusive type-traits like templated system, which
allows compile-time validation that the command table and the command
enum match up.
When you are downloading a map, all the commands are queued up
for you. Clients joining/leaving is done by the network protocol,
and as such are processed immediately. This means that by the
time you are processing the commands, a client that triggered
it, might already have left.
So, all commands that do something with ClientID, shouldn't
error on an invalid ClientID when DC_EXEC is set, but
gracefully handle the command anyway, to make sure the
game-state is kept in sync with all the clients that did
execute the DoCommand while the now-gone client was still
there.
Additionally, in the small chance a client disconnects between
the server validating a DoCommand and the command being
executed, also just process the command as if the client was
still there. Otherwise, lag or latency can cause clients that
did not receive the disconnect yet to desync.
This switch has been a pain for years. Often disabling broke
compilation, as no developer compiles OpenTTD without, neither do
any of our official binaries.
Additionaly, it has grown so hugely in our codebase, that it
clearly shows that the current solution was a poor one. 350+
instances of "#ifdef ENABLE_NETWORK" were in the code, of which
only ~30 in the networking code itself. The rest were all around
the code to do the right thing, from GUI to NewGRF.
A more proper solution would be to stub all the functions, and
make sure the rest of the code can simply assume network is
available. This was also partially done, and most variables were
correct if networking was disabled. Despite that, often the #ifdefs
were still used.
With the recent removal of DOS, there is also no platform anymore
which we support where networking isn't working out-of-the-box.
All in all, it is time to remove the ENABLE_NETWORK switch. No
replacement is planned, but if you feel we really need this option,
we welcome any Pull Request which implements this in a way that
doesn't crawl through the code like this diff shows we used to.
Patric Stout