Peter M. Groen / TrueMQTT-cpp

Useful if you want to take action when a connection to the broker
is established or lost. For example, after a reconnection you can
expect to receive all retained messages again on the topics you are
subscribed too.

This is a breaking API change.

The whole interface of this library now uses "std::string_view"
instead of "std::string" / "std::string &". In result, we can
now promise to only copy data once thoughout the library.

For subscriptions, this is a copy once to read the data from the
socket into a buffer.

For publish, this is a copy once to write the data in a buffer
to send over the socket.

For publish, this doesn't change the memory footprint, as because
"std::string &" was already taking care of this. For subscriptions
however, it reduces the memory usage by a factor of three. And as
result it helps with the throughput.

Otherwise, on reconnect a whole bunch of older samples are sent
out.

Some brokers work a lot better if you don't hammer everything on
a single topic. Others are less fuzzed about it.

Also, check if we get a response every ping-request, so we know
the connection with the broker is still good.

As the last-will is retained, multiple runs gave different results.
By picking the topics better, this is no longer the case.

Doing it every 100us consumes a lot of CPU, and was not the
intention.

Additionally, increase the read interval to 100ms, as 10ms is using
quite a lot of CPU for not really a good reason other than reacting
quickly on error situations. That really is not the right balance
to strike.

Secrets aren't available for builds triggered from forks. As such,
this is a bit pointless, as it will only work with people who
have write access to the repository.

This means the socket can be blocking, which makes administration
easier. The drawback is that there is now a queue, including
signalling, between the main thread and write thread. This
consumes a bit more CPU; but in return the main thread is never
blocked.

Before this commit, we had one small buffer telling the packet
type and length, and another buffer with the payload. They were
send to the kernel one by one. For small packets, this is a
problem, as NODELAY causes the first buffer to be send on the
IP stack, and the payload after. This increases the IP overhead
for no good reason.

Now instead, already reserve room in the packet to write the
header, and send it as one single unit to the kernel.

send() no longer is blocking, and all sendNNN calls now return
false if the call couldn't be executed.

Additionally, the library now recovers much better from issues,
like unexpected broker disconnects.

This avoids copying the pointers from Client::Impl into Connection,
which is just administrative work. Now we can access Client::Impl,
and have all variables available to us.

Especially select() can take 100ms (as that is the timeout),
after which a lot could have changed. So re-check the state if
we aren't asked to disconnect before continueing.

Last commit the hope was the {} alone was enough, but SonarCloud,
somewhat rightfully, still complains.

It wasn't all that clear that this was the case.

Protocol specification of MQTT 3.1.1 doesn't allow unsubscribes
to fail.

The specifications say that the broker MAY deduplicate messages
on overlapping subscriptions. In result, some do, and some don't.

It is now, by documentation, left to the user of this library to
handle overlapping subscriptions properly, and depending on their
broker, they may receive one or more times the same message when
the subscriptions overlap.

Subscriptions are now stored in a tree-like structure, to quickly
find the correct callbacks. This not only reduces the complexity
from O(n) to O(logn), but also doesn't require stuff like regex.

It does however require slightly more memory.

std::move() is doing the right thing either way.

Just for small strings it will copy, and only large strings will
actually be moved.

Fixes #1.

This includes CONNACK and SUBACK.

By using Happy Eyeballs, we stagger connections of a host resolves
into multiple IPs. This is useful for IPv6 / IPv4 hosts, where
one of the two can stutter.

Sadly, creating a connection is rather complex, with many odd
things that can happen along the way. For example, a writeable
socket doesn't mean it is actually connected; it can also mean
the socket is in an error state.

This implementation is inspired by my own work on OpenTTD's
variant of this.

All administration should been taken care of now, and the only
thing remaining is creating connect/disconnect and implementing
the sendXXX functions.