== echo server

We'll be using the `use_op` (https://www.boost.org/doc/libs/master/doc/html/boost_asio/overview/model/completion_tokens.html[asio completion]) token everywhere,
so we're using a https://www.boost.org/doc/libs/master/doc/html/boost_asio/reference/asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens[default completion token], so that we can skip the last parameters.

.example/echo_server.cpp declarations
[example]
[source,cpp]
----
include::../../example/echo_server.cpp[tag=decls]
----

We're writing the echo function as a <<promise, promise>> coroutine.
It's an eager coroutine and recommended as the default;
in case a lazy coro is needed, <<task, task>> is available.

.example/echo_server.cpp echo function
[example]
[source,cpp]
----
include::../../example/echo_server.cpp[tag=echo]
----
<1> When using the use_op completion token, I/O errors are translated into C++ exceptions. Additionally,
if the coroutine gets cancelled (e.g. because the user hit Ctrl-C),
an exception will be raised, too. Under these conditions, we print the error and exit the loop.
<2> We run the loop until we get cancelled (exception) or the user closes the connection.
<3> Read as much as is available.
<4> Write all the read bytes.

Note that promise is eager. Calling `echo` will immediately execute code until `async_read_some`
and then return control to the caller.

Next, we also need an acceptor function. Here, we're using a <<generator,generator>> to manage the acceptor state.
This is a coroutine that can be co_awaited multiple times, until a `co_return` expression is reached.

.example/echo_server.cpp listen function
[example]
[source,cpp]
----
include::../../example/echo_server.cpp[tag=listen]
----
<1> Cancellation will also lead to an exception here being thrown from the `co_await`
<2> Asynchronously accept the connection
<3> Yield it to the awaiting coroutine
<4> `co_return` a value for C++ conformance.


With those two functions we can now write the server:

.example/echo_server.cpp run_server function
[example]
[source,cpp]
----
include::../../example/echo_server.cpp[tag=run_server]
----
<1> Construct the listener generator coroutine. When the object is destroyed,
    the coroutine will be cancelled, performing all required cleanup.
<2> When we have more than 10 workers, we wait for one to finish
<3> Accept a new connection & launch it.

The <<wait_group, wait_group>> is used to manage the running echo functions.
This class will cancel & await the running `echo` coroutines.

We do not need to do the same for the `listener`, because it will just stop on its own, when `l` gets destroyed.
The destructor of a generator will cancel it.

Since the `promise` is eager, just calling it is enough to launch.
We then put those promises into a <<wait_group, wait_group>> which will allow us to tear down all the workers on scope exit.



.example/echo_server.cpp co_main function
[example]
[source,cpp]
----
include::../../example/echo_server.cpp[tag=main]
----
<1> Run `run_server` with an async scope.

The <<with, with>> function shown above, will run a function with a resource such as <<wait_group, wait_group>>.
On scope exit `with` will invoke & `co_await` an asynchronous teardown function.
This will cause all connections to be properly shutdown before `co_main` exists.

