[Runtime] don't allocate threads array twice

See merge request !58

handle UnboundedBlockingMpscQueue spurious wake-ups

Closes #4

See merge request !57

Add meson option for scheduling strategy and according CI jobs

See merge request !53

A spurious wake-up can be produced by the new UnblockOnMainActorTest which
triggers the assert(!mpscQueue.empty()) in UnboundedBlockingMpscQueue::get.

Those spurious wake-ups are possible because the push and wake-up pair in
UnboundedBlockingMpscQueue::put are not atomic.
The following sequence diagram demonstrates a spurious wake-up:

   T1          T2            Q
   .           .            { }
  put(e)       .            { }
 push 54-57    .            {e}
   .         get()          {e}
   .        consume e       { }
   .           .            { }
   .         get()          { }
   .         block          { }
 unblock       .            { }
   .           .            { }
   .         wakeup         { }
   .           .            { }
               X
       assert(!queue.Empty())

To deal with spurious wake-ups we recheck the wake-up condition (a non empty queue)
and block again if we find it empty.
We assume spurious wake-ups are rare because it was difficult to reproduce them
even with a dedicated Test (the new UnblockOnMainActorTest) therefore we declare
the empty queue branch as unlikely.

Fixes #4.

the threads array is initialized using the Runtime::Runtime initializer list
and afterwards again in the constructor.

Co-authored-by: Florian Fischer <florian.fl.fischer@fau.de>

Various small changes

See merge request !55

Fix memory leak: Ensure that the current fiber is recycled in discardAndResume()

See merge request !51

Improve code readability of UnboundedBlockingMpscQueue

See merge request !52

The current Fiber is now always stored in the context, not just on
debug builds. This also means that we can remove the currentFiber
thread local variable, as using only a thread local storage would
yield wrong results in case a blocked context is resumed on another
worker thread.

[build] Switch to C++2a

See merge request !47

Improve debug output, create Worker class

See merge request !42

There is no need to call load() on std::atomic if the default memory
order is used.

[doc] make doxygen search for input recursively

See merge request !50

If RECURSIVE is set to NO doxygen will only process files in emper/
when set to YES it will see all files reachable from emper/.

We are unable to switch to C++20 until
https://github.com/mesonbuild/meson/issues/8084 is fixed.

Add emper::getFullVersion()

See merge request !49

This also solves a dependency declaration issue in WorkerSleepExample:
Prior to this change, a clean build could potentially result in

ninja  -C build
ninja: Entering directory `build'
[13/57] Compiling C++ object apps/worker_sleep_example.p/WorkerSleepExample.cpp.o
FAILED: apps/worker_sleep_example.p/WorkerSleepExample.cpp.o
ccache c++ -Iapps/worker_sleep_example.p -Iapps -I../apps -Iemper -I../emper -Iemper/include -I../emper/include -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -Wextra -Wpedantic -Werror -std=c++17 -O2 -g -Wno-non-virtual-dtor -MD -MQ apps/worker_sleep_example.p/WorkerSleepExample.cpp.o -MF apps/worker_sleep_example.p/WorkerSleepExample.cpp.o.d -o apps/worker_sleep_example.p/WorkerSleepExample.cpp.o -c ../apps/WorkerSleepExample.cpp
../apps/WorkerSleepExample.cpp:12:10: fatal error: emper-version.h: No such file or directory
   12 | #include "emper-version.h"        // for EMPER_FULL_VERSION
      |          ^~~~~~~~~~~~~~~~~
compilation terminated.
[17/57] Generating emper-version.h with a custom command
ninja: build stopped: subcommand failed.
make: *** [Makefile:23: build] Error 1

because worker_sleep_example_exec should have also depend on
emper_version_h. However this is obviously error prone, as users
easily forget to add this dependency. Instead we add
emper::getFullVersion() which is part of the EMPER shared object (not
just of a single header).

Use lib::adt::LockedUnboundedQueue in Scheduler

See merge request !45

Re-use Runtime::getWorkerId() when possible

See merge request !44

Allow an actor to be startet from anywhere

See merge request !43

This class was previously unused, but can be used in Scheduler after
minor modifcations.

Runtime::getWorkerId() may be an expensive operation, hence re-use the
result of the method when possible.

Schedule from anywhere

See merge request !41

In debug builds, the schedule() method is now guarded by an assert(),
as due to this change, the schedule() method will no longer work from
everywhere.

This also improves the worker sleep method.

This directory is used, for example, by clangd.

[gitlab-ci] Bump CI container to flowdalic/debian-testing-dev:1.4

See merge request !40