tinc is using a separate thread to read from the TAP device on Windows.
The rationale was that the notification mechanism for packets arriving
on the virtual network device is based on Win32 events, and the event
loop did not support listening to these events.
Thanks to recent improvements, this event loop limitation has been
lifted. Therefore we can get rid of the separate thread and simply add
the Win32 "incoming packet" event to the event loop, just like a socket.
The result is cleaner code that's easier to reason about.
Currently, when the tinc service handler callback (which runs in a
separate thread) receives a service shutdown request, it calls
event_exit() to request the event loop to exit.
This approach has a few issues:
- The event loop will only notice the exit request when the next event
fires. This slows down tinc service shutdown. In some extreme cases
(DeviceStandby enabled, long PingTimeout and no connections),
shutdown can take ages.
- Strictly speaking, because of the absence of memory barriers, there
is no guarantee that the event loop will even notice an exit request
coming from another thread. I suppose marking the "running" variable
as "volatile" is supposed to alleviate that, but it's unclear whether
that provides any guarantees with modern systems and compilers.
This commit fixes the issue by leveraging the new event loop Windows
interface, using a custom Windows event that is manually set when
shutdown is requested.
This commit changes the event loop to use WSAEventSelect() and
WSAWaitForMultipleEvents() on Windows. This paves the way for making the
event loop more flexible on Windows by introducing the required
infrastructure to make the event loop wait on any Win32 event.
This commit only affects the internal implementation of the event
module. Externally visible behavior remains strictly unchanged (for
now).
When using socket functions, "sockerrno" is supposed to be used to
retrieve the error code as opposed to "errno", so that it is translated
to the correct call on Windows (WSAGetLastError() - Windows does not
update errno on socket errors). Unfortunately, the use of sockerrno is
inconsistent throughout the tinc codebase, as errno is often used
incorrectly on socket-related calls.
This commit fixes these oversights, which improves socket error
handling on Windows.
There are several reasons for this:
- MacOS/X doesn't support polling the tap device using kqueue, requiring a
workaround to fall back to select().
- On Windows only sockets are properly handled, therefore tinc uses a second
thread that does a blocking ReadFile() on the TAP-Win32/64 device. However,
this does not mix well with libevent.
- Libevent, event just the core, is quite large, and although it is easy to get
and install on many platforms, it can be a burden.
- Libev is more lightweight and seems technically superior, but it doesn't
abstract away all the platform differences (for example, async events are not
supported on Windows).
This relieves some confusion and problems during the libevent transition.
In particular, "event_add" was defined by both.
(The 't' stands for 'timeout', 'tinc', 'temporary', or some such.)
- Convert cp to cp(); so that automatic indenters work.
- Convert constructions like if(x == NULL) to if(!x).
- Move all assignments out of conditions.
