Add a synchronous signal handler for graceful shutdown

[mrdimidium]

Signals in Unix can be asynchronous (interrupting one of the threads and calling the sighandle callback) or synchronous (blocking the process on the sigwait call).

Asynchronous signals seem easier to use, but they have a problem: since the system switches to sigaction at an arbitrary moment, the process can be stopped at an arbitrary point (for example, while acquiring a mutex or writing to a global buffer). Therefore, posix defines a specific list of functions that are safe to use in a handler. Using any other code (even the simplest ones like print) results in UB.

The code below circumvents this problem using blocking signal handling:

• set a mask as early as possible to prevent receiving signals on the main thread (this property will be inherited when creating new threads);
• create a new thread that specifies that it can receive SIGINT/SIGTERM;
• block the new thread on sigwait, and call cleanup callbacks when a signal is received.

Since the thread is blocked on sigwait, it consumes no resources (except a few MB for its stack). If the program terminates, the thread will be killed when the main thread terminates.

Using atomic to get shutdown is necessary regardless of the thread I create here, posix states that the signal sent to the process can be handled on any of the threads.

[karlseguin]

Doesn't seem to work on MacOS.

> zig build run
INFO  app : server running . . . . . . . . . . . . . . . . .  [+0ms]
      address = 127.0.0.1:9222

^Cinfo: Received termination signal...

> zig build run
FATAL app : server run error . . . . . . . . . . . . . . . .  [+0ms]
      err = AddressInUse

FATAL app : exit . . . . . . . . . . . . . . . . . . . . . .  [+0ms]
      err = AddressInUse

I have to kill it a couple times before it's gone.

[karlseguin]

We don't use Self for normal struct, only for generics.

[karlseguin]

This is fancy, and it's fine. But it would also be ok to have a shutdown function in App/Server/whatever that takes an *anyopaque and knows to cast it back to its own type. I guess you wouldn't need an allocation anymore.

[mrdimidium]

This trick is only needed for type checking. You should copy the pointer to anyopaque anyway, so you won't waste any extra memory with .{server}. Since memory is allocated in the arena, the allocation here is almost equivalent to a pointer next to a pointer to a function.

[krichprollsch]

Does it work on macos? I tested it on my linux and in docker, it looks good to me in these cases.

[mrdimidium]

@krichprollsch, sorry, I didn't have MacOS at hand, I'll set it up this evening and check.

[krichprollsch]

np, we can also wait for a test from @karlseguin or @nikneym

[nikneym]

Just tested on MacOS, seems I have to run kill 2 times after to properly exit.

[mrdimidium]

Interesting, this is actually a problem on macos, it doesn't interrupt accept after calling posix.shutdown, it needs an explicit posix.close.

[mrdimidium]

Sorry for the delay, I fixed it for Mac. The issue was related to different shutdown behavior on Linux and Mac (the handler itself works fine), so I left a comment in the code.