# dista.signals

> A module of Dista's API.

SIGTERM: the head exits, and a worker drains, even as a container's PID 1.

Kubernetes stops a pod with SIGTERM, and the kernel drops any signal sent to
a container's PID 1, as dista's process is, unless it has a handler for it.
The head's handler exits at once, as the signal's default action would. A
worker's writes a byte to a pipe its event loop polls, so the worker can
drain: finish what it's running, hand what it holds to other workers, and
leave (see `worker.mojo`). A second SIGTERM, or one that arrives before the
worker's loop runs, ends it at once.

A signal handler can't reach a Mojo variable, so the pipe's write end is
duplicated onto a fixed descriptor, `WAKE_FD`, which the handler writes to.

## Functions

### `exit_on_sigterm`

```mojo
def exit_on_sigterm()
```

Makes SIGTERM end this process, even where it would be dropped.

### `wake_on_sigterm`

```mojo
def wake_on_sigterm() -> c_int
```

Makes SIGTERM write a byte to a pipe, and returns its read end; or -1, with SIGTERM ending the process, if `WAKE_FD` is taken or the pipe can't be made. Both ends block (setting O_NONBLOCK needs a variadic `fcntl`, which `external_call` can't make on Apple's arm64), so read it only once `poll` says it's readable.

### `take_wakes`

```mojo
def take_wakes(fd: Int32) -> Int
```

Reads, once, the bytes SIGTERMs wrote to `fd`, which `poll` found readable; how many there were.

## Aliases

### `SIGTERM`

```mojo
comptime SIGTERM = Int32(15)
```

### `WAKE_FD`

```mojo
comptime WAKE_FD = Int32(250)
```

The descriptor a worker's handler writes to: below macOS's default limit of 256 open files, and above what a process has open when it starts.