# 11.1 Wiring

_Peios / Advanced Peios / peinit / Output Handling_

> peinit is not a logging system but it holds the pipes at birth — where a service's output goes, how it is tagged, and terminal-attached services.

peinit is not a logging system — eventd stores, indexes and queries
logs. But peinit holds the pipes at birth. It decides where a service's
output goes, and it has to cover the window before eventd exists.

## 11.1.1 The pipes

Before exec, peinit creates a pipe for stdout and one for stderr with
`pipe2(O_CLOEXEC)`. The child's streams are redirected onto the write
ends; peinit keeps the read ends and watches them with epoll.

The blocking discipline is asymmetric, deliberately:

- The parent's **read** ends are non-blocking. PID 1 cannot afford to
  block on a read.
- The child's **write** ends keep ordinary blocking semantics.

That second half is what makes backpressure work. When a service
produces faster than peinit consumes, the kernel pipe buffer fills and
the service's own `write()` blocks, so the service slows down. Making
the write ends non-blocking would turn that into `EAGAIN` failures in
the service, converting a flow-control mechanism into an error the
service has to handle.

The child's stdin is redirected to `/dev/null`. peinit provides no
interactive input channel; a service needing input obtains it explicitly
— through a socket, or a stored descriptor — never through inherited
stdin.

The epoll instance itself is close-on-exec and is never inherited.

## 11.1.2 Tagging

peinit reads output line by line and tags each line with:

- the origin,
- the stream — stdout or stderr,
- a `CLOCK_REALTIME` timestamp,
- the job's identifier.

The origin is the service name for a main process. For a hook it is the
service name, the hook kind and its index — `jellyfin/ExecStartPre[0]`.
Reload commands are `<service>/ExecReload` and health checks
`<service>/HealthCheck`.

Health check output is captured, which is usually the only way to find
out why a health check failed.

## 11.1.3 Terminal-attached services

A service with a `TTYPath` has all three streams on its terminal, and
both pipe pairs are closed in the child (§5.4). Its output is not
captured at all — it goes to the terminal, which is what asking for one
means.