# Waiting and retrying

Source: https://libtmux.org/en/rs/latest/topics/waiting-and-retry/

> Polling a condition instead of guessing a sleep, and tmux's own wait-for signal channel as the alternative to polling.

After sending input or starting a process, wait for the state your next step
requires. [Pane
interaction](https://libtmux.org/en/rs/latest/topics/pane-interaction/#waiting-for-something-to-finish) covers
waiting for screen text. This page covers arbitrary conditions and tmux's named
[`wait-for`](https://libtmux.org/en/tmux/latest/manual/wait-for/) signal channels.

## Polling a condition

Polling checks a condition repeatedly until it succeeds or a deadline expires.
Set a deadline and choose an interval that limits unnecessary tmux commands.

**Helper:** [`libtmux::test::retry_until(within, condition)`](<https://libtmux.org/en/rs/latest/reference/test-retry_until/>)

**Where it lives:** [`crates/libtmux/src/test.rs`](<https://github.com/libtmux/libtmux-rs/blob/84de9258a07d10bdd5f6da01b016a9ae06fe1210/crates/libtmux/src/test.rs>), enabled with the `test-support` Cargo
feature.

### Examples

```rust
libtmux::test::retry_until(std::time::Duration::from_secs(5), async || {
    session.windows().await.map(|ws| ws.iter().any(|w| w.name() == "build")).unwrap_or(false)
})
.await?;
```

## tmux's own [wait-for](https://libtmux.org/en/tmux/latest/manual/wait-for/) channel

Use [`tmux wait-for -S <channel>`](https://libtmux.org/en/tmux/latest/manual/wait-for/) to signal and [`tmux wait-for <channel>`](https://libtmux.org/en/tmux/latest/manual/wait-for/) to
block until signalled. This avoids repeated screen captures when the command can
announce its own completion:

[`Server.signal_channel`](<https://libtmux.org/en/rs/latest/reference/server-server-signal_channel/>) signals a channel.
[`Server.wait_for_channel`](<https://libtmux.org/en/rs/latest/reference/server-server-wait_for_channel/>) waits with a timeout and returns a [`ChannelWait`](<https://libtmux.org/en/rs/latest/reference/server-channelwait/>)
indicating whether it was signalled or timed out.

```rust
server.signal_channel("built").await?;
let outcome = server.wait_for_channel("built", std::time::Duration::from_secs(5)).await?;
```

tmux remembers a signal sent before a waiter starts. The next wait on that
channel returns immediately, so completion is not lost when the command finishes
first.

A raw [`wait-for`](https://libtmux.org/en/tmux/latest/manual/wait-for/) client can exit zero when the server dies, as well as when the
channel is signalled. Verify server liveness when a lost server must be treated
as a failed task.

Use a channel name specific to the task. A remembered signal can otherwise
satisfy an unrelated later wait.

<details>
<summary>tmux manual and source</summary>

The tmux manual describes [completion channels](https://github.com/tmux/tmux/blob/94796f6b1182507efac8a272fc309a79e22e58a5/tmux.1#L8715).
The [channel implementation](https://github.com/tmux/tmux/blob/94796f6b1182507efac8a272fc309a79e22e58a5/cmd-wait-for.c#L388)
retains an early signal until a waiter consumes it. Use a fresh channel name
for each operation.

</details>