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docs: Add ERROR_HANDLING
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that explains how we plan to change error handling in zellij and invites
new contributors to join the fun. Details the currently existent error
handling capabilities and gives a bunch of examples taken from zellij-org#1670.
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# Help wanted

As of writing this, zellij relies on `unwrap()` to catch errors (terminate
execution) in many functions, rather than returning a [`Result`][Result] to
propagate these errors further up. While we don't consider `unwrap` to be a bad
thing in general, it hides the underlying error and leaves the user only with a
stack trace to go on. Worse than this, it will crash the application. This is
particularly bad when the user is using long-running sessions to perform tasks.

Hence, we would like to eliminate `unwrap()` statements from the code where
possible, and apply better error handling instead. This way, functions higher
up in the call stack can react to errors from underlying functions and either
try to recover, or give some meaningful error messages if recovery isn't
possible.

Since the zellij codebase is pretty big and growing rapidly, this endeavor
will continue to be pursued over time, as zellij develops. The idea is that
modules or single files are converted bit by bit, preferably in small PRs that
each target a specific module or file. **If you are looking to contribute to
zellij, this may be an ideal start for you!** This way you get to know the
codebase and get an idea which modules are used at which other places in the
code.

If you have an interest in this, don't hesitate to get in touch with us.


# Error handling facilities

## Displaying panic messages

Panics are generally handled via the `Panic` error type and the
[`handle_panic`][handle_panic] panic handler function. The fancy formatting
is performed by the [`miette`][miette] crate.


## Propagating errors

We use the [`anyhow`][anyhow] crate to propagate errors up the call stack. At
the moment, zellij doesn't have custom error types, so we wrap whatever errors
the underlying libraries give us, if any. [`anyhow`][anyhow] serves the purpose
of providing [`context`][context] about where (i.e. under which circumstances)
an error happened.

A critical requirement for propagating errors is that all functions involved
must return the [`Result`][Result] type. This allows convenient error handling
with the `?` operator.

At some point you will likely stop propagating errors and decide what to do
with the error. Generally you can:

1. Try to recover from the error, or
2. Report the error to the user and either
1. Terminate program execution (See [`fatal`][fatal]), or
2. Continue program execution (See [`non_fatal`][non_fatal])


## Handling errors

Ideally, when the program encounters an error it will try to recover as good as
it can. This can mean falling back to some sane default if a specific value
(e.g. an environment variable) cannot be found. Note that this isn't always
applicable. If in doubt, don't hesitate to ask.

Recovery usually isn't an option if an operation has changed the internal state
(i.e. the value or content of specific variables) of objects in the code. In
this case, if an error is encountered, it is best to declare the program state
corrupted and terminate the whole application. This can be done by `unwrap`ing
on the [`Result`][Result] type. Always try to propagate the error as good as
you can and attach meaningful context before `unwrap`ing. This gives the user
an idea what went wrong and can also help developers in quickly identifying
which parts of the code to debug if necessary.

When you encounter such a fatal error and cannot propagate it further up (e.g.
because the current function cannot be changed to return a [`Result`][Result],
or because it is the "root" function of a program thread), use the
[`fatal`][fatal] function to panic the application. It will attach some small
context to the error and finally `unwrap` it. Using this function over the
regular `unwrap` has the added benefit that other developers seeing this in the
code know that someone has previously spent some thought about error handling
at this location.

If you encounter a non-fatal error, use the [`non_fatal`][non_fatal] function
to handle it. Instead of `panic`ing the application, the error is written to
the application log and execution continues. Please use this sparingly, as an
error usually calls for actions to be taken rather than ignoring it.


# Examples of applied error handling

You can have a look at the commit that introduced error handling to the
`zellij_server::screen` module [right here][1] (look at the changes in
`zellij-server/src/screen.rs`). We'll use this to demonstrate a few things in
the following text.


## Converting a function to return a `Result` type

Here's an example of the `Screen::render` function as it looked before:

```rust
pub fn render(&mut self) {
// ...
let serialized_output = output.serialize();
self.bus
.senders
.send_to_server(ServerInstruction::Render(Some(serialized_output)))
.unwrap();
}
```

It performs a few actions (not shown here for brevity) and then sends an IPC
message to the server. As you can see it calls `unwrap()` on the result from
sending a message to the server. This means: If sending the message to the
server fails, execution is terminated and the program crashes. Let's assume
that crashing the application in this case is a reasonable course of action.

In total (as of writing this), the `render()` function is called 80 times from
various places in the code of the `Screen` struct. Hence, if sending the
message fails, we only see that the application crashed trying to send an IPC
message to the server. We won't know which of the 80 different code paths lead
to the execution of this function.

So what can we do? Instead of `unwrap`ing on the `Result` type here, we can
pass it up to the calling functions. Each of the callers can then decide for
themselves what to do: Continue regardless, propagate the error further up or
terminate execution.

Here's what the function looked like after the change linked above:

```rust
pub fn render(&mut self) -> Result<()> {
let err_context = || "failed to render screen".to_string();
// ...

let serialized_output = output.serialize();
self.bus
.senders
.send_to_server(ServerInstruction::Render(Some(serialized_output)))
.with_context(err_context)
}
```

We leverage the [`Context`][context] trait from [`anyhow`][anyhow] to attach a
context message to the error and make the function return a `Result` type
instead. As you can see, the `Result` here contains a `()`, which is the empty
type. It's primary purpose here is allowing us to propagate errors to callers
of this function.

Hence, for example the `resize_to_screen` function changed from this:

```rust
pub fn resize_to_screen(&mut self, new_screen_size: Size) {
// ...
self.render();
}
```

to this:

```rust
pub fn resize_to_screen(&mut self, new_screen_size: Size) -> Result<()> {
// ...
self.render()
.context("failed to resize to screen size: {new_screen_size:#?}")
}
```

Note how it returns a `Result` type now, too. This way we can pass the error up
to callers of `resize_to_screen` and keep going like this until we decide it's
time to do something about the error.

In general, any function calling `unwrap` or `expect` is a good candidate to be
rewritten to return a `Result` type instead.


## Attaching context

[Anyhow][anyhow]s [`Context`][context] trait gives us two methods to attach
context to an error: `context` and `with_context`. In functions where `Result`s
can be returned in multiple places, it is preferable to use `with_context`, as
it saves you from duplicating the same context message in multiple places in
the code. This was shown in the `render` method above, but the call to the
other function returning a `Result` wasn't shown:

```rust
pub fn render(&mut self) -> Result<()> {
let err_context = || "failed to render screen".to_string();
// ...

for tab_index in tabs_to_close {
// ...
self.close_tab_at_index(tab_index)
.with_context(err_context)?;
}
// ...
self.bus
.senders
.send_to_server(ServerInstruction::Render(Some(serialized_output)))
.with_context(err_context)
}
```

When there is only a single `Result` to be returned from your function, use
`context` as shown above for `resize_to_screen`.


## Choosing helpful context messages

When attaching context to an error, usually you want to express what you were
doing when the error occurred, i.e. in what context the error occurred. In the
`render` method, we could have done something like this instead:

```rust
pub fn render(&mut self) -> Result<()> {
// ...

for tab_index in tabs_to_close {
// ...
self.close_tab_at_index(tab_index)
.context("Failed to close tab at index: {tab_index}")?;
}
// ...
self.bus
.senders
.send_to_server(ServerInstruction::Render(Some(serialized_output)))
.context("Failed to send message to server")
}
```

Why do we add the message "failed to render screen" instead? Because that is
what we were trying to do when we received the error from the underlying
functions (`close_tab_at_index` and `send_to_server` in this case). Functions
from libraries usually already return an error that describes what went wrong
(Example: When we try to open a file that doesn't exist, the std library will
give us a [`NotFound`][2] error), so we don't have to repeat that.

In case of doubt, look at the name of the function you're currently working in
and write a context message somehow mentioning this.


## Terminating execution

We want to propagate errors as far up as we can. This way, every function along
the way can at least attach a context message giving us an idea what chain of
events lead to the error. Where do we terminate execution in `Screen`? If you
study the code in `screen.rs`, you'll notice all the components of zellij
interact with the `Screen` instance by means of IPC messages. These messages
are handled in the `screen_thread_main` function. Here's an excerpt:

```rust
ScreenInstruction::Render => {
screen.render()?;
},
ScreenInstruction::NewPane(pid, client_or_tab_index) => {
// ...
screen.update_tabs()?;

screen.render()?;
},
ScreenInstruction::OpenInPlaceEditor(pid, client_id) => {
// ...
screen.update_tabs()?;

screen.render()?;
},
```

The code goes on like this for quite a while, so there are many places where an
error may occur. In this case, since all the functions are called from this
central location, we forego attaching a context message to every error.
Instead, we propagate the errors to the caller of this function, which happens
to be the function `init_session` in `zellij-server/src/lib.rs`. We see that
`screen_thread_main` is spawned to run in a separate thread. Hence, we cannot
propagate the error further up and terminate execution at this point:

```rust
// ...
screen_thread_main(
screen_bus,
max_panes,
client_attributes_clone,
config_options,
)
.fatal();
```

Remember the call to [`fatal`][fatal] will log the error and afterwards panic
the application (i.e. crash zellij). Since we made sure to attach context
messages to the errors on their way up, we will see these messages in the
resulting output!


[handle_panic]: https://docs.rs/zellij-utils/latest/zellij_utils/errors/fn.handle_panic.html
[miette]: https://crates.io/crates/miette
[anyhow]: https://crates.io/crates/anyhow
[context]: https://docs.rs/anyhow/latest/anyhow/trait.Context.html
[Result]: https://doc.rust-lang.org/stable/std/result/enum.Result.html
[fatal]: https://docs.rs/zellij-utils/latest/zellij_utils/errors/trait.FatalError.html#tymethod.fatal
[non_fatal]: https://docs.rs/zellij-utils/latest/zellij_utils/errors/trait.FatalError.html#tymethod.non_fatal
[1]: https://github.com/zellij-org/zellij/commit/99e2bef8c68bd166cf89e90c8ffe8c02272ab4d3
[2]: https://doc.rust-lang.org/stable/std/io/enum.ErrorKind.html#variant.NotFound

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