BAT_PAGER="builtin" && --help parsing causes panic

[imsakg]

export BAT_PAGER="builtin"
alias -g -- --help='--help 2>&1 | bat --language=help --style=plain'

thread 'main' panicked at src/output.rs:212:65:ary/Caches/Homebrew/cargo_cache/registry/src/index.crates.io-1called `Result::unwrap()` on an `Err` value: Any { .. }

[lmmx]

The panic occurs in the Drop implementation for OutputType in src/output.rs. The problematic code is:

bat/src/output.rs

Lines 210 to 214 in 4e13c3e

	OutputType::BuiltinPager(ref mut pager) => {
	if pager.handle.is_some() {
	let _ = pager.handle.take().unwrap().join().unwrap();
	}
	}

char 65 would be the final .unwrap

bat/src/output.rs

Line 19 in 4e13c3e

handle: Option<JoinHandle<Result<()>>>,

• The first one is safe as we checked is_some(), this takes the JoinHandle out of the Option
• The second is the problematic one. JoinHandle<T>::join() returns a Result<T>, since T here is Result<()> then it returns Result<Result<()>>. This would be failing either from the spawned thread panicking in turn or the thread returning an Err result, not sure if it’s possible to say which from this? but I’d guess the latter

The thread is created like this

bat/src/output.rs

Lines 26 to 31 in 4e13c3e

	let handle = {
	let pager = pager.clone();
	Some(spawn(move || {
	minus::dynamic_paging(pager).map_err(Error::from)
	}))
	};

The thread is running minus::dynamic_paging() (docs) which can return an error. If that function fails or panics, the join().unwrap() will panic during cleanup.

Indeed its docs say

Panics

This function will panic if another instance of minus is already running.

Errors

The function will return with an error if it encounters a error during paging.

…so still not sure which is causing it here.

If that could always potentially contain an Err then we shouldn’t be unwrapping it

Since we don’t actually use that result (we assign to _, drop never returns anything) I am wondering why we’re unwrapping it in the first place (or if we have to at all).

Could we just take, unwrap once, and join?

[lmmx]

Cannot repro this namely as my alias doesn't have a -g flag

@imsakg can you repro with any other style of calling it, e.g. by setting BAT_PAGER='builtin' before the call to bat like this

echo "hello" | BAT_PAGER='builtin' bat

It doesn't panic when I call it like this:

echo "hello" | BAT_PAGER='builtin' bat --language=help --style=plain

The info you gave above doesn't include the call you used that panicked, as the alias has bat wrapped in a string. More info would be helpful to debug