derive: Protect against unbounded recursion when u.is_empty()

[fitzgen]

Fixes #107

[fitzgen]

I'll publish a new release with this fix.

[fitzgen]

Published 1.1.1

[jcaesar]

Does this work as intended, i.e. is it ok to skip decreasing count if #expr aborts through ??

Generated code for my struct, for consideration

fn arbitrary(u: &mut Unstructured) -> Result<Self> {
    RECURSIVE_COUNT_Nah.with(|count| {
        if count.get() > 0 && u.is_empty() {
            return Err(arbitrary::Error::NotEnoughData);
        }
        count.set(count.get() + 1);
        let result = {
            Ok(
                match (u64::from(<u32 as Arbitrary>::arbitrary(u)?) * 2u64) >> 32 {
                    0u64 => Nah::Foo(
                        Arbitrary::arbitrary(u)?,
                        Arbitrary::arbitrary(u)?,
                        Arbitrary::arbitrary(u)?,
                    ),
                    1u64 => Nah::Bar(Arbitrary::arbitrary(u)?, Arbitrary::arbitrary(u)?),
                    _ => panic!("internal error: entered unreachable code"),
                },
            )
        };
        count.set(count.get() - 1);
        result
    })
}

[jcaesar]

Thinking about it further, wouldn't it be better to only modify count when the unstructured is already empty?

• No need to access the thread local under normal conditions (performance micro-optimization, I know.)
• It's entirely possible that a recursive struct is benign and does finish generating on zeros only (after recursing a few times from the Unstructured input). But this currently aborts before trying that.

[fitzgen]

Happy to review a pull request if you want to make one!