async-std@1.6.0-beta.1: cannot run an executor inside another executor

[GopherJ]

after upgrading to 1.6.0-beta.1, I start to get this error message. Previous version: 1.5.0

[dignifiedquire]

This sounds like you are running block_on inside another block_on, which was not really well supported before, but is now being detected and not ignored anymore.

[GopherJ]

@dignifiedquire I removed block_on inside block_on but all my tests still fail, the compiler doesn't complain about the changes but I'm failling tests, would you mind having a look? I would like to test the new version of async-std for https://github.com/casbin/casbin-rs because maybe wasm compiles so that we can finally resolve: casbin/casbin-rs#101

https://github.com/GopherJ/casbin-rs/tree/upgrade-async-std

[GopherJ]

Also this error is a runtime error which makes it harder to identify during compile time, is it possible to locate it during compile time?

cargo check always ok but when we run/test it, errors come

[andysalerno]

I'm receiving the same error. At first it was throwing from a block_on, so I removed the block_on and have none remaining in (my) codebase, then it began throwing from another line.

From inspecting the trace, it looks like there's a block_on in async-std-1.6.0-beta.1/src/fs/file.rs:318.

In my code, the File is going out of scope, I guess invoking the drop() of the File, which leads to a stack trace like this, which I guess involves a block-in-a-block (since my main() is decorated with the async_std::main attribute):

  21: async_std::task::block_on::block_on
             at /home/andy/.cargo/registry/src/github.com-1ecc6299db9ec823/async-std-1.6.0-beta.1/src/task/block_on.rs:33
  22: <async_std::fs::file::File as core::ops::drop::Drop>::drop
             at /home/andy/.cargo/registry/src/github.com-1ecc6299db9ec823/async-std-1.6.0-beta.1/src/fs/file.rs:318
  23: core::ptr::drop_in_place
             at /rustc/8d69840ab92ea7f4d323420088dd8c9775f180cd/src/libcore/ptr/mod.rs:177
  24: core::ptr::drop_in_place
             at /rustc/8d69840ab92ea7f4d323420088dd8c9775f180cd/src/libcore/ptr/mod.rs:177
  25: toygrep::buffer::async_line_buffer::AsyncLineBufferReader<R>::take_line_buffer

https://github.com/async-rs/async-std/blob/master/src/fs/file.rs#L318

Agree with @GopherJ -- if blocking inside a blocking scope is now illegal, then hopefully there is some static way to detect this, since any instance of a block_on anywhere in your dependencies will start throwing at runtime (even within the async-std libraries itself, apparently).

[andysalerno]

(this comment previously had an old investigation with some incorrect info, now I have updated it to be accurate)

I just tested, and this is indeed failing:

fn main() {
    async_std::task::block_on(async move {
        async_std::fs::File::open(".").await.unwrap();
    });
}

thread 'main' panicked at 'cannot run an executor inside another executor', <::std::macros::panic macros>:2:4

[dignifiedquire]

Thank you @andysalerno for the test and report. #768 fixes the case for File dropping.