BlockingLayer example in async context panics

[jorgehermo9]

While working on #5678 I had the necessity of calling an async function within a sync function.

I saw the BlockingLayer did something very similar of what I wanted in those two lines:

opendal/core/src/layers/blocking.rs

Line 137 in fc767b2

handle: Handle::try_current()

and

opendal/core/src/layers/blocking.rs

Line 232 in fc767b2

self.handle.block_on(async {

However, when I tried the following BlockingLayer example at

opendal/core/src/layers/blocking.rs

Line 43 in fc767b2

/// async fn main() -> Result<()> {

with the following main code based on that example:

#[tokio::main]
async fn main() -> Result<()> {
    // Build an `BlockingOperator` with blocking layer to start operating the storage.
    let op: BlockingOperator = Operator::new(services::Memory::default())?
        .layer(BlockingLayer::create()?)
        .finish()
        .blocking();
    op.write("test", "content")?;
    Ok(())
}

It panicked with the following error:

thread 'main' panicked at /home/jorge/github/opendal/core/src/layers/blocking.rs:232:21:
Cannot start a runtime from within a runtime. This happens because a function (like `block_on`) attempted to block the current thread while the thread is being used to drive asynchronous tasks.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It seems that using Handle::current().block_on(..) does not work propertly within async contexts... Or am I doing something wrong?

I am trying to do something very similar at #5678 and I wonder what should be the correct approach in these cases

[Xuanwo]

I had the necessity of calling an async function within a sync function.

I'm guessing you need this because of the cache layer. However, this is not a good pattern and shouldn't happen. Operators with BlockingLayer should only be used in a blocking context, such as cross-language bindings.

We can only implement CacheLayer for async operations.

[jorgehermo9]

I'm guessing you need this because of the cache layer. However, this is not a good pattern and shouldn't happen. Operators with BlockingLayer should only be used in a blocking context, such as cross-language bindings.

Perfect, thanks for your explanation!

And about this BlockingLayer example, is the panic expected? or is the documentation example wrong?

[Xuanwo]

Not a bug to fix, closing.