In Tokio, the futures for tasks are stored on the stack unless they are
explicitly boxed, either by the user or auto-boxed by Tokio when they
are especially large. Auto-boxing now also occurs in release mode
(since #6826).
Having very large futures can be problematic as it can cause a stack
overflow. In some cases it might be desireable to have smaller futures,
even if they are placed on the heap.
This change adds the size of the future driving an async task or the
function driving a blocking task to the tracing instrumentation. In the
case of a future that is auto-boxed by Tokio, both the final size as well
the original size before boxing is included.
To do this, a new struct `SpawnMeta` gets passed down from where a
future might get boxed to where the instrumentation is added. This
contains the task name (optionally) and the original future or function
size. If the `tokio_unstable` cfg flag and the `tracing` feature aren't both
enabled, then this struct will be zero sized, which is a small improvement
on the previous behavior of unconditionally passing down an `Option<&str>`
for the name.
This will make this information immediately available in Tokio Console,
and will enable new lints which will warn users if they have large futures
(just for async tasks).
We have some tests under the `tracing-instrumentation` crate which test
that the `size.bytes` and `original_size.bytes` fields are set correctly.
The minimal version of `tracing` required for Tokio has been bumped from
0.1.25 to 0.1.29 to get the `Value` impl on `Option<T>`. Given that the current
version is 0.1.40, this seems reasonable, especially given that Tracing's MSRV
is still lower than Tokio's in the latest version.
This impl had a bound on `StreamReader<S, E>`; this is incorrect
because:
- The second generic parameter to `StreamReader` is not an error type;
it's a buffer type.
- The `Stream` error type in `StreamReader` should not need to be the
same as the `Sink` error type.
This "passthrough" `Sink` impl was effectively unusable because it
required the `Sink` error type be the same as the `StreamReader` buffer
type.
Resolve this by allowing the `StreamReader` buffer to be anything in
this impl.
