Basic working multi-driver demo using async bbq

[jamesmunns]

This implements a quad-waitcell version of bbq, as well as a basic PoC with two sort-of drivers. This data structure is intended for drivers such as a serial port.

[hawkw]

this is awesome!

i had some tiny unimportant suggestions, take them or leave them

[hawkw]

👍 on the Arc, this is definitely a lot nicer...we can probably bring back the NonNulls as an optimization to avoid the field access, if it's necessary, but this feels like a safer starting point.

some tiny suggestions on the tracing stuff, but no blockers from me

[hawkw]

nit, take it or leave it: maybe worth sticking

Suggested change

	"Got bbqueue max write grant",
	);
	return GrantW {
	grant: wgr,
	side: self.side,
	storage: self.storage.clone(),
	}
	},
	Err(_) => {
	trace!("awaiting bbqueue max write grant");
	// Uh oh! Couldn't get a send grant. We need to wait for the OTHER reader
	// to release some bytes first.
	match self.side {
	Side::ASide => &self.storage.b_wait,
	Side::BSide => &self.storage.a_wait,
	}.release_waitcell.wait().await.unwrap();
	trace!("awoke for bbqueue max write grant");
	side = ?self.side,
	"Got bbqueue max write grant",
	);
	return GrantW {
	grant: wgr,
	side: self.side,
	storage: self.storage.clone(),
	}
	},
	Err(_) => {
	trace!(side = ?self.side, max, "awaiting bbqueue max write grant");
	// Uh oh! Couldn't get a send grant. We need to wait for the OTHER reader
	// to release some bytes first.
	match self.side {
	Side::ASide => &self.storage.b_wait,
	Side::BSide => &self.storage.a_wait,
	}.release_waitcell.wait().await.unwrap();
	trace!(side = ?self.side, max, "awoke for bbqueue max write grant");

[hawkw]

Suggested change

	trace!(
	size = size,
	"Got bbqueue exact write grant",
	);
	return GrantW {
	grant: wgr,
	side: self.side,
	storage: self.storage.clone(),
	}
	},
	Err(_) => {
	trace!("awaiting bbqueue exact write grant");
	// Uh oh! Couldn't get a send grant. We need to wait for the OTHER reader
	// to release some bytes first.
	match self.side {
	Side::ASide => &self.storage.b_wait,
	Side::BSide => &self.storage.a_wait,
	}.release_waitcell.wait().await.unwrap();
	trace!("awoke for bbqueue exact write grant");
	},
	trace!(
	size = size,
	side = ?self.side,
	"Got bbqueue exact write grant",
	);
	return GrantW {
	grant: wgr,
	side: self.side,
	storage: self.storage.clone(),
	}
	},
	Err(_) => {
	trace!(side = ?self.side, size, "awaiting bbqueue exact write grant");
	// Uh oh! Couldn't get a send grant. We need to wait for the OTHER reader
	// to release some bytes first.
	match self.side {
	Side::ASide => &self.storage.b_wait,
	Side::BSide => &self.storage.a_wait,
	}.release_waitcell.wait().await.unwrap();
	trace!(side = ?self.side, size, "awoke for bbqueue exact write grant");
	},

[hawkw]

Suggested change

	size = rgr.len(),
	"Got bbqueue read grant",
	);
	return GrantR {
	grant: rgr,
	side: self.side,
	storage: self.storage.clone(),
	}
	},
	Err(_) => {
	trace!("awaiting bbqueue read grant");
	// Uh oh! Couldn't get a read grant. We need to wait for the OTHER writer
	// to commit some bytes first.
	match self.side {
	Side::ASide => &self.storage.b_wait.commit_waitcell,
	Side::BSide => &self.storage.a_wait.commit_waitcell,
	}.wait().await.unwrap();
	trace!("awoke for bbqueue read grant");
	size = rgr.len(),
	side = ?self.side,
	"Got bbqueue read grant",
	);
	return GrantR {
	grant: rgr,
	side: self.side,
	storage: self.storage.clone(),
	}
	},
	Err(_) => {
	trace!(side = ?self.side, "awaiting bbqueue read grant");
	// Uh oh! Couldn't get a read grant. We need to wait for the OTHER writer
	// to commit some bytes first.
	match self.side {
	Side::ASide => &self.storage.b_wait.commit_waitcell,
	Side::BSide => &self.storage.a_wait.commit_waitcell,
	}.wait().await.unwrap();
	trace!(side = ?self.side, "awoke for bbqueue read grant");

[hawkw]

tiny nit: this could just be

Suggested change

	size = size,
	size,

[hawkw]

nit: could just be

Suggested change

	max = max,
	max,