Reintroduce Locker API (multi-thread support)

[bnoordhuis]

Lockers were removed in #272 but it's become clear they're necessary to access isolates from different threads and to make things like --prof work, that require an ambient isolate.

The straw-man proposal is something like this:

1. Isolate becomes Send but is basically an opaque object with no functionality.

2. Creating a Locker consumes or borrows the Isolate and gives you a LocalIsolate (name TBD) that's !Send + !Sync.

3. Dropping the locker gives you back the Isolate.

There's a complementary Unlocker API for yielding the isolate. It'd be nice to have but it might be hard to model in Rust.

Stretch goal: make HandleScope::new() work with both Isolate and LocalIsolate so downstream users by and large won't have to update their code?

Would fix #486, #639, and some others. Stands a good chance of obsoleting #602. Removes the need for 83052c45351768c2e8923d1bf557728e03fecbf6.

[nicksrandall]

I would love to help out and work on this but I'd new to this repo so I'd need some guidance. Would step 1 be reverting #272 to re-introduce the locker api?

[bnoordhuis]

@nicksrandall That sounds like a good first step, yes. PR welcome!

[ry]

@nicksrandall Help is much appreciated.

I think we need to figure out what the API would be still.

I was thinking we should have an API that consumes an OwnedIsolate to provide a Send object (which can then be sent to a different thread) and then something that consumes the object to turn it back into an OwnedIsolate.

Maybe something like

fn lock(i: v8::OwnedIsolate) -> v8::LockedIsolate;
fn unlock(i: v8::LockedIsolate) -> v8::OwnedIsolate;

[nicksrandall]

Thanks for the help, I'll get started on this. Just to make sure I understand, do we still want to use v8's locking mechanism defined here or do we want to implement the lock entirely on our end?

@ry I'll trust your opinion on this but I was thinking that for the API we might be able to attach a lock method to the v8::OwnedIsolate struct and create a new struct v8::LockedIsolate which would have a corresponding unlock method.

Something like:

let locked = owned.lock();
let owned = locked.unlock();

Should I try for the method approach or would you prefer that I stick with adding new functions to the crate?

[piscisaureus]

Maybe something like

fn lock(i: v8::OwnedIsolate) -> v8::LockedIsolate;
fn unlock(i: v8::LockedIsolate) -> v8::OwnedIsolate;

IMO it’s dubious to use a consume operation for this - that kind of defeats the purpose of a lock, after all you want to be able to lock the isolate and also send it to another thread.

We have a type called IsolateHandle that implements Send/Sync. Maybe it’s better to build on that.

[ObsidianMinor]

There's a complementary Unlocker API for yielding the isolate. It'd be nice to have but it might be hard to model in Rust.

Could it modeled this using a method on a locked isolate that takes a mut ref to the lock? It wouldn't be as flexible, but it would satisfy what the API is designed for (perhaps an unsafe way to construct an unlocker on an isolate would also be provided?)