Allow loading non-static snapshots

[mraerino]

I'm using deno_core inside my own library.

I would like to be able to load snapshots from binary blobs at runtime.
They could be coming from a local file or the network.
This is currently blocked by the fact that Snapshots need a static lifetime.

In v8::CreateParams.set_snapshot_blob there is a restriction for the lifetime of a snapshot and I imagine that's why a static lifetime was chosen for v8::StartupData:

To avoid unnecessary copies of data, V8 will point directly into the given data blob, so pretty please keep it around until V8 exit.

I think it should be enough to make it live as long as the isolate, right?

From a comment in #4403:

So I'd much rather see a solution where an Isolate can take ownership of a Box<[u8]> that contains the snapshot.

[kitsonk]

Makes total sense... Great idea and contribution!

[mathiasrw]

@mraerino Thank you for the contribution.

Been looking into making a proof of concept for #986 (compile ts code into a single executable binary)

Would love to have a look at how you deal with loading the snapshot. Is your project open on github?

[mraerino]

@mathiasrw FTR: I'm not sure if my change will help a lot with that feature.

I don't have a public project using this. We're using Deno internally in a very large project.

The basic usage of this change is like this:

use deno_core::{CoreIsolate, StartupData, Snapshot};

let bytes: Vec<u8> = /* load me from file or somewhere else */;
let boxed = bytes.into_boxed_slice();
let isolate = CoreIsolate::new(StartupData::Snapshot(Snapshot:Boxed(boxed)), false);

[mathiasrw]

@mraerino Thank you for the input