Support loading snapshots before snapshotting

[NickGeek]

Hi, I noticed that in runtime.rs there is a TODO around supporting loading snapshots and snapshotting on the same instance. It's a feature I'd find quite useful so I did some digging.

It looks like V8 supports receiving a StartupData in the constructor of SnapshotCreator (source). I tried naively updating rusty_v8 to use that parameter, but I get an assertion failure:

# Fatal error in ../../../../v8/src/api/api.cc, line 636
# Debug check failed: data->created_.

Is there anything I can do to help get that todo item done?

[bartlomieju]

@NickGeek I've transferred the issue to rusty_v8 repo as it must first be implemented here before we can add that feature to deno itself.

[bartlomieju]

@NickGeek please open a PR with your code; the error you got suggests that snapshot was not taken before dropping the isolate.

[NickGeek]

@bartlomieju Sweet, thanks. I'll try and get that up tonight. Just a warning: it's definitely in a draft state, I was mostly just seeing if I could get something to work.

[NickGeek]

I've made a PR with the changes I tried at #555

[bartlomieju]

This is now supported.