Fix memory overflow if WriteString exceed memory capacity

[ben-page]

• The memory allocated for the initial GetData wasn't reused if different data was loaded.
• FastEvaluate() is called when _dataHeapPtr is near the end of the memory store, it can overflow the store. I created a method to check the memory capacity and grow as necessary.

[christophwille]

https://github.com/open-policy-agent/npm-opa-wasm/blob/3e618c84a07ba3af8d7bab75eb77395bd79d7c54/src/opa.js#L267

this library mimics what npm-opa-wasm does in terms of execution order.

[ben-page]

https://github.com/open-policy-agent/npm-opa-wasm/blob/3e618c84a07ba3af8d7bab75eb77395bd79d7c54/src/opa.js#L267

this library mimics what npm-opa-wasm does in terms of execution order.

I can take out the first commit, if that helps. It's a small amount of wasted memory. The second commit is the important one.

[christophwille]

Yes (and I just saw that npm-opa-wasm has applied memory fixes recently, so I'll have to take a look at their recent history too)

Edit: opened #38 for mem fix note above.

[christophwille]

Can we somehow unit test the behavior?

[ben-page]

Can we somehow unit test the behavior?

I think so. Give a me a couple days.

[christophwille]

open-policy-agent/npm-opa-wasm@f1be838 corresponding JS impl

[christophwille]

@ben-page I started a new PR #58 but that doesn't work yet - it is basically your code from this PR plus a few amenities and tests - and the tests actually fail. Did your solution work for you in prod?