ensure function args outlive prepare_call helper

[jedireza]

We have a suspicious failing test:

  1) JsFunction call a JsFunction built in JS that implements x => x + 1:
     AssertionError: expected 1 to equal 17
      at Context.<anonymous> (lib/functions.js:14:12)

We expect the number 16 to be passed to our JavaScript function but it's mostly being passed 0. I say mostly because sometimes it's a random number. 😳

I've also witnessed random segmentation faults when running tests such as:

[1]    18077 segmentation fault (core dumped)  npm t

It turned out that when we pass args to the prepare_call helper, that values were being freed at the end of it's scope. The docs for as_mut_ptr have a clue about this:

The caller must ensure that the slice outlives the pointer this function returns, or else it will end up pointing to garbage.

With the help of @alexcrichton, we now pass prepare_call a reference instead which allows the value to live long enough.

The segmentation fault seems to be resolved too, which makes sense. I haven't seen it since implementing this change.

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Update: I believe this bug was introduced in 7fc8d98 when some logic in JsFunction::call was abstracted out into prepare_call to support #61.

[dherman]

r+

Looks perfect – thanks for chasing this down!