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Currently, slow call path will always create a dangling pointer to
replace a null pointer when called with eg. a `new Uint8Array()`
parameter, which V8 initialises as a null pointer backed buffer.
However, the fast call path will never change the pointer value and will
thus expose a null pointer. Thus, it's possible that the pointer value
that a native call sees coming from Deno changes between two sequential
invocations of the same function with the exact same parameters.
Since null pointers can be quite important, and `Uint8Array` is the
chosen fast path for Deno FFI `"buffer"` parameters, I think it is
fairly important that the null pointer be properly exposed to the native
code. Thus this PR.
### `*mut c_void`
While here, I also changed the type of our pointer values to `*mut
c_void`. This is mainly due to JS buffers always being `*mut`, and
because we offer a way to turn a pointer into a JS `ArrayBuffer`
(`op_ffi_get_buf`) which is read-write. I'm not exactly sure which way
we should really go here, we have pointers that are definitely mut but
we also cannot assume all of our pointers are. So, do we go with the
maxima or the minima?
### `optimisedCall(new Uint8Array())`
V8 seems to have a bug where calling an optimised function with a newly
created empty `Uint8Array` (no argument or 0) will not see the data
pointer being null but instead it's some stable pointer, perhaps
pointing to some internal null-backing-store. The pointer value is also
an odd (not even) number, so it might specifically be a tagged pointer.
This will probably be an issue for some users, if they try to use eg.
`method(cstr("something"), new Uint8Array())` as a way to do a fast call
to `method` with a null pointer as the second parameter.
If instead of a `new Uint8Array()` the user instead uses some `const
NULL = new Uint8Array()` where the `NULL` buffer has been passed to a
slow call previously, then the fast call will properly see a null
pointer.
I'll take this up with some V8 engineers to see if this couldn't be
fixed.
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