Make it easier to write into ByteBuffer via writable pointers.

[Lukasa]

The Problem

Plenty of C APIs expose APIs where they write into a buffer provided by the caller. For example, consider OpenSSL's SSL_read, declared like this:

int SSL_read(SSL *ssl, void *buf, int num);

In this model, OpenSSL will write into the pointer at buf. To support this with ByteBuffer, the following code has to be written:

let rc = outputBuffer.writeWithUnsafeMutableBytes { pointer in
    SSL_read(self.ssl, pointer.baseAddress, CInt(pointer.count))
}

The issue here is that this call will give you a pointer to only the writable bytes that actually exist in your ByteBuffer. If there are only 2 bytes, or even 0 bytes, that's all you're going to get. That can make using this API extremely tricky, and requires calls to the (very flawed, see #537) function changeCapacity before using it.

It'd be nice to have a function like ByteBuffer.writeWithUnsafeMutableBytes(minimumWritableSpace: Int, _ block: (UnsafeMutableRawBufferPointer) -> Int). This function would guarantee that the minimum size of the pointer in the block would be appropriately set, performing any CoW or other operation as needed.

This is not a trivial patch, but if you're interested in the internals of ByteBuffer this would be a good thing to build.

[2bjake]

This is a dup of #1105 right? If so, #1175 addressed this.

[Lukasa]

Sure is.