Zero-copy writing const data under Seastar API level 9?

[niekbouman]

Dear @xemul ,

I have a piece of code that constructs a scattered_message and passes this into the write() method of an output_stream<char>.
In particlar, I use the

void append_static (const char_type *buf, size_t size)

method of scattered_message.

I am trying to port my code to Seastar's new API level 9, by constructing a vector of temporary_buffer<char>,
where for each buffer I use the zero-copy constructor:

temporary_buffer (CharType *buf, size_t size, deleter d) noexcept

The problem is that this takes a non-const pointer, which means I would need to use a const_cast when taking this route, which can, if I remember correctly, give Undefined Behavior, depending on the constness of the storage to which this pointer points.

In fact, I now see that scattered_message::append_static in fact does the same internally (via a C-style cast)

    void append_static(const char_type* buf, size_t size) {
        if (size) {
            _p = packet(std::move(_p), fragment{(char_type*)buf, size}, deleter());
        }
    }

Would you know of a way to support zero-copy-writes from storage pointed to by a const char* that avoids the need for a (possibly unsafe) const_cast ?

[avikivity]

Perhaps we need a temporary_buffer::from_static()

[xemul]

Or maybe it's worth re-visiting the code that tries to append const char* buffer to output_stream. Where does this raw buffer come from in your code?

[niekbouman]

Where does this raw buffer come from in your code?

From a third-party library that gives a const pointers to the data:
https://github.com/capnproto/capnproto/blob/v2/c%2B%2B/src/capnp/message.h#L233

In principle, I guess one could construct a temporary_buffer<const char> and an output_stream<const char> etc, but this will probably cause an undesired (and unsupported?) bifurcation / virality of constness..?

[avikivity]

Yes, temporary_buffer should not have been a template.