.. there are some additional FIXME nags in net_tcp (L 1012) about blocking
because libuv is holding unsafe ptrs to task local data. the proposed
fix going is not really feasible w/ the current design, IMO, but i'll
leave it there in case someone really wants to make the case without
creating more hassle than it's worth.
- we now have two interfaces for the TCP/IP server/listener workflow,
based on different user approaches surrounding how to deal with the
flow of accept a new tcp connection:
1. the "original" API closely mimics the low-level libuv API, in that we
have an on_connect_cb that the user provides *that is ran on the libuv
thread*. In this callback, the user can accept() a connection, turning it
into a tcp_socket.. of course, before accepting, they have the option
of passing it to a new task, provided they *make the cb block until
the accept is done* .. this is because, in libuv, you have to do the
uv_accept call in the span of that on_connect_cb callback that gets fired
when a new connection comes in. thems the breaks..
I wanted to just get rid of this API, because the general proposition of
users always running code on the libuv thread sounds like an invitation
for many future headaches. the API restriction to have to choose to
immediately accept a connection (and allow the user to block libuv as
needed) isn't too bad for power users who could conceive of circumstances
where they would drop an incoming TCP connection and know what they're
doing, in general.
but as a general API, I thought this was a bit cumbersome, so I ended up
devising..
2. an API that is initiated with a call to `net::tcp::new_listener()` ..
has a similar signature to `net::tcp::listen()`, except that is just
returns an object that sort of behaves like a `comm::port`. Users can
block on the `tcp_conn_port` to receive new connections, either in the
current task or in a new task, depending on which API route they take
(`net::tcp::conn_recv` or `net::tcp::conn_recv_spawn` respectively).. there
is also a `net::tcp::conn_peek` function that will do a peek on the
underlying port to see if there are pending connections.
The main difference, with this API, is that the low-level libuv glue is
going to *accept every connection attempt*, along with the overhead that
that brings. But, this is a much more hassle-free API for 95% of use
cases and will probably be the one that most users will want to reach for.
I need these in the context of doing various malloc/free operations for
libuv structs that need to live in the heap, because of API workflow
(there's no stack to put them in). This has cropped up several times
when impl'ing the high-level API for things like timers, but I've decided
to take the plunge and use this approach for the net::tcp module.
Technically, this can be avoided by spawning a new
task that contains the needed memory structures on its stack and then
having it block for the duration of the time we need that memory to be
valid (this is what I did in std::timer). Exposing this API provides a
much lower overhead way to address
the issue, albeit with safety concerns. The main mitigation policy should
be to use malloc/free with libuv handles only when the handles, are then
associated with a resource or class-with-dtor. So we have a finite lifetime
for the object and can gaurantee a free(), barring a runtime crash (in
which case you have bigger problems!)
.. up next: windows!
.. impl'd uv::direct::read_stop() and uv::direct::close() to wrap things up
.. demonstrated sending data out of the uv_read_cb via a channel (which
we block on to recv all of it, complete w/ EOF notification) that is
read from after the loop exits.
.. helpers to read the guts of a uv_buf_t
.. an idea im kicking around: starting to pile up all of these hideous
data accessor functions in uv::direct .. I might make impl/iface pairs
for the various uv_* types that I'm using, in order to encapsulate those
data access functions and, perhaps, make the access look a little cleaner
(it still won't be straight field access, but it'll be a lot better)
.. formatting cleanup to satisfy make check
so we're now adhering the libuv C api and passing structs by-val where
it is expected, instead of pulling pointer trickery (or worse having to
malloc structs in c++ to be passed back to rust and then into C again)
have to use ++ sigil in rust-side extern fn decls in order to have rust
actually copy the struct, by value, onto the C stack. gotcha, indeed.
also adding a helper method to verify/remind how to pass a struct by-val
into C... check out the rust fn sig for rust_uv_ip4_test_verify_port_val()
for more infos
