The existing jsoncomm is a work of beautiy. For very big arguments
however the used `SOCK_SEQPACKET` hits the limitations of the
kernel network buffer size (see also [0]). This lead to various
workarounds in #824,#1331,#1836 where parts of the request are
encoded as part of the json method call and parts are done via
a side-channel via fd-passing.
This commit changes the code so that the fd channel is automatically
and transparently created and the workarounds are removed. A test
is added that ensures that very big messages can be passed.
[0] https://github.com/osbuild/osbuild/pull/1833
When `jsoncomm` fails because the message is too big it currently
does not indicate just how big the message was. This commit adds
this information so that it's easier for us to determine what to
do about it.
We could also include a pointer to `/proc/sys/net/core/wmem_defaults`
but it seems we want to not require fiddling with that so let's
not do it for now.
See also https://github.com/osbuild/osbuild/pull/1838
Often, a message is being sent and followed by a call to `recv`
to wait for a reply. Create a simple helper `send_and_recv` that
does both in one method.
Add a simple check for that helper to the tests.
Add a new constructor method that allows creating a `Socket` from
an existing file-descriptor of a socket. This might be need when
the socket was passed to a child process.
Add a simple test for the new constructor method.
Add a new constructor method, `Socket.new_pair`, to create a pair
of connected sockets (via `socketpair`) and wrap both sides via
`jsoncomm.Socket`.
Add a simple test to check it.
Now that jsoncomm.Socket is using a connection-oriented socket,
the destination in `socket.sendmsg` is ignored and thus can and
should be dropped from the `jsoncomm.Socket.send` method.
Adjust the tests accordingly.
Switch to use a connection oriented datagram based protocol, i.e.
`SOCK_SEQPACKET`, instead of `SOCK_DGRAM`. It sill preserves
message boundaries, but since it is connection oriented the client
nor the server do not need to specify the destination addresses
of the peer in sendmsg/recvmesg. Moreover, the host will be able
to send messages to the client, even if the latter is sandboxed
with a separate network namespace. In the `SOCK_DRAM` case the
auto-bound address of the client would not be visible to the host
and thus sending messages would to it would fail.
Adapt the jsoncomm tests as well as `BaseAPI`.
Add support for `util.types.PathLike` paths for socket addresses,
instead of just plain strings. Test it by using pathlib.Path to
create the address in the corresponding test.
