systemfd
systemfd is the 1% of systemd that's useful for development. It's a tiny process that
opens a bunch of sockets and passes them to another process so that that process can
then restart itself without dropping connections. For that it uses the systemd socket
passing protocol (LISTEN_FDS + LISTEN_PID) environment variables on macOS and Linux
and a custom protocol on Windows. Both are supported by the
listenfd crate.
When combined with cargo-watch or
watchexec you can get automatically
reloading development servers:
$ systemfd --no-pid -s http::5000 -- cargo watch -x run
$ systemfd --no-pid -s http::5000 -- watchexec -r -- cargo run
The --no-pid flag disables passing the LISTEN_PID variable on unix (it has no effect
on Windows). This makes listenfd skip the pid check which would fail with
cargo watch otherwise. To see how to implement a server ready for systemfd
see below.
Installation
You can install a precompiled version of systemfd with curl to bash:
curl -sSfL https://github.com/mitsuhiko/systemfd/releases/latest/download/systemfd-installer.sh | sh
On windows you can use powershell to install it:
powershell -c "irm https://github.com/mitsuhiko/systemfd/releases/latest/download/systemfd-installer.ps1 | iex"
Alternatively you can get systemfd by installing it with cargo:
cargo install systemfd
Usage
systemfd creates one or more sockets as specified on the command line and then
invokes another application. Each time you pass the -s (or --socket)
parameter a new socket is created. The value for the parameter is a socket
specification in the format [TYPE::]VALUE where TYPE defaults to tcp or
unix depending on the value. The following types exist:
tcp: creates a tcp listenerhttp: creates a tcp listener for http usage (prints a http url in the info output)https: creates a tcp listener for https usage (prints a https url in the info output)unix: creates a unix listener socketudp: creates a udp socket
VALUE depends on the socket type. The following formats are supported:
<port>: an integer port value, assumes 127.0.0.1 as host<host>:<port>: binds to a specific network interface and port<unix-path>: requests a specific unix socket
Examples
$ systemfd -s http::5000 -- my-server-executable
$ systemfd -s http::5000 -s https::5443 -- my-server-executable
$ systemfd -s 5000 -- my-server-executable
$ systemfd -s udp::1567 -- my-game-server-executable
When systemfd starts it will print out the socket it created. This can be disabled
by passing -q. Additionally if a port is set to 0 a random port is picked.
Windows Protocol
On Windows, passing of sockets is significantly more complex than on Unix. To
achieve this, this utility implements a custom socket passing system that is also
implemented by the listenfd crate. When the sockets are created, an additional
local RPC server is spawned that gives out duplicates of the sockets to other
processes. The RPC server uses TCP and is communicated to the child with the
SYSTEMFD_SOCKET_SERVER environment variable. The RPC calls themselves are
protected with a SYSTEMFD_SOCKET_SECRET secret key.
The only understood command is SECRET|PID with secret and the processes' PID
inserted. The server replies with N WSAPROTOCOL_INFOW structures. The client
is expected to count the number of bytes and act accordingly.
This protocol is currently somewhat of a hack and might change. It only
exists to support the listenfd crate.
License and Links
- Issue Tracker
- Example
- License: Apache-2.0
