Creating IP/TCP/*Conn from fd and socket activated services

[Niklas Schnelle]

Hi fellow Gophers,

when reading http://0pointer.de/blog/projects/inetd.html I began to wonder how hard it would be to
support socket activation in a Go server. I couldn't find anything in the net package on how to create a XXConn from an
fd though. Is this supported? It would be kind of cool to have systemd listen on port 80 and just passing connected sockets to a Go server
especially since this also solves the problem of listening on privileged ports.

Greetings Nik

[minux]

first convert the fd to *os.File via os.NewFile,

and then convert the *os.File to a net.Conn via net.FileConn.

[Albert Strasheim]

Hello

I contributed the FileConn patch long ago to be able to do exactly this.

Enjoy!

Regards

Albert

[Albert Strasheim]

Hello again

On Saturday, January 26, 2013 1:57:03 AM UTC+2, minux wrote:

On Sat, Jan 26, 2013 at 3:38 AM, Niklas Schnelle <niklas....@gmail.com> wrote:

when reading http://0pointer.de/blog/projects/inetd.html I began to wonder how hard it would be to
support socket activation in a Go server. I couldn't find anything in the net package on how to create a XXConn from an
fd though. Is this supported? It would be kind of cool to have systemd listen on port 80 and just passing connected sockets to a Go server
especially since this also solves the problem of listening on privileged ports.

first convert the fd to *os.File via os.NewFile,

and then convert the *os.File to a net.Conn via net.FileConn.

You might also find this snippet useful:

https://gist.github.com/4640792

Regards

Albert

[al...@polvi.net]

Here is a complete example using that snippet.

https://github.com/polvi/go-socket-activated-example

-Alex

[Dave Cheney]

Very nice. You don't need to play with O_CLOEXEC, net.FileListener
will ensure the fd is in the correct state.

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[Naitik Shah]

I've also built a library that makes it possible to optionally use
systemd socket activation:

https://github.com/daaku/go.grace/blob/master/grace.go#L144

(Though as I re-read the code I realized the TCPListener assumption is
unnecessary and it should just be any FileListener).


-Naitik

--
-Naitik

[Albert Strasheim]

Hello

mattharden made a good point which made me realise that the original
snippet isn't quite doing what sd_listen_fds is doing.

Looking at the code again:

http://cgit.freedesktop.org/systemd/systemd/plain/src/libsystemd-daemon/sd-daemon.c

sd_listen_fds is checking the CLOEXEC flag and if it's set, doesn't
set it again. However, all the file descriptors are still "returned"
to the caller.

I mistakenly made the snippet skip file descriptors without CLOEXEC
set. Not enough coffee that day.

Aside: it might be nice to write a sd_notify in Go too. Check out the
WATCHDOG=1 section here:

http://www.freedesktop.org/software/systemd/man/sd_notify.html

Regards

Albert