net.DialContext() stops things when the context is cancelled
These days, a number of core Go standard
packages support functions that take a
context.Context argument and abort their
operation if the context is cancelled. This is an interesting trick
in Go, because normally you can't gracefully interrupt a goroutine
doing network IO (which leads to problems in practice). When I started looking into the relevant
standard library code I expected to find that things like
net.Dialer.DialContext() had special hooks into the runtime's
network poller (netpoller) to
do this. This turns out to not be the case; instead dialing uses
an interesting and elegant approach that's open to everyone doing
In order to abort an outstanding dial operation if the context is cancelled, the net package simply sets an expired (write) deadline. In order to do this asynchronously, it starts a background goroutine to listen for the context being cancelled (and then there's some complexity involved to clean everything up properly and handle potential races; races caused a number of issues, eg issue 16523). Setting read and write deadlines is already explicitly documented as affecting currently pending reads (and writes), not just future ones, so dialing is reusing a general mechanism that already needs to exist.
(This reuse is a little bit tricky for dialing, which is taking advantage of a customary and useful property where the underlying OS only reports a network socket as writeable once it's connected. This means that you generally check for a connection having completed by seeing if it's now writeable, and in turn this means you can sensibly limit or abort this check by setting a write deadline.)
Now that I've discovered this use of deadlines in
it's clear that I can do the same thing to abort outstanding network
reads or writes in my own code.
As a bonus, this will probably return a fairly distinctive error,
or I can wrap this in something that implements 'read with context'
or 'write with context', probably with some of the race precautions
seen in the net package's code.
PS: I was going to say that this is also how
handles its context being cancelled, but then I went to look at the code
and now I have no idea how that actually works.
PPS: If the context you pass to
DialContext() already has a
DialContext() immediately sets a write deadline on the
underlying network connection, in addition to its handling of
cancellation. There's also some complexity in the code to stop
as soon as possible if the context is cancelled immediately,
before it starts up the whole extra goroutine infrastructure to