HTTP/3: Potential race between AddConn and RoundTrip #457
Description
I believe there's a race between the two paths when using HTTP/3 for the transport. I found this when hitting a panic here in internal/http3/transport.go:
return conn, t.newClientConn(conn), nil
I had a suspicion here because the wrapping path (transport.go) was recently refactored to use http3.Transport instead of http3.RoundTripper. This only happens intermittently but it is prevalent when using imroc/req in a highly trafficked environment that generates a bunch of custom clients.
Anyway, this t.newClientConn is initialized by:
func (t *Transport) init() error
... which normally is initialized inside roundTripOpt:
t.initOnce.Do(func() { t.initErr = t.init() })
Except this doesn't happen when calling this separate path:
// AddConn add a http3 connection, dial new conn if not exists.
func (t *Transport) AddConn(ctx context.Context, addr string) error {
addr = authorityAddr(addr)
cl, _, err := t.getClient(ctx, addr, false)
if err == nil {
cl.useCount.Add(-1)
}
return err
}
The whole chain to handlePendingAltSvc seems like it can potentially race after launching the Goroutine, I think. Let me know what you think.