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wgengine/magicsock: fix deadlock on shutdown

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This fixes a deadlock on shutdown.
One goroutine is waiting to send on c.derpRecvCh before unlocking c.mu.
The other goroutine is waiting to lock c.mu before receiving from c.derpRecvCh.

#3736 has a more detailed explanation of the sequence of events.

Fixes #3736

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
This commit is contained in:
Josh Bleecher Snyder 2022-01-14 13:43:24 -08:00 committed by Josh Bleecher Snyder
parent 390490e7b1
commit de4696da10
1 changed files with 3 additions and 1 deletions
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4
wgengine/magicsock/magicsock.go
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@ -265,6 +265,7 @@ type Conn struct {
stunReceiveFunc atomic.Value // of func(p []byte, fromAddr *net.UDPAddr)
// derpRecvCh is used by receiveDERP to read DERP messages.
// It must have buffer size > 0; see issue 3736.
derpRecvCh chan derpReadResult
// bind is the wireguard-go conn.Bind for Conn.
@ -529,7 +530,7 @@ func (o *Options) derpActiveFunc() func() {
// of NewConn. Mostly for tests.
func newConn() *Conn {
c := &Conn{
derpRecvCh: make(chan derpReadResult),
derpRecvCh: make(chan derpReadResult, 1), // must be buffered, see issue 3736
derpStarted: make(chan struct{}),
peerLastDerp: make(map[key.NodePublic]int),
peerMap: newPeerMap(),
@ -2647,6 +2648,7 @@ func (c *connBind) Close() error {
}
// Send an empty read result to unblock receiveDERP,
// which will then check connBind.Closed.
// connBind.Closed takes c.mu, but c.derpRecvCh is buffered.
c.derpRecvCh <- derpReadResult{}
return nil
}