commit 6baa3c4d5f70fd50223dc41febf67576267d039b Author: Cecylia Bocovich cohosh@torproject.org Date: Thu Oct 15 14:47:51 2020 -0400

Add synchronization to prevent post-melt collects

This fixes a race condition in which snowflakes.End() is called while snowflakes.Collect() is in progress resulting in a write to a closed channel. We now wait for all in-progress collections to finish and add an extra check before proceeding with a collection. --- client/lib/peers.go | 15 +++++++++++++-- client/lib/snowflake.go | 1 - 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/client/lib/peers.go b/client/lib/peers.go index d864fc8..d02eed3 100644 --- a/client/lib/peers.go +++ b/client/lib/peers.go @@ -5,6 +5,7 @@ import ( "errors" "fmt" "log" + "sync" )

// Container which keeps track of multiple WebRTC remote peers. @@ -25,7 +26,10 @@ type Peers struct { snowflakeChan chan *WebRTCPeer activePeers *list.List

- melt chan struct{} + melt chan struct{} + melted bool + + collection sync.WaitGroup }

// Construct a fresh container of remote peers. @@ -45,6 +49,11 @@ func NewPeers(tongue Tongue) (*Peers, error) { // As part of |SnowflakeCollector| interface. func (p *Peers) Collect() (*WebRTCPeer, error) { // Engage the Snowflake Catching interface, which must be available. + p.collection.Add(1) + defer p.collection.Done() + if p.melted { + return nil, fmt.Errorf("Snowflakes have melted") + } if nil == p.Tongue { return nil, errors.New("missing Tongue to catch Snowflakes with") } @@ -110,8 +119,10 @@ func (p *Peers) purgeClosedPeers() {

// Close all Peers contained here. func (p *Peers) End() { - close(p.snowflakeChan) close(p.melt) + p.melted = true + p.collection.Wait() + close(p.snowflakeChan) cnt := p.Count() for e := p.activePeers.Front(); e != nil; { next := e.Next() diff --git a/client/lib/snowflake.go b/client/lib/snowflake.go index 0ba5667..e888160 100644 --- a/client/lib/snowflake.go +++ b/client/lib/snowflake.go @@ -181,7 +181,6 @@ func Handler(socks net.Conn, tongue Tongue) error { // transfer to the Tor SOCKS handler when needed. func connectLoop(snowflakes SnowflakeCollector) { for { - // Check if ending is necessary. _, err := snowflakes.Collect() if err != nil { log.Printf("WebRTC: %v Retrying in %v...",