magicsock: add description of testClosingContext

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/wgengine/magicsock/magicsock_test.go b/wgengine/magicsock/magicsock_test.go
index 794d01e7a..4aac0cdad 100644
--- a/wgengine/magicsock/magicsock_test.go
+++ b/wgengine/magicsock/magicsock_test.go
@@ -543,6 +543,11 @@ func TestDeviceStartStop(t *testing.T) {
 	dev.Close()
 }
 
+// A context used in TestConnClosing() which seeks to test that code which calls
+// Err() to see if a connection is already being closed does not then proceed to
+// try to acquire the mutex, as this would lead to deadlock. When Err() is called
+// this context acquires the lock itself, in order to force a deadlock (and test
+// failure on timeout).
 type testConnClosingContext struct {
 	parent context.Context
 	mu     *sync.Mutex