event: document select case slice use and add edge case test (#16680)

Feed keeps active subscription channels in a slice called 'f.sendCases'.
The Send method tracks the active cases in a local variable 'cases'
whose value is f.sendCases initially. 'cases' shrinks to a shorter
prefix of f.sendCases every time a send succeeds, moving the successful
case out of range of the active case list.

This can be confusing because the two slices share a backing array. Add
more comments to document what is going on. Also add a test for removing
a case that is in 'f.sentCases' but not 'cases'.

event/feed.go

@@ -148,7 +148,9 @@ func (f *Feed) Send(value interface{}) (nsent int) {
 		f.sendCases[i].Send = rvalue
 	}
 
-	// Send until all channels except removeSub have been chosen.
+	// Send until all channels except removeSub have been chosen. 'cases' tracks a prefix
+	// of sendCases. When a send succeeds, the corresponding case moves to the end of
+	// 'cases' and it shrinks by one element.
 	cases := f.sendCases
 	for {
 		// Fast path: try sending without blocking before adding to the select set.
@@ -170,6 +172,7 @@ func (f *Feed) Send(value interface{}) (nsent int) {
 			index := f.sendCases.find(recv.Interface())
 			f.sendCases = f.sendCases.delete(index)
 			if index >= 0 && index < len(cases) {
+				// Shrink 'cases' too because the removed case was still active.
 				cases = f.sendCases[:len(cases)-1]
 			}
 		} else {

event/feed_test.go

@@ -235,6 +235,45 @@ func TestFeedUnsubscribeBlockedPost(t *testing.T) {
 	wg.Wait()
 }
 
+// Checks that unsubscribing a channel during Send works even if that
+// channel has already been sent on.
+func TestFeedUnsubscribeSentChan(t *testing.T) {
+	var (
+		feed Feed
+		ch1  = make(chan int)
+		ch2  = make(chan int)
+		sub1 = feed.Subscribe(ch1)
+		sub2 = feed.Subscribe(ch2)
+		wg   sync.WaitGroup
+	)
+	defer sub2.Unsubscribe()
+
+	wg.Add(1)
+	go func() {
+		feed.Send(0)
+		wg.Done()
+	}()
+
+	// Wait for the value on ch1.
+	<-ch1
+	// Unsubscribe ch1, removing it from the send cases.
+	sub1.Unsubscribe()
+
+	// Receive ch2, finishing Send.
+	<-ch2
+	wg.Wait()
+
+	// Send again. This should send to ch2 only, so the wait group will unblock
+	// as soon as a value is received on ch2.
+	wg.Add(1)
+	go func() {
+		feed.Send(0)
+		wg.Done()
+	}()
+	<-ch2
+	wg.Wait()
+}
+
 func TestFeedUnsubscribeFromInbox(t *testing.T) {
 	var (
 		feed Feed