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 {