From 53a18d2e2734d078200ec607055ae551245ae74b Mon Sep 17 00:00:00 2001
From: Felix Lange <fjl@users.noreply.github.com>
Date: Thu, 10 May 2018 12:26:36 +0200
Subject: 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 | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
(limited to 'event/feed.go')
diff --git a/event/feed.go b/event/feed.go
index 78fa3d98d..f578f00c1 100644
--- a/event/feed.go
+++ b/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 {
--
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