runtime: drop chan circular linked list in favor of circular buffer

The list elements are already being allocated out of a
single memory buffer.  We can drop the Link* pointer
following and the memory it requires, replacing it with
index operations.

The change also keeps a channel from containing a pointer
back into its own allocation block, which would create a
cycle.  Blocks involved in cycles are not guaranteed to be
finalized properly, and channels depend on finalizers to
free OS-level locks on some systems.  The self-reference
was keeping channels from being garbage collected.

runtime-gdb.py will need to be updated in order to dump
the content of buffered channels with the new data structure.

Fixes #1676.

R=ken2, r
CC=golang-dev
https://golang.org/cl/4411045

1 files changed, 41 insertions, 0 deletions

diff --git a/test/gc2.go b/test/gc2.go
new file mode 100644
index 0000000000..c5c6cbe4bb
--- /dev/null
+++ b/test/gc2.go
@@ -0,0 +1,41 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check that buffered channels are garbage collected properly.
+// An interesting case because they have finalizers and used to
+// have self loops that kept them from being collected.
+// (Cyclic data with finalizers is never finalized, nor collected.)
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+)
+
+func main() {
+	const N = 10000
+	st := runtime.MemStats
+	for i := 0; i < N; i++ {
+		c := make(chan int, 10)
+		_ = c
+		if i%100 == 0 {
+			for j := 0; j < 4; j++ {
+				runtime.GC()
+				runtime.Gosched()
+				runtime.GC()
+				runtime.Gosched()
+			}
+		}
+	}
+	
+	obj := runtime.MemStats.HeapObjects - st.HeapObjects
+	if obj > N/5 {
+		fmt.Println("too many objects left:", obj)
+		os.Exit(1)
+	}
+}