runtime: fix memory leak in parallel garbage collector

The work buffer management used by the garbage
collector during parallel collections leaks buffers.
This CL tests for and fixes the leak.

R=golang-dev, dvyukov, r
CC=golang-dev
https://golang.org/cl/5254059

src/pkg/runtime/gc_test.go

+package runtime_test
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestGcSys(t *testing.T) {
+	for i := 0; i < 1000000; i++ {
+		workthegc()
+	}
+
+	// Should only be using a few MB.
+	runtime.UpdateMemStats()
+	sys := runtime.MemStats.Sys
+	t.Logf("using %d MB", sys>>20)
+	if sys > 10e6 {
+		t.Fatalf("using too much memory: %d MB", sys>>20)
+	}
+}
+
+func workthegc() []byte {
+	return make([]byte, 1029)
+}

src/pkg/runtime/mgc0.c

@@ -501,7 +501,7 @@ putempty(Workbuf *b)
 
 	runtime·lock(&work.emu);
 	b->next = work.empty;
-	work.empty = b->next;
+	work.empty = b;
 	runtime·unlock(&work.emu);
 }
 

src/run.bash

@@ -41,6 +41,10 @@ fi
 gomake testshort
 ) || exit $?
 
+(xcd pkg/runtime;
+gotest -short -cpu=1,2,4
+) || exit $?
+
 (xcd pkg/sync;
 GOMAXPROCS=10 gomake testshort
 ) || exit $?