runtime: simplify TestSelectStackAdjust

Currently this test attempts to trigger a concurrent GC in a very
indirect way, but the way it does so is extremely error-prone. This test
is virtually always prone to flaking based on test order. For example if
the test that executed immediately before this one made a big heap but
didn't clean it up, then this test could easily fail to trigger a GC.
I was able to prove this with a small reproducer.

This roundabout way of triggering a GC is also way overkill for this
test. It just wants to get goroutines in a select and shrink their
stacks. Every GC will schedule a stack for shrinking if it can.

Replace all the complicated machinery with a single runtime.GC call.
I've confirmed that the test consistently triggers a stack shrink,
noting that both shrinkstack's copystack call is made and that
syncadjustsudogs (the relevant function that's being indirectly tested)
are both called.

Fixes #44610.

Change-Id: Ib1c091e0d1475bf6c596f56dc9b85eaea366fc73
Reviewed-on: https://go-review.googlesource.com/c/go/+/313109
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

diff --git a/src/runtime/chan_test.go b/src/runtime/chan_test.go
index 003d6a9fb357fe8d0b2470dd6bcfadd4107aa8cd..c9ce3ac643184f99a6776aa2b81971b9ddcf992d 100644 (file)
--- a/src/runtime/chan_test.go
+++ b/src/runtime/chan_test.go
@@ -708,8 +708,6 @@ func TestSelectDuplicateChannel(t *testing.T) {
        c <- 8 // wake up B.  This operation used to fail because c.recvq was corrupted (it tries to wake up an already running G instead of B)
 }
 
-var selectSink interface{}
-
 func TestSelectStackAdjust(t *testing.T) {
        // Test that channel receive slots that contain local stack
        // pointers are adjusted correctly by stack shrinking.
@@ -766,20 +764,8 @@ func TestSelectStackAdjust(t *testing.T) {
        <-ready2
        time.Sleep(10 * time.Millisecond)
 
-       // Force concurrent GC a few times.
-       var before, after runtime.MemStats
-       runtime.ReadMemStats(&before)
-       for i := 0; i < 100; i++ {
-               selectSink = new([1 << 20]byte)
-               runtime.ReadMemStats(&after)
-               if after.NumGC-before.NumGC >= 2 {
-                       goto done
-               }
-               runtime.Gosched()
-       }
-       t.Fatal("failed to trigger concurrent GC")
-done:
-       selectSink = nil
+       // Force concurrent GC to shrink the stacks.
+       runtime.GC()
 
        // Wake selects.
        close(d)