sync: set GOMAXPROCS to 1 in TestPoolGC

This test expects to be able to drain a Pool using only Get. This isn't
actually possible in the general case, since a pooled value could get
stuck in some P's private slot. However, if GOMAXPROCS=1, there's only 1
P we could be running on, so getting stuck becomes impossible.

This test isn't checking any concurrent properties of Pool, so this is
fine. Just set GOMAXPROCS=1 for this one particular test.

Fixes #73728.

Change-Id: I9053e28118060650f2cd7d0d58f5a86d630b36f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/673375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

diff --git a/src/sync/pool_test.go b/src/sync/pool_test.go
index 7f60ed70265bdcbcc8579eaea6106af8733340db..f0f0d3d40a30afdc91768e8ac6cc049de6a018b4 100644 (file)
--- a/src/sync/pool_test.go
+++ b/src/sync/pool_test.go
@@ -102,6 +102,18 @@ func TestPoolRelease(t *testing.T) {
 }
 
 func testPool(t *testing.T, drain bool) {
+       if drain {
+               // Run with GOMAXPROCS=1 if drain is set. The code below implicitly
+               // assumes it can remove all the pool-cached values with cleanups
+               // with Get, but this isn't necessarily true if a value gets stuck
+               // in the private slot for some P. This is especially likely when
+               // running with mayMoreStackPreempt. We can make this exact, however,
+               // by setting GOMAXPROCS to 1, so there's only 1 P. This is fine for
+               // this test, since we're not trying to check any concurrent properties
+               // of Pool anyway.
+               defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
+       }
+
        var p Pool
        const N = 100
        for try := 0; try < 3; try++ {