runtime/race: run tests with GOMAXPROCS=1

We set GOMAXPROCS=1 to prevent test flakiness.
There are two sources of flakiness:
1. Some tests rely on particular execution order.
   If the order is different, race does not happen at all.
2. Ironically, ThreadSanitizer runtime contains a logical race condition
   that can lead to false negatives if racy accesses happen literally at the same time.
Tests used to work reliably in the good old days of GOMAXPROCS=1.
So let's set it for now. A more reliable solution is to explicitly annotate tests
with required execution order by means of a special "invisible" synchronization primitive
(that's what is done for C++ ThreadSanitizer tests). This is issue #14119.

This reduces flakes on RaceAsFunc3 test from 60/3000 to 1/3000.

Fixes #14086
Fixes #14079
Fixes #14035

Change-Id: Ibaec6b2b21e27b62563bffbb28473a854722cf41
Reviewed-on: https://go-review.googlesource.com/18968Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/race/output_test.go

@@ -51,7 +51,10 @@ func TestOutput(t *testing.T) {
 			}
 			cmd.Env = append(cmd.Env, env)
 		}
-		cmd.Env = append(cmd.Env, "GORACE="+test.gorace)
+		cmd.Env = append(cmd.Env,
+			"GOMAXPROCS=1", // see comment in race_test.go
+			"GORACE="+test.gorace,
+		)
 		got, _ := cmd.CombinedOutput()
 		if !regexp.MustCompile(test.re).MatchString(string(got)) {
 			t.Fatalf("failed test case %v, expect:\n%v\ngot:\n%s",

src/runtime/race/race_test.go

@@ -155,7 +155,20 @@ func runTests() ([]byte, error) {
 		}
 		cmd.Env = append(cmd.Env, env)
 	}
-	cmd.Env = append(cmd.Env, `GORACE=suppress_equal_stacks=0 suppress_equal_addresses=0 exitcode=0`)
+	// We set GOMAXPROCS=1 to prevent test flakiness.
+	// There are two sources of flakiness:
+	// 1. Some tests rely on particular execution order.
+	//    If the order is different, race does not happen at all.
+	// 2. Ironically, ThreadSanitizer runtime contains a logical race condition
+	//    that can lead to false negatives if racy accesses happen literally at the same time.
+	// Tests used to work reliably in the good old days of GOMAXPROCS=1.
+	// So let's set it for now. A more reliable solution is to explicitly annotate tests
+	// with required execution order by means of a special "invisible" synchronization primitive
+	// (that's what is done for C++ ThreadSanitizer tests). This is issue #14119.
+	cmd.Env = append(cmd.Env,
+		"GOMAXPROCS=1",
+		"GORACE=suppress_equal_stacks=0 suppress_equal_addresses=0 exitcode=0",
+	)
 	return cmd.CombinedOutput()
 }