gopls/internal/regtest: only run in 'singleton' mode on certain GOOS

The gopls regtests have highly variable performance on certain operating
systems, we believe due to kernel contention. Flakes are a problem, and
it's better to have coverage of our default execution mode than no
coverage at all.

For golang/go#44099
For golang/go#42789

Change-Id: Ie9f3f93bb950930401aac3ce55cdffb889d0f0e1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/289692
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

Author: findleyr

gopls/internal/regtest/regtest.go

@@ -62,11 +62,24 @@ func (r RunMultiple) Run(t *testing.T, files string, f TestFunc) {
 	}
 }
 
+// The regtests run significantly slower on these operating systems, due to (we
+// believe) kernel locking behavior. Only run in singleton mode on these
+// operating system when using -short.
+var slowGOOS = map[string]bool{
+	"darwin":  true,
+	"openbsd": true,
+	"plan9":   true,
+}
+
 func DefaultModes() Mode {
+	normal := Singleton | Experimental
+	if slowGOOS[runtime.GOOS] && testing.Short() {
+		normal = Singleton
+	}
 	if *runSubprocessTests {
-		return NormalModes | SeparateProcess
+		return normal | SeparateProcess
 	}
-	return NormalModes
+	return normal
 }
 
 // Main sets up and tears down the shared regtest state.

gopls/internal/regtest/runner.go

@@ -47,9 +47,6 @@ const (
 	// Experimental enables all of the experimental configurations that are
 	// being developed. Currently, it enables the workspace module.
 	Experimental
-	// NormalModes are the global default execution modes, when unmodified by
-	// test flags or by individual test options.
-	NormalModes = Singleton | Experimental
 )
 
 // A Runner runs tests in gopls execution environments, as specified by its