test: honor rust env in dual volume harness

test/volume_server/framework/cluster_dual.go

@@ -11,7 +11,7 @@ import (
 type DualVolumeCluster = MultiVolumeCluster
 
 // StartDualVolumeCluster starts a cluster with 2 volume servers.
-// Deprecated: Use StartMultiVolumeCluster(t, profile, 2) directly.
-func StartDualVolumeCluster(t testing.TB, profile matrix.Profile) *DualVolumeCluster {
-	return StartMultiVolumeCluster(t, profile, 2)
+// Deprecated: Use StartMultiVolumeClusterAuto(t, profile, 2) directly.
+func StartDualVolumeCluster(t testing.TB, profile matrix.Profile) MultiCluster {
+	return StartMultiVolumeClusterAuto(t, profile, 2)
 }

test/volume_server/framework/cluster_interface.go

@@ -22,12 +22,16 @@ type TestCluster interface {
 	Stop()
 }
 
+func useRustVolumeServer() bool {
+	return os.Getenv("VOLUME_SERVER_IMPL") == "rust"
+}
+
 // StartVolumeCluster starts a single-volume cluster using either the Go or
 // Rust volume server, depending on the VOLUME_SERVER_IMPL environment variable.
 // Set VOLUME_SERVER_IMPL=rust to use the Rust volume server.
 func StartVolumeCluster(t testing.TB, profile matrix.Profile) TestCluster {
 	t.Helper()
-	if os.Getenv("VOLUME_SERVER_IMPL") == "rust" {
+	if useRustVolumeServer() {
 		return StartRustVolumeCluster(t, profile)
 	}
 	return StartSingleVolumeCluster(t, profile)
@@ -52,7 +56,7 @@ type MultiCluster interface {
 // Set VOLUME_SERVER_IMPL=rust to use Rust volume servers.
 func StartMultiVolumeClusterAuto(t testing.TB, profile matrix.Profile, count int) MultiCluster {
 	t.Helper()
-	if os.Getenv("VOLUME_SERVER_IMPL") == "rust" {
+	if useRustVolumeServer() {
 		return StartRustMultiVolumeCluster(t, profile, count)
 	}
 	return StartMultiVolumeCluster(t, profile, count)

test/volume_server/framework/cluster_interface_test.go

@@ -0,0 +1,20 @@
+package framework
+
+import "testing"
+
+func TestUseRustVolumeServer(t *testing.T) {
+	t.Setenv("VOLUME_SERVER_IMPL", "rust")
+	if !useRustVolumeServer() {
+		t.Fatalf("expected rust selection when VOLUME_SERVER_IMPL=rust")
+	}
+
+	t.Setenv("VOLUME_SERVER_IMPL", "go")
+	if useRustVolumeServer() {
+		t.Fatalf("expected go selection when VOLUME_SERVER_IMPL=go")
+	}
+
+	t.Setenv("VOLUME_SERVER_IMPL", "")
+	if useRustVolumeServer() {
+		t.Fatalf("expected go selection when VOLUME_SERVER_IMPL is unset")
+	}
+}