storage/sql/postgres: expose stdlib tunables, set them for tests

- adapted TestUnmarshalConfig to ensure the fields are read in
- added a test to see that at least MaxOpenConns works:
  - this is only exposed through (*db).Stats() in go 1.11, so this test
    has a build tag
  - the other two configurables can't be read back, so we've got to
    trust that the mechanism works given the one instance that's tested..

Signed-off-by: Stephan Renatus <srenatus@chef.io>

storage/sql/postgres_test.go

@@ -0,0 +1,48 @@
+// +build go1.11
+
+package sql
+
+import (
+	"os"
+	"testing"
+)
+
+func TestPostgresTunables(t *testing.T) {
+	host := os.Getenv(testPostgresEnv)
+	if host == "" {
+		t.Skipf("test environment variable %q not set, skipping", testPostgresEnv)
+	}
+	baseCfg := &Postgres{
+		Database: getenv("DEX_POSTGRES_DATABASE", "postgres"),
+		User:     getenv("DEX_POSTGRES_USER", "postgres"),
+		Password: getenv("DEX_POSTGRES_PASSWORD", "postgres"),
+		Host:     host,
+		SSL: PostgresSSL{
+			Mode: sslDisable, // Postgres container doesn't support SSL.
+		}}
+
+	t.Run("with nothing set, uses defaults", func(t *testing.T) {
+		cfg := *baseCfg
+		c, err := cfg.open(logger, cfg.createDataSourceName())
+		if err != nil {
+			t.Fatalf("error opening connector: %s", err.Error())
+		}
+		defer c.db.Close()
+		if m := c.db.Stats().MaxOpenConnections; m != 5 {
+			t.Errorf("expected MaxOpenConnections to have its default (5), got %d", m)
+		}
+	})
+
+	t.Run("with something set, uses that", func(t *testing.T) {
+		cfg := *baseCfg
+		cfg.MaxOpenConns = 101
+		c, err := cfg.open(logger, cfg.createDataSourceName())
+		if err != nil {
+			t.Fatalf("error opening connector: %s", err.Error())
+		}
+		defer c.db.Close()
+		if m := c.db.Stats().MaxOpenConnections; m != 101 {
+			t.Errorf("expected MaxOpenConnections to be set to 101, got %d", m)
+		}
+	})
+}