itest: test "lnd --debuglevel=show" command

Tests that "lnd --debuglevel=show" command works and prints the list of
supported subsystems.

itest/list_on_test.go

@@ -702,4 +702,8 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "send to route failed htlc timeout",
 		TestFunc: testSendToRouteFailHTLCTimeout,
 	},
+	{
+		Name:     "debuglevel show",
+		TestFunc: testDebuglevelShow,
+	},
 }

itest/lnd_config.go

@@ -0,0 +1,26 @@
+package itest
+
+import (
+	"os/exec"
+
+	"github.com/lightningnetwork/lnd/lntest"
+	"github.com/stretchr/testify/require"
+)
+
+// testDebuglevelShow tests that "lnd --debuglevel=show" command works and
+// prints the list of supported subsystems.
+func testDebuglevelShow(ht *lntest.HarnessTest) {
+	// We can't use ht.NewNode, because it adds more arguments to the
+	// command line (e.g. flags configuring bitcoin backend), but we want to
+	// make sure that "lnd --debuglevel=show" works without any other flags.
+	lndBinary := getLndBinary(ht.T)
+	cmd := exec.Command(lndBinary, "--debuglevel=show")
+	stdoutStderrBytes, err := cmd.CombinedOutput()
+	require.NoError(ht.T, err, "failed to run 'lnd --debuglevel=show'")
+
+	// Make sure that the output contains the list of supported subsystems
+	// and that the list is not empty. We search PEER subsystem.
+	stdoutStderr := string(stdoutStderrBytes)
+	require.Contains(ht.T, stdoutStderr, "Supported subsystems")
+	require.Contains(ht.T, stdoutStderr, "PEER")
+}