config: warn the user if client_addr is empty

if the provided value is empty string then the client services
(DNS, HTTP, HTTPS, GRPC) are not listening and the user is not notified
in any way about what's happening.
Also, since a not provided client_addr defaults to 127.0.0.1, we make sure
we are not getting unwanted warnings

Signed-off-by: Alessandro De Blasis <alex@deblasis.net>

agent/config/builder.go

@@ -552,6 +552,9 @@ func (b *builder) build() (rt RuntimeConfig, err error) {
 
 	// determine client addresses
 	clientAddrs := b.expandIPs("client_addr", c.ClientAddr)
+	if len(clientAddrs) == 0 {
+		b.warn("client_addr is empty, client services (DNS, HTTP, HTTPS, GRPC) will not be listening for connections")
+	}
 	dnsAddrs := b.makeAddrs(b.expandAddrs("addresses.dns", c.Addresses.DNS), clientAddrs, dnsPort)
 	httpAddrs := b.makeAddrs(b.expandAddrs("addresses.http", c.Addresses.HTTP), clientAddrs, httpPort)
 	httpsAddrs := b.makeAddrs(b.expandAddrs("addresses.https", c.Addresses.HTTPS), clientAddrs, httpsPort)

agent/config/builder_test.go

@@ -247,3 +247,47 @@ func TestLoad_HTTPMaxConnsPerClientExceedsRLimit(t *testing.T) {
 	require.Error(t, err)
 	assert.Contains(t, err.Error(), "but limits.http_max_conns_per_client: 16777217 needs at least 16777237")
 }
+
+func TestLoad_EmptyClientAddr(t *testing.T) {
+
+	type testCase struct {
+		name                   string
+		clientAddr             *string
+		expectedWarningMessage *string
+	}
+
+	fn := func(t *testing.T, tc testCase) {
+		opts := LoadOpts{
+			FlagValues: Config{
+				ClientAddr: tc.clientAddr,
+				DataDir:    pString("dir"),
+			},
+		}
+		patchLoadOptsShims(&opts)
+		result, err := Load(opts)
+		require.NoError(t, err)
+		if tc.expectedWarningMessage != nil {
+			require.Len(t, result.Warnings, 1)
+			require.Contains(t, result.Warnings[0], *tc.expectedWarningMessage)
+		}
+	}
+
+	var testCases = []testCase{
+		{
+			name:                   "empty string",
+			clientAddr:             pString(""),
+			expectedWarningMessage: pString("client_addr is empty, client services (DNS, HTTP, HTTPS, GRPC) will not be listening for connections"),
+		},
+		{
+			name:                   "nil pointer",
+			clientAddr:             nil, // defaults to 127.0.0.1
+			expectedWarningMessage: nil, // expecting no warnings
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			fn(t, tc)
+		})
+	}
+}