Rule: Error out earlier when building alertmanager config (#5405)

* Error out earlier when building alertmanager config

Signed-off-by: Jéssica Lins <jessicaalins@gmail.com>

* Add test case for empty host

Signed-off-by: Jéssica Lins <jlins@redhat.com>

pkg/alert/config.go

@@ -95,7 +95,15 @@ func BuildAlertmanagerConfig(address string, timeout time.Duration) (Alertmanage
 	}
 
 	scheme := parsed.Scheme
+	if scheme == "" {
+		return AlertmanagerConfig{}, errors.New("alertmanagers.url contains empty scheme")
+	}
+
 	host := parsed.Host
+	if host == "" {
+		return AlertmanagerConfig{}, errors.New("alertmanagers.url contains empty host")
+	}
+
 	for _, qType := range []dns.QType{dns.A, dns.SRV, dns.SRVNoA} {
 		prefix := string(qType) + "+"
 		if strings.HasPrefix(strings.ToLower(scheme), prefix) {

pkg/alert/config_test.go

@@ -132,6 +132,14 @@ func TestBuildAlertmanagerConfiguration(t *testing.T) {
 			address: "://user:pass@localhost:9093",
 			err:     true,
 		},
+		{
+			address: "http://user:pass@",
+			err:     true,
+		},
+		{
+			address: "dnssrv+_http._tcp.example.com",
+			err:     true,
+		},
 	} {
 		t.Run(tc.address, func(t *testing.T) {
 			cfg, err := BuildAlertmanagerConfig(tc.address, time.Duration(0))