fix composite alert crash

.go-version

@@ -1 +1 @@
-1.95.3
+1.95.4

lightstep/resource_alert_test.go

@@ -1312,3 +1312,78 @@ resource "lightstep_alert" "test" {
 		},
 	})
 }
+
+func TestCompositeCrash(t *testing.T) {
+	var condition client.UnifiedCondition
+
+	conditionConfig := `
+resource "lightstep_alert" "test" {
+  name         = "test"
+  project_name = "` + testProject + `"
+
+  composite_alert {
+
+    alert {
+      name  = "A"
+      title = "cpu"
+
+      query {
+        hidden       = false
+        query_name   = "a"
+        query_string = "metric a | delta 30s | group_by[], sum"
+        display      = "line"
+        hidden_queries = { "a" = "%s" }
+      }
+
+      expression {
+        operand = "above"
+
+        thresholds {
+          critical = 20
+        }
+      }
+    }
+	alert {
+      name  = "B"
+      title = "b"
+
+      query {
+        hidden       = false
+        query_name   = "a"
+        query_string = "metric b | delta 30s | group_by[], sum"
+        display      = "line" 
+      }
+
+      expression {
+        operand = "above"
+
+        thresholds {
+          critical = 20
+        }
+      }
+    }
+  }
+}
+`
+
+	resourceName := "lightstep_alert.test"
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccMetricConditionDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: fmt.Sprintf(conditionConfig, "false"),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckLightstepAlertExists(resourceName, &condition),
+				),
+			},
+			{
+				Config: fmt.Sprintf(conditionConfig, "true"),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckLightstepAlertExists(resourceName, &condition),
+				),
+			},
+		},
+	})
+}

lightstep/resource_metric_condition.go

@@ -1105,8 +1105,13 @@ func buildCompositeAlert(d *schema.ResourceData) (*client.CompositeAlert, error)
 		if !ok {
 			return nil, fmt.Errorf("could not parse alert")
 		}
-
-		subAlertExpression, err := buildSubAlertExpression(subAlertIn["expression"].([]interface{})[0].(map[string]interface{}))
+		subAlertExpressionsUntyped := subAlertIn["expression"].([]interface{})
+		if len(subAlertExpressionsUntyped) == 0 {
+			// if a subalert is being dropped and recreated, we will get a zero-value
+			// subalert in the list. ignore it.
+			continue
+		}
+		subAlertExpression, err := buildSubAlertExpression(subAlertExpressionsUntyped[0].(map[string]interface{}))
 		if err != nil {
 			return nil, err
 		}