fix: do not panic if monitors undefined (#442)

Fixes: #432

Author: mtojek

provider/agent.go

@@ -383,16 +383,28 @@ func agentResource() *schema.Resource {
 			}
 
 			if rd.HasChange("resources_monitoring") {
-				monitors, ok := rd.Get("resources_monitoring").(*schema.Set)
+				rmResource, ok := rd.Get("resources_monitoring").(*schema.Set)
 				if !ok {
-					return xerrors.Errorf("unexpected type %T for resources_monitoring.0.volume, expected []any", rd.Get("resources_monitoring.0.volume"))
+					return xerrors.Errorf("unexpected type %T for resources_monitoring, expected []any", rd.Get("resources_monitoring"))
 				}
 
-				monitor := monitors.List()[0].(map[string]any)
+				rmResourceAsList := rmResource.List()
+				if len(rmResourceAsList) == 0 {
+					return xerrors.Errorf("developer error: resources_monitoring cannot be empty")
+				}
+				rawMonitors := rmResourceAsList[0]
+				if rawMonitors == nil {
+					return xerrors.Errorf("resources_monitoring must define at least one monitor")
+				}
+
+				monitors, ok := rawMonitors.(map[string]any)
+				if !ok {
+					return xerrors.Errorf("unexpected type %T for resources_monitoring.0.volume, expected []any", rawMonitors)
+				}
 
-				volumes, ok := monitor["volume"].(*schema.Set)
+				volumes, ok := monitors["volume"].(*schema.Set)
 				if !ok {
-					return xerrors.Errorf("unexpected type %T for resources_monitoring.0.volume, expected []any", monitor["volume"])
+					return xerrors.Errorf("unexpected type %T for resources_monitoring.0.volume, expected []any", monitors["volume"])
 				}
 
 				paths := map[string]bool{}
@@ -403,7 +415,6 @@ func agentResource() *schema.Resource {
 					}
 
 					// print path for debug purpose
-
 					path, ok := obj["path"].(string)
 					if !ok {
 						return xerrors.Errorf("unexpected type %T for volume path, expected string", obj["path"])

provider/agent_test.go

@@ -352,6 +352,26 @@ func TestAgent_ResourcesMonitoring(t *testing.T) {
 		})
 	})
 
+	t.Run("MissingMonitors", func(t *testing.T) {
+		resource.Test(t, resource.TestCase{
+			ProviderFactories: coderFactory(),
+			IsUnitTest:        true,
+			Steps: []resource.TestStep{{
+				Config: `
+					provider "coder" {
+						url = "https://example.com"
+					}
+					resource "coder_agent" "dev" {
+						os = "linux"
+						arch = "amd64"
+						resources_monitoring {}
+					}`,
+				Check:       nil,
+				ExpectError: regexp.MustCompile(`resources_monitoring must define at least one monitor`),
+			}},
+		})
+	})
+
 	t.Run("DuplicatePaths", func(t *testing.T) {
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: coderFactory(),