Merge pull request #21879 from hashicorp/jbardin/sdk-panics

prevent sdk panics in 2 specific cases

builtin/providers/test/resource_list.go

@@ -137,6 +137,11 @@ func testResourceList() *schema.Resource {
 					},
 				},
 			},
+			"map_list": {
+				Type:     schema.TypeList,
+				Optional: true,
+				Elem:     &schema.Schema{Type: schema.TypeMap},
+			},
 		},
 	}
 }

builtin/providers/test/resource_list_test.go

@@ -103,6 +103,37 @@ resource "test_resource_list" "foo" {
 	})
 }
 
+func TestResourceList_mapList(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+variable "map" {
+  type = map(string)
+  default = {}
+}
+
+resource "test_resource_list" "foo" {
+	map_list = [
+	  {
+	    a = "1"
+	  },
+	  var.map
+	]
+}
+				`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr(
+						"test_resource_list.foo", "map_list.1", "",
+					),
+				),
+			},
+		},
+	})
+}
+
 func TestResourceList_sublist(t *testing.T) {
 	resource.UnitTest(t, resource.TestCase{
 		Providers:    testAccProviders,

helper/schema/field_reader_config.go

@@ -219,6 +219,9 @@ func (r *ConfigFieldReader) readMap(k string, schema *Schema) (FieldReadResult,
 			v, _ := r.Config.Get(key)
 			result[ik] = v
 		}
+	case nil:
+		// the map may have been empty on the configuration, so we leave the
+		// empty result
 	default:
 		panic(fmt.Sprintf("unknown type: %#v", mraw))
 	}

helper/schema/field_reader_diff.go

@@ -95,7 +95,9 @@ func (r *DiffFieldReader) readMap(
 		return FieldReadResult{}, err
 	}
 	if source.Exists {
-		result = source.Value.(map[string]interface{})
+		// readMap may return a nil value, or an unknown value placeholder in
+		// some cases, causing the type assertion to panic if we don't assign the ok value
+		result, _ = source.Value.(map[string]interface{})
 		resultSet = true
 	}