simple list diffs may also have missing elements

Like was done for list blocks, simple lists of strings may be missing
empty string elements, and any list may be implicitly truncated.

builtin/providers/test/resource.go

@@ -171,7 +171,7 @@ func testResourceUpdate(d *schema.ResourceData, meta interface{}) error {
 	if errMsg != "" {
 		return errors.New(errMsg)
 	}
-	return nil
+	return testResourceRead(d, meta)
 }
 
 func testResourceDelete(d *schema.ResourceData, meta interface{}) error {

builtin/providers/test/resource_list.go

@@ -30,6 +30,13 @@ func testResourceList() *schema.Resource {
 							Optional: true,
 							ForceNew: true,
 						},
+						"sublist": {
+							Type:     schema.TypeList,
+							Optional: true,
+							Elem: &schema.Schema{
+								Type: schema.TypeString,
+							},
+						},
 						"sublist_block": {
 							Type:     schema.TypeList,
 							Optional: true,

builtin/providers/test/resource_list_test.go

@@ -218,3 +218,61 @@ resource "test_resource_list" "foo" {
 		},
 	})
 }
+
+func TestResourceList_emptyStrings(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_list" "foo" {
+  list_block {
+    sublist = ["a", ""]
+  }
+
+  list_block {
+    sublist = [""]
+  }
+
+  list_block {
+    sublist = ["", "c", ""]
+  }
+}
+				`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.0.sublist.0", "a"),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.0.sublist.1", ""),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.1.sublist.0", ""),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.2.sublist.0", ""),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.2.sublist.1", "c"),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.2.sublist.2", ""),
+				),
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_list" "foo" {
+  list_block {
+    sublist = [""]
+  }
+
+  list_block {
+    sublist = []
+  }
+
+  list_block {
+    sublist = ["", "c"]
+  }
+}
+			`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.0.sublist.#", "1"),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.0.sublist.0", ""),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.1.sublist.#", "0"),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.2.sublist.1", "c"),
+					resource.TestCheckResourceAttr("test_resource_list.foo", "list_block.2.sublist.#", "2"),
+				),
+			},
+		},
+	})
+}

terraform/diff.go

@@ -624,7 +624,7 @@ func (d *InstanceDiff) applyBlockDiff(path []string, attrs map[string]string, sc
 						if err != nil {
 							// this shouldn't happen since we added these
 							// ourself, but make note of it just in case.
-							log.Printf("[ERROR] bas list index in %q: %s", k, err)
+							log.Printf("[ERROR] bad list index in %q: %s", k, err)
 							continue
 						}
 						if i >= length {
@@ -811,10 +811,61 @@ func (d *InstanceDiff) applyCollectionDiff(path []string, attrs map[string]strin
 		}
 	}
 
+	// Just like in nested list blocks, for simple lists we may need to fill in
+	// missing empty strings.
+	countKey := name + "." + idx
+	count := result[countKey]
+	length, _ := strconv.Atoi(count)
+
+	if count != "" && count != hcl2shim.UnknownVariableValue &&
+		attrSchema.Type.Equals(cty.List(cty.String)) {
+		// insert empty strings into missing indexes
+		for i := 0; i < length; i++ {
+			key := fmt.Sprintf("%s.%d", name, i)
+			if _, ok := result[key]; !ok {
+				result[key] = ""
+			}
+		}
+	}
+
+	// now check for truncation in any type of list
+	if attrSchema.Type.IsListType() {
+		for key := range result {
+			if key == countKey {
+				continue
+			}
+
+			if len(key) <= len(name)+1 {
+				// not sure what this is, but don't panic
+				continue
+			}
+
+			index := key[len(name)+1:]
+
+			// It is possible to have nested sets or maps, so look for another dot
+			dot := strings.Index(index, ".")
+			if dot > 0 {
+				index = index[:dot]
+			}
+
+			// This shouldn't have any more dots, since the element type is only string.
+			num, err := strconv.Atoi(index)
+			if err != nil {
+				log.Printf("[ERROR] bad list index in %q: %s", currentKey, err)
+				continue
+			}
+
+			if num >= length {
+				delete(result, key)
+			}
+		}
+	}
+
 	// Fill in the count value if it was missing for some reason:
-	if result[name+"."+idx] == "" {
-		result[name+"."+idx] = countFlatmapContainerValues(name+"."+idx, result)
+	if result[countKey] == "" {
+		result[countKey] = countFlatmapContainerValues(countKey, result)
 	}
+
 	return result, nil
 }