helper/schema: Normalize bools to "true"/"false" in diffs

For a long time now, the diff logic has relied on the behavior of
`mapstructure.WeakDecode` to determine how various primitives are
converted into strings.  The `schema.DiffString` function is used for
all primitive field types: TypeBool, TypeInt, TypeFloat, and TypeString.

The `mapstructure` library's string representation of booleans is "0"
and "1", which differs from `strconv.FormatBool`'s "false" and "true"
(which is used in writing out boolean fields to the state).

Because of this difference, diffs have long had the potential for
cosmetically odd but semantically neutral output like:

    "true" => "1"
    "false" => "0"

So long as `mapstructure.Decode` or `strconv.ParseBool` are used to
interpret these strings, there's no functional problem.

We had our first clear functional problem with #6005 and friends, where
users noticed diffs like the above showing up unexpectedly and causing
troubles when `ignore_changes` was in play.

This particular bug occurs down in Terraform core's EvalIgnoreChanges.
There, the diff is modified to account for ignored attributes, and
special logic attempts to handle properly the situation where the
ignored attribute was going to trigger a resource replacement. That
logic relies on the string representations of the Old and New fields in
the diff to be the same so that it filters properly.

So therefore, we now get a bug when a diff includes `Old: "0", New:
"false"` since the strings do not match, and `ignore_changes` is not
properly handled.

Here, we introduce `TypeBool`-specific normalizing into `finalizeDiff`.
I spiked out a full `diffBool` function, but figuring out which pieces
of `diffString` to duplicate there got hairy. This seemed like a simpler
and more direct solution.

Fixes #6005 (and potentially others!)

builtin/providers/test/resource.go

@@ -21,6 +21,10 @@ func testResource() *schema.Resource {
 				Type:     schema.TypeString,
 				Optional: true,
 			},
+			"optional_bool": &schema.Schema{
+				Type:     schema.TypeBool,
+				Optional: true,
+			},
 			"optional_force_new": &schema.Schema{
 				Type:     schema.TypeString,
 				Optional: true,

builtin/providers/test/resource_test.go

@@ -124,6 +124,47 @@ resource "test_resource" "foo" {
 	})
 }
 
+// Covers specific scenario in #6005, handled by normalizing boolean strings in
+// helper/schema
+func TestResource_ignoreChangesForceNewBoolean(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+  required           = "yep"
+  optional_force_new = "one"
+  optional_bool      = true
+  lifecycle {
+    ignore_changes = ["optional_force_new"]
+  }
+}
+				`),
+				Check: func(s *terraform.State) error {
+					return nil
+				},
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+  required           = "yep"
+  optional_force_new = "two"
+  optional_bool      = true
+  lifecycle {
+    ignore_changes = ["optional_force_new"]
+  }
+}
+				`),
+				Check: func(s *terraform.State) error {
+					return nil
+				},
+			},
+		},
+	})
+}
+
 func TestResource_ignoreChangesMap(t *testing.T) {
 	resource.UnitTest(t, resource.TestCase{
 		Providers:    testAccProviders,

helper/schema/schema.go

@@ -243,6 +243,20 @@ func (s *Schema) finalizeDiff(
 		return d
 	}
 
+	if s.Type == TypeBool {
+		normalizeBoolString := func(s string) string {
+			switch s {
+			case "0":
+				return "false"
+			case "1":
+				return "true"
+			}
+			return s
+		}
+		d.Old = normalizeBoolString(d.Old)
+		d.New = normalizeBoolString(d.New)
+	}
+
 	if d.NewRemoved {
 		return d
 	}

helper/schema/schema_test.go

@@ -491,7 +491,7 @@ func TestSchemaMap_Diff(t *testing.T) {
 				Attributes: map[string]*terraform.ResourceAttrDiff{
 					"port": &terraform.ResourceAttrDiff{
 						Old:         "",
-						New:         "0",
+						New:         "false",
 						RequiresNew: true,
 					},
 				},
@@ -2344,6 +2344,56 @@ func TestSchemaMap_Diff(t *testing.T) {
 
 			Err: false,
 		},
+
+		"Bools can be set with 0/1 in config, still get true/false": {
+			Schema: map[string]*Schema{
+				"one": &Schema{
+					Type:     TypeBool,
+					Optional: true,
+				},
+				"two": &Schema{
+					Type:     TypeBool,
+					Optional: true,
+				},
+				"three": &Schema{
+					Type:     TypeBool,
+					Optional: true,
+				},
+			},
+
+			State: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"one":   "false",
+					"two":   "true",
+					"three": "true",
+				},
+			},
+
+			Config: map[string]interface{}{
+				"one": "1",
+				"two": "0",
+			},
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"one": &terraform.ResourceAttrDiff{
+						Old: "false",
+						New: "true",
+					},
+					"two": &terraform.ResourceAttrDiff{
+						Old: "true",
+						New: "false",
+					},
+					"three": &terraform.ResourceAttrDiff{
+						Old:        "true",
+						New:        "false",
+						NewRemoved: true,
+					},
+				},
+			},
+
+			Err: false,
+		},
 	}
 
 	for tn, tc := range cases {