fix bug in ignore_changes Transform

The cty.Transform for ignore_changes could return early when building a
map that had multiple ignored keys.

Refactor the function to try and separate the fast-path a little better,
and hopefully make it easier to follow.

terraform/eval_diff.go

@@ -664,47 +664,58 @@ func processIgnoreChangesIndividual(prior, config cty.Value, ignoreChanges []hcl
 	}
 
 	ret, _ := cty.Transform(config, func(path cty.Path, v cty.Value) (cty.Value, error) {
+		// Easy path for when we are only matching the entire value. The only
+		// values we break up for inspection are maps.
+		if !v.Type().IsMapType() {
+			for _, ignored := range ignoredValues {
+				if path.Equals(ignored.path) {
+					return ignored.value, nil
+				}
+			}
+			return v, nil
+		}
+		// We now know this must be a map, so we need to accumulate the values
+		// key-by-key.
+
+		if !v.IsNull() && !v.IsKnown() {
+			// since v is not known, we cannot ignore individual keys
+			return v, nil
+		}
+
+		// The configMap is the current configuration value, which we will
+		// mutate based on the ignored paths and the prior map value.
+		var configMap map[string]cty.Value
+		switch {
+		case v.IsNull() || v.LengthInt() == 0:
+			configMap = map[string]cty.Value{}
+		default:
+			configMap = v.AsValueMap()
+		}
+
 		for _, ignored := range ignoredValues {
 			if !path.Equals(ignored.path) {
-				return v, nil
+				continue
 			}
 
-			// no index, so we can return the entire value
 			if ignored.key.IsNull() {
+				// The map address is confirmed to match at this point,
+				// so if there is no key, we want the entire map and can
+				// stop accumulating values.
 				return ignored.value, nil
 			}
-
-			// we have an index key, so make sure we have a map
-			if !v.Type().IsMapType() {
-				// we'll let other validation catch any type mismatch
-				return v, nil
-			}
-
 			// Now we know we are ignoring a specific index of this map, so get
 			// the config map and modify, add, or remove the desired key.
-			var configMap map[string]cty.Value
-			var priorMap map[string]cty.Value
-
-			if !v.IsNull() {
-				if !v.IsKnown() {
-					// if the entire map is not known, we can't ignore any
-					// specific keys yet.
-					continue
-				}
-				configMap = v.AsValueMap()
-			}
-			if configMap == nil {
-				configMap = map[string]cty.Value{}
-			}
 
 			// We also need to create a prior map, so we can check for
-			// existence while getting the value. Value.Index will always
-			// return null.
-			if !ignored.value.IsNull() {
-				priorMap = ignored.value.AsValueMap()
-			}
-			if priorMap == nil {
+			// existence while getting the value, because Value.Index will
+			// return null for a key with a null value and for a non-existent
+			// key.
+			var priorMap map[string]cty.Value
+			switch {
+			case ignored.value.IsNull() || ignored.value.LengthInt() == 0:
 				priorMap = map[string]cty.Value{}
+			default:
+				priorMap = ignored.value.AsValueMap()
 			}
 
 			key := ignored.key.AsString()
@@ -718,14 +729,13 @@ func processIgnoreChangesIndividual(prior, config cty.Value, ignoreChanges []hcl
 			default:
 				configMap[key] = priorElem
 			}
-
-			if len(configMap) == 0 {
-				return cty.MapValEmpty(v.Type().ElementType()), nil
-			}
-
-			return cty.MapVal(configMap), nil
 		}
-		return v, nil
+
+		if len(configMap) == 0 {
+			return cty.MapValEmpty(v.Type().ElementType()), nil
+		}
+
+		return cty.MapVal(configMap), nil
 	})
 	return ret, nil
 }

terraform/eval_diff_test.go

@@ -136,6 +136,53 @@ func TestProcessIgnoreChangesIndividual(t *testing.T) {
 				"b": cty.StringVal("b value"),
 			}),
 		},
+		"map_index_multiple_keys": {
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.MapVal(map[string]cty.Value{
+					"a0": cty.StringVal("a0 value"),
+					"a1": cty.StringVal("a1 value"),
+					"a2": cty.StringVal("a2 value"),
+					"a3": cty.StringVal("a3 value"),
+				}),
+				"b": cty.StringVal("b value"),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.NullVal(cty.Map(cty.String)),
+				"b": cty.StringVal("new b value"),
+			}),
+			[]string{`a["a1"]`, `a["a2"]`, `a["a3"]`, `b`},
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.MapVal(map[string]cty.Value{
+					"a1": cty.StringVal("a1 value"),
+					"a2": cty.StringVal("a2 value"),
+					"a3": cty.StringVal("a3 value"),
+				}),
+				"b": cty.StringVal("b value"),
+			}),
+		},
+		"map_index_redundant": {
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.MapVal(map[string]cty.Value{
+					"a0": cty.StringVal("a0 value"),
+					"a1": cty.StringVal("a1 value"),
+					"a2": cty.StringVal("a2 value"),
+				}),
+				"b": cty.StringVal("b value"),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.NullVal(cty.Map(cty.String)),
+				"b": cty.StringVal("new b value"),
+			}),
+			[]string{`a["a1"]`, `a`, `b`},
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.MapVal(map[string]cty.Value{
+					"a0": cty.StringVal("a0 value"),
+					"a1": cty.StringVal("a1 value"),
+					"a2": cty.StringVal("a2 value"),
+				}),
+				"b": cty.StringVal("b value"),
+			}),
+		},
 		"missing_map_index": {
 			cty.ObjectVal(map[string]cty.Value{
 				"a": cty.MapVal(map[string]cty.Value{