don't retain removed map values

Make sure values removed from a map during apply are not copied into the
new map. The broken test is no longer valid in this case, and the
updated diff.Apply should prevent the case it used to cover.

helper/plugin/grpc_provider.go

@@ -1110,7 +1110,6 @@ func stripSchema(s *schema.Schema) *schema.Schema {
 // very uncommon nor was it reliable before 0.12 either.
 func normalizeNullValues(dst, src cty.Value, preferDst bool) cty.Value {
 	ty := dst.Type()
-
 	if !src.IsNull() && !src.IsKnown() {
 		// While this seems backwards to return src when preferDst is set, it
 		// means this might be a plan scenario, and it must retain unknown
@@ -1154,7 +1153,12 @@ func normalizeNullValues(dst, src cty.Value, preferDst bool) cty.Value {
 
 		srcMap := src.AsValueMap()
 		for key, v := range srcMap {
-			dstVal := dstMap[key]
+			dstVal, ok := dstMap[key]
+			if !ok && !preferDst && ty.IsMapType() {
+				// don't transfer old map values to dst during apply
+				continue
+			}
+
 			if dstVal == cty.NilVal {
 				if preferDst && ty.IsMapType() {
 					// let plan shape this map however it wants

helper/plugin/grpc_provider_test.go

@@ -795,26 +795,6 @@ func TestNormalizeNullValues(t *testing.T) {
 				}))),
 			}),
 		},
-		{
-			// Retain the zero value within the map
-			Src: cty.ObjectVal(map[string]cty.Value{
-				"map": cty.MapVal(map[string]cty.Value{
-					"a": cty.StringVal("a"),
-					"b": cty.StringVal(""),
-				}),
-			}),
-			Dst: cty.ObjectVal(map[string]cty.Value{
-				"map": cty.MapVal(map[string]cty.Value{
-					"a": cty.StringVal("a"),
-				}),
-			}),
-			Expect: cty.ObjectVal(map[string]cty.Value{
-				"map": cty.MapVal(map[string]cty.Value{
-					"a": cty.StringVal("a"),
-					"b": cty.StringVal(""),
-				}),
-			}),
-		},
 		{
 			// Retain don't re-add unexpected planned values in a map
 			Src: cty.ObjectVal(map[string]cty.Value{
@@ -835,6 +815,26 @@ func TestNormalizeNullValues(t *testing.T) {
 			}),
 			Plan: true,
 		},
+		{
+			// Remove extra values after apply
+			Src: cty.ObjectVal(map[string]cty.Value{
+				"map": cty.MapVal(map[string]cty.Value{
+					"a": cty.StringVal("a"),
+					"b": cty.StringVal("b"),
+				}),
+			}),
+			Dst: cty.ObjectVal(map[string]cty.Value{
+				"map": cty.MapVal(map[string]cty.Value{
+					"a": cty.StringVal("a"),
+				}),
+			}),
+			Expect: cty.ObjectVal(map[string]cty.Value{
+				"map": cty.MapVal(map[string]cty.Value{
+					"a": cty.StringVal("a"),
+				}),
+			}),
+			Plan: false,
+		},
 		{
 			Src: cty.ObjectVal(map[string]cty.Value{
 				"a": cty.StringVal("a"),