fix unknowns added to maps by schemaMap

The legacy diff process inserts unknown values into an optional+computed
map. Fix these up in post-plan normalization process, by looking for
known strings that were changed to unknown.

helper/plugin/grpc_provider.go

@@ -1184,11 +1184,8 @@ func normalizeNullValues(dst, src cty.Value, preferDst bool) cty.Value {
 			dstMap = map[string]cty.Value{}
 		}
 
-		ei := src.ElementIterator()
+		srcMap := src.AsValueMap()
-		for ei.Next() {
+		for key, v := range srcMap {
-			k, v := ei.Element()
-			key := k.AsString()
-
 			dstVal := dstMap[key]
 			if dstVal == cty.NilVal {
 				if preferDst && ty.IsMapType() {
@@ -1211,6 +1208,24 @@ func normalizeNullValues(dst, src cty.Value, preferDst bool) cty.Value {
 		}
 
 		if ty.IsMapType() {
+			// helper/schema will populate an optional+computed map with
+			// unknowns which we have to fixup here.
+			// It would be preferable to simply prevent any known value from
+			// becoming unknown, but concessions have to be made to retain the
+			// broken legacy behavior when possible.
+			for k, srcVal := range srcMap {
+				if !srcVal.IsNull() && srcVal.IsKnown() {
+					dstVal, ok := dstMap[k]
+					if !ok {
+						continue
+					}
+
+					if !dstVal.IsNull() && !dstVal.IsKnown() {
+						dstMap[k] = srcVal
+					}
+				}
+			}
+
 			return cty.MapVal(dstMap)
 		}
 

helper/plugin/grpc_provider_test.go

@@ -909,7 +909,7 @@ func TestNormalizeNullValues(t *testing.T) {
 				}),
 			}),
 		},
-		// the empty list should be transferred, but the new unknown show not be overridden
+		// the empty list should be transferred, but the new unknown should not be overridden
 		{
 			Src: cty.ObjectVal(map[string]cty.Value{
 				"network_interface": cty.ListVal([]cty.Value{
@@ -942,6 +942,28 @@ func TestNormalizeNullValues(t *testing.T) {
 			}),
 			Plan: true,
 		},
+		{
+			// fix unknowns added to a 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"),
+					"b": cty.UnknownVal(cty.String),
+				}),
+			}),
+			Expect: cty.ObjectVal(map[string]cty.Value{
+				"map": cty.MapVal(map[string]cty.Value{
+					"a": cty.StringVal("a"),
+					"b": cty.StringVal(""),
+				}),
+			}),
+			Plan: true,
+		},
 	} {
 		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
 			got := normalizeNullValues(tc.Dst, tc.Src, tc.Plan)