Add MergeOpt MergeEmptyMaps to allow an empty map to be merged. (#685)

Some users may want to use this as a different meaning.

Added the following note in the doc comments:
```
// NOTE: Since YANG doesn't distinguish between a nil map and an empty map,
// please consider another approach before using this option.
```

Author: wenovus

ygot/struct_validation_map.go

@@ -559,6 +559,20 @@ type MergeOverwriteExistingFields struct{}
 // IsMergeOpt marks MergeStructOpt as a MergeOpt.
 func (*MergeOverwriteExistingFields) IsMergeOpt() {}
 
+// MergeEmptyMaps is a MergeOpt that allows control of the merge behaviour
+// of MergeStructs and MergeStructInto functions.
+//
+// When used, if both the destination struct and the source struct has an empty
+// map field, but it is non-nil in either one, then that map field in the
+// destination will always be populated with an empty, non-nil map value.
+//
+// NOTE: Since YANG doesn't distinguish between a nil map and an empty map,
+// please consider another approach before using this option.
+type MergeEmptyMaps struct{}
+
+// IsMergeOpt marks MergeEmptyMaps as a MergeOpt.
+func (*MergeEmptyMaps) IsMergeOpt() {}
+
 // MergeStructs takes two input GoStruct and merges their contents,
 // returning a new GoStruct. If the input structs a and b are of
 // different types, an error is returned.
@@ -622,6 +636,18 @@ func fieldOverwriteEnabled(opts []MergeOpt) bool {
 	return false
 }
 
+// mergeEmptyMapsEnabled returns true if MergeEmptyMaps
+// is present in the slice of MergeOpt.
+func mergeEmptyMapsEnabled(opts []MergeOpt) bool {
+	for _, o := range opts {
+		switch o.(type) {
+		case *MergeEmptyMaps:
+			return true
+		}
+	}
+	return false
+}
+
 // copyStruct copies the fields of srcVal into the dstVal struct in-place.
 func copyStruct(dstVal, srcVal reflect.Value, opts ...MergeOpt) error {
 	if srcVal.Type() != dstVal.Type() {
@@ -795,8 +821,11 @@ func copyMapField(dstField, srcField reflect.Value, opts ...MergeOpt) error {
 	}
 
 	// Skip cases where there are empty maps in both src and dst.
+	// Exception: user wants an empty map to be merged as well.
 	if srcField.Len() == 0 && dstField.Len() == 0 {
-		return nil
+		if !mergeEmptyMapsEnabled(opts) || srcField.IsNil() {
+			return nil
+		}
 	}
 
 	m, err := validateMap(srcField, dstField)

ygot/struct_validation_map_test.go

@@ -1839,17 +1839,93 @@ func TestCopyStruct(t *testing.T) {
 		inSrc:   &copyTest{StringSlice: []string{"mikkeler-draft-bear"}},
 		inDst:   &copyTest{StringSlice: []string{"mikkeler-draft-bear"}},
 		wantDst: &copyTest{StringSlice: []string{"mikkeler-draft-bear"}},
-	},
-		{
-			name:  "overwrite, slice fields not unique",
-			inSrc: &copyTest{StringSlice: []string{"mikkeler-draft-bear"}},
-			inDst: &copyTest{StringSlice: []string{"kingfisher"}},
-			inOpts: []MergeOpt{
-				&MergeOverwriteExistingFields{},
+	}, {
+		name:  "overwrite, slice fields not unique",
+		inSrc: &copyTest{StringSlice: []string{"mikkeler-draft-bear"}},
+		inDst: &copyTest{StringSlice: []string{"kingfisher"}},
+		inOpts: []MergeOpt{
+			&MergeOverwriteExistingFields{},
+		},
+		wantDst: &copyTest{StringSlice: []string{"kingfisher", "mikkeler-draft-bear"}},
+	}, {
+		name: "dst struct pointer with no populated field",
+		inSrc: &copyTest{
+			StringField: String("lagunitas-aunt-sally"),
+			StructPointer: &copyTest{
+				StructPointer: &copyTest{},
 			},
-			wantDst: &copyTest{StringSlice: []string{"kingfisher", "mikkeler-draft-bear"}},
 		},
-	}
+		inDst: &copyTest{},
+		wantDst: &copyTest{
+			StringField: String("lagunitas-aunt-sally"),
+			StructPointer: &copyTest{
+				StructPointer: &copyTest{},
+			},
+		},
+	}, {
+		name:  "src struct pointer with no populated field",
+		inSrc: &copyTest{},
+		inDst: &copyTest{
+			StringField: String("lagunitas-aunt-sally"),
+			StructPointer: &copyTest{
+				StructPointer: &copyTest{},
+			},
+		},
+		wantDst: &copyTest{
+			StringField: String("lagunitas-aunt-sally"),
+			StructPointer: &copyTest{
+				StructPointer: &copyTest{},
+			},
+		},
+	}, {
+		name: "dst single-key map with no elements",
+		inSrc: &copyTest{
+			StringMap: map[string]*copyTest{},
+		},
+		inDst: &copyTest{},
+		inOpts: []MergeOpt{
+			&MergeEmptyMaps{},
+		},
+		wantDst: &copyTest{
+			StringMap: map[string]*copyTest{},
+		},
+	}, {
+		name:  "dst single-key map with no elements",
+		inSrc: &copyTest{},
+		inDst: &copyTest{
+			StringMap: map[string]*copyTest{},
+		},
+		inOpts: []MergeOpt{
+			&MergeEmptyMaps{},
+		},
+		wantDst: &copyTest{
+			StringMap: map[string]*copyTest{},
+		},
+	}, {
+		name: "dst struct map with no elements",
+		inSrc: &copyTest{
+			StructMap: map[copyMapKey]*copyTest{},
+		},
+		inDst: &copyTest{},
+		inOpts: []MergeOpt{
+			&MergeEmptyMaps{},
+		},
+		wantDst: &copyTest{
+			StructMap: map[copyMapKey]*copyTest{},
+		},
+	}, {
+		name:  "src struct map with no elements",
+		inSrc: &copyTest{},
+		inDst: &copyTest{
+			StructMap: map[copyMapKey]*copyTest{},
+		},
+		inOpts: []MergeOpt{
+			&MergeEmptyMaps{},
+		},
+		wantDst: &copyTest{
+			StructMap: map[copyMapKey]*copyTest{},
+		},
+	}}
 
 	for _, tt := range tests {
 		dst, src := reflect.ValueOf(tt.inDst).Elem(), reflect.ValueOf(tt.inSrc).Elem()
@@ -1867,7 +1943,7 @@ func TestCopyStruct(t *testing.T) {
 			continue
 		}
 
-		if diff := pretty.Compare(dst.Interface(), wantDst.Interface()); diff != "" {
+		if diff := cmp.Diff(dst.Interface(), wantDst.Interface()); diff != "" {
 			t.Errorf("%s: copyStruct(%v, %v): did not get expected copied struct, diff(-got,+want):\n%s", tt.name, tt.inSrc, tt.inDst, diff)
 		}
 	}