feat(decoder-configs) add AllowUnsetPointer option to skip unset errors for pointer fields

Author: rostislaved

mapstructure.go

@@ -222,6 +222,12 @@ type DecoderConfig struct {
 	// will affect all nested structs as well.
 	ErrorUnset bool
 
+	// AllowUnsetPointer, if set to true, will prevent fields with pointer types
+	// from being reported as unset, even if ErrorUnset is true and the field was
+	// not present in the input data. This allows pointer fields to be optional
+	// without triggering an error when they are missing.
+	AllowUnsetPointer bool
+
 	// ZeroFields, if set to true, will zero fields before writing them.
 	// For example, a map will be emptied before decoded values are put in
 	// it. If this is false, a map will be merged.
@@ -1459,9 +1465,9 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 			}
 
 			if !rawMapVal.IsValid() {
-				// There was no matching key in the map for the value in
-				// the struct. Remember it for potential errors and metadata.
-				targetValKeysUnused[fieldName] = struct{}{}
+				if !(d.config.AllowUnsetPointer && fieldValue.Kind() == reflect.Ptr) {
+					targetValKeysUnused[fieldName] = struct{}{}
+				}
 				continue
 			}
 		}

mapstructure_test.go

@@ -1675,6 +1675,32 @@ func TestDecoder_ErrorUnset(t *testing.T) {
 	}
 }
 
+func TestDecoder_ErrorUnset_AllowUnsetPointer(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"vstring": "hello",
+		"foo":     "bar",
+	}
+
+	var result BasicPointer
+	config := &DecoderConfig{
+		ErrorUnset:        true,
+		AllowUnsetPointer: true,
+		Result:            &result,
+	}
+
+	decoder, err := NewDecoder(config)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	err = decoder.Decode(input)
+	if err != nil {
+		t.Fatal("error not expected")
+	}
+}
+
 func TestMap(t *testing.T) {
 	t.Parallel()