fix leading newline issue

goyaml.v3/emitterc.go

@@ -1160,8 +1160,15 @@ func yaml_emitter_process_head_comment(emitter *yaml_emitter_t) bool {
 }
 
 // Write an line comment.
-func yaml_emitter_process_line_comment(emitter *yaml_emitter_t) bool {
+func yaml_emitter_process_line_comment_linebreak(emitter *yaml_emitter_t, linebreak bool) bool {
 	if len(emitter.line_comment) == 0 {
+		// The next 3 lines are needed to resolve an issue with leading newlines
+		// See https://github.com/go-yaml/yaml/issues/755
+		// When linebreak is set to true, put_break will be called and will add
+		// the needed newline.
+		if linebreak && !put_break(emitter) {
+			return false
+		}
 		return true
 	}
 	if !emitter.whitespace {
@@ -1910,7 +1917,7 @@ func yaml_emitter_write_literal_scalar(emitter *yaml_emitter_t, value []byte) bo
 	if !yaml_emitter_write_block_scalar_hints(emitter, value) {
 		return false
 	}
-	if !yaml_emitter_process_line_comment(emitter) {
+	if !yaml_emitter_process_line_comment_linebreak(emitter, true) {
 		return false
 	}
 	//emitter.indention = true
@@ -1947,7 +1954,7 @@ func yaml_emitter_write_folded_scalar(emitter *yaml_emitter_t, value []byte) boo
 	if !yaml_emitter_write_block_scalar_hints(emitter, value) {
 		return false
 	}
-	if !yaml_emitter_process_line_comment(emitter) {
+	if !yaml_emitter_process_line_comment_linebreak(emitter, true) {
 		return false
 	}
 

goyaml.v3/patch.go

@@ -31,3 +31,9 @@ func (e *Encoder) CompactSeqIndent() {
 func (e *Encoder) DefaultSeqIndent() {
 	e.encoder.emitter.compact_sequence_indent = false
 }
+
+// yaml_emitter_process_line_comment preserves the original signature and delegates to
+// yaml_emitter_process_line_comment_linebreak passing false for linebreak
+func yaml_emitter_process_line_comment(emitter *yaml_emitter_t) bool {
+	return yaml_emitter_process_line_comment_linebreak(emitter, false)
+}

goyaml.v3/patch_test.go

@@ -144,3 +144,17 @@ func (s *S) TestEncoderCompactIndents(c *C) {
 		c.Assert(buf.String(), Equals, expected)
 	}
 }
+
+func (s *S) TestNewLinePreserved(c *C) {
+	obj := &marshalerValue{}
+	obj.Field.value = "a:\n        b:\n                c: d\n"
+	data, err := yaml.Marshal(obj)
+	c.Assert(err, IsNil)
+	c.Assert(string(data), Equals, "_: |\n    a:\n            b:\n                    c: d\n")
+
+	obj.Field.value = "\na:\n        b:\n                c: d\n"
+	data, err = yaml.Marshal(obj)
+	c.Assert(err, IsNil)
+	// the newline at the start of the file should be preserved
+	c.Assert(string(data), Equals, "_: |4\n\n    a:\n            b:\n                    c: d\n")
+}