go/doc: fix ToText

Fixes #6769.

LGTM=bradfitz
R=bgarcia, rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/84220044

src/pkg/go/doc/comment.go

@@ -392,7 +392,9 @@ func ToText(w io.Writer, text string, indent, preIndent string, width int) {
 		case opPre:
 			w.Write(nl)
 			for _, line := range b.lines {
-				if !isBlank(line) {
+				if isBlank(line) {
+					w.Write([]byte("\n"))
+				} else {
 					w.Write([]byte(preIndent))
 					w.Write([]byte(line))
 				}

src/pkg/go/doc/comment_test.go

@@ -44,6 +44,7 @@ func TestIsHeading(t *testing.T) {
 var blocksTests = []struct {
 	in   string
 	out  []block
+	text string
 }{
 	{
 		in: `Para 1.
@@ -59,6 +60,22 @@ Para 3.
 	pre1
 
 Para 4.
+
+	pre
+	pre1
+
+	pre2
+
+Para 5.
+
+
+	pre
+
+
+	pre1
+	pre2
+
+Para 6.
 	pre
 	pre2
 `,
@@ -69,8 +86,44 @@ Para 4.
 			{opPara, []string{"Para 3.\n"}},
 			{opPre, []string{"pre\n", "pre1\n"}},
 			{opPara, []string{"Para 4.\n"}},
+			{opPre, []string{"pre\n", "pre1\n", "\n", "pre2\n"}},
+			{opPara, []string{"Para 5.\n"}},
+			{opPre, []string{"pre\n", "\n", "\n", "pre1\n", "pre2\n"}},
+			{opPara, []string{"Para 6.\n"}},
 			{opPre, []string{"pre\n", "pre2\n"}},
 		},
+		text: `.   Para 1. Para 1 line 2.
+
+.   Para 2.
+
+
+.   Section
+
+.   Para 3.
+
+$	pre
+$	pre1
+
+.   Para 4.
+
+$	pre
+$	pre1
+
+$	pre2
+
+.   Para 5.
+
+$	pre
+
+
+$	pre1
+$	pre2
+
+.   Para 6.
+
+$	pre
+$	pre2
+`,
 	},
 }
 
@@ -83,6 +136,17 @@ func TestBlocks(t *testing.T) {
 	}
 }
 
+func TestToText(t *testing.T) {
+	var buf bytes.Buffer
+	for i, tt := range blocksTests {
+		ToText(&buf, tt.in, ".   ", "$\t", 40)
+		if have := buf.String(); have != tt.text {
+			t.Errorf("#%d: mismatch\nhave: %s\nwant: %s\nhave vs want:\n%q\n%q", i, have, tt.text, have, tt.text)
+		}
+		buf.Reset()
+	}
+}
+
 var emphasizeTests = []struct {
 	in  string
 	out string