gofmt: make sure certain 2-line comments are stable

       under repeated application of gofmt

R=agl, agl1
CC=golang-dev
https://golang.org/cl/212046

src/pkg/go/printer/printer.go

@@ -429,6 +429,7 @@ func stripCommonPrefix(lines [][]byte) {
 	if len(lines) < 2 {
 		return // at most one line - nothing to do
 	}
+	// len(lines) >= 2
 
 	// The heuristic in this function tries to handle a few
 	// common patterns of /*-style comments: Comments where
@@ -441,19 +442,29 @@ func stripCommonPrefix(lines [][]byte) {
 	// Compute maximum common white prefix of all but the first,
 	// last, and blank lines, and replace blank lines with empty
 	// lines (the first line starts with /* and has no prefix).
+	// In case of two-line comments, consider the last line for
+	// the prefix computation since otherwise the prefix would
+	// be empty.
+	//
+	// Note that the first and last line are never empty (they
+	// contain the opening /* and closing */ respectively) and
+	// thus they can be ignored by the blank line check.
 	var prefix []byte
-	for i, line := range lines {
+	if len(lines) > 2 {
+		for i, line := range lines[1 : len(lines)-1] {
 			switch {
-		case i == 0 || i == len(lines)-1:
-			// ignore
 			case isBlank(line):
-			lines[i] = nil
+				lines[i+1] = nil
 			case prefix == nil:
 				prefix = commonPrefix(line, line)
 			default:
 				prefix = commonPrefix(prefix, line)
 			}
 		}
+	} else { // len(lines) == 2
+		line := lines[1]
+		prefix = commonPrefix(line, line)
+	}
 
 	/*
 	 * Check for vertical "line of stars" and correct prefix accordingly.

src/pkg/go/printer/testdata/comments.golden

@@ -217,6 +217,80 @@ func _() {
 		aligned line */
 }
 
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line */
+}
+
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line */
+}
 
 /*
  * line
@@ -232,6 +306,24 @@ func _() {
  *	of
  *	stars */
 
+/* a line of
+ * stars */
+
+/*	and another line of
+ *	stars */
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
 /*
 aligned in middle
 here

src/pkg/go/printer/testdata/comments.input

@@ -217,6 +217,80 @@ func _() {
 		aligned line */
 }
 
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	   */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+		*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line */
+}
+
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	   */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+		*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line */
+}
 
 /*
  * line
@@ -232,6 +306,24 @@ func _() {
  *	of
  *	stars */
 
+/* a line of
+ * stars */
+
+/*	and another line of
+ *	stars */
+
+/* a line of stars
+*/
+
+/*	and another line of
+*/
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
 /*
 aligned in middle
 here