go/printer: set prefix correctly when all comment lines blank

In stripCommonPrefix, the prefix was correctly calculated in all cases,
except one. That unhandled case is when there are more than 2 lines,
but all lines are blank (other than the first and last lines,
which contain /* and */ respectively).
This change detects that case and correctly sets the prefix calculated
from the last line. This is consistent with the (correct) behavior
that happens when there's at least one non-blank line.
That fixes issue #9751 that occurs for problematic input,
where cmd/gofmt and go/source would insert extra indentation on
every format operation. It also allows go/printer itself to print
such parsed files in an expected way.

Fixes #9751.

Change-Id: Id3dfb945beb59ffad3705085a3c285fca30a5f87
Reviewed-on: https://go-review.googlesource.com/3684Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/printer/printer.go

@@ -496,9 +496,10 @@ func stripCommonPrefix(lines []string) {
 	// 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.
+	// In cases where only the first and last lines are not blank,
+	// such as two-line comments, or comments where all inner lines
+	// are blank, 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
@@ -517,6 +518,10 @@ func stripCommonPrefix(lines []string) {
 				prefix = commonPrefix(prefix, line)
 			}
 		}
+		if first { // all lines were blank (except first and last)
+			line := lines[len(lines)-1]
+			prefix = commonPrefix(line, line)
+		}
 	} else { // len(lines) == 2, lines cannot be blank (contain /* and */)
 		line := lines[1]
 		prefix = commonPrefix(line, line)

src/go/printer/testdata/comments.golden

@@ -413,6 +413,68 @@ func _() {
 		aligned line */
 }
 
+// Issue 9751.
+func _() {
+	/*a string
+
+	b string*/
+
+	/*A string
+
+
+
+	Z string*/
+
+	/*a string
+
+	b string
+
+	c string*/
+
+	{
+		/*a string
+		b string*/
+
+		/*a string
+
+		b string*/
+
+		/*a string
+
+		b string
+
+		c string*/
+	}
+
+	{
+		/*a string
+		b string*/
+
+		/*a string
+
+		b string*/
+
+		/*a string
+
+		b string
+
+		c string*/
+	}
+
+	/*
+	 */
+
+	/*
+
+	 */
+
+	/*
+
+	 * line
+
+	 */
+}
+
 /*
  * line
  * of

src/go/printer/testdata/comments.input

@@ -418,6 +418,68 @@ func _() {
 		aligned line */
 }
 
+// Issue 9751.
+func _() {
+	/*a string
+
+	b string*/
+
+	/*A string
+
+
+
+	Z string*/
+
+	/*a string
+
+	b string
+
+	c string*/
+
+	{
+		/*a string
+b string*/
+
+		/*a string
+
+b string*/
+
+		/*a string
+
+b string
+
+c string*/
+	}
+
+	{
+		/*a string
+				b string*/
+
+		/*a string
+
+				b string*/
+
+		/*a string
+
+				b string
+
+				c string*/
+	}
+
+	/*
+	*/
+
+	/*
+
+	*/
+
+	/*
+
+	 * line
+
+	*/
+}
+
 /*
  * line
  * of