go/scanner: don't eat \r in comments if that shortens the comment

For consistent formatting across platforms we strip \r's from
comments. This happens in the go/scanner which already does
this for raw string literals where it is mandated by the spec.
But if a (sequence of) \r appears in a regular (/*-style) comment
between a * and a /, removing that (sequence of) \r shortens that
text segment to */ which terminates the comment prematurely.

Don't do it.

As an aside, a better approach would be to not touch comments,
(and raw string literals for that matter) in the scanner and
leave the extra processing to clients. That is the approach
taken by the cmd/compile/internal/syntax package. However, we
probably can't change the semantics here too much, so just do
the minimal change that doesn't produce invalid comments. It's
an esoteric case after all.

Fixes #11151.

Change-Id: Ib4dcb52094f13c235e840c9672e439ea65fef961
Reviewed-on: https://go-review.googlesource.com/87498
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/printer/printer_test.go

@@ -710,3 +710,26 @@ type bar int	// comment2
 		t.Errorf("got %q, want %q", buf.String(), bar)
 	}
 }
+
+func TestIssue11151(t *testing.T) {
+	const src = "package p\t/*\r/1\r*\r/2*\r\r\r\r/3*\r\r+\r\r/4*/\n"
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	Fprint(&buf, fset, f)
+	got := buf.String()
+	const want = "package p\t/*/1*\r/2*\r/3*+/4*/\n" // \r following opening /* should be stripped
+	if got != want {
+		t.Errorf("\ngot : %q\nwant: %q", got, want)
+	}
+
+	// the resulting program must be valid
+	_, err = parser.ParseFile(fset, "", got, 0)
+	if err != nil {
+		t.Errorf("%v\norig: %q\ngot : %q", err, src, got)
+	}
+}

src/go/scanner/scanner.go

@@ -204,7 +204,7 @@ func (s *Scanner) scanComment() string {
 exit:
 	lit := s.src[offs:s.offset]
 	if hasCR {
-		lit = stripCR(lit)
+		lit = stripCR(lit, lit[1] == '*')
 	}
 
 	return string(lit)
@@ -480,11 +480,16 @@ func (s *Scanner) scanString() string {
 	return string(s.src[offs:s.offset])
 }
 
-func stripCR(b []byte) []byte {
+func stripCR(b []byte, comment bool) []byte {
 	c := make([]byte, len(b))
 	i := 0
-	for _, ch := range b {
-		if ch != '\r' {
+	for j, ch := range b {
+		// In a /*-style comment, don't strip \r from *\r/ (incl.
+		// sequences of \r from *\r\r...\r/) since the resulting
+		// */ would terminate the comment too early unless the \r
+		// is immediately following the opening /* in which case
+		// it's ok because /*/ is not closed yet (issue #11151).
+		if ch != '\r' || comment && i > len("/*") && c[i-1] == '*' && j+1 < len(b) && b[j+1] == '/' {
 			c[i] = ch
 			i++
 		}
@@ -514,7 +519,7 @@ func (s *Scanner) scanRawString() string {
 
 	lit := s.src[offs:s.offset]
 	if hasCR {
-		lit = stripCR(lit)
+		lit = stripCR(lit, false)
 	}
 
 	return string(lit)

src/go/scanner/scanner_test.go

@@ -45,6 +45,8 @@ var tokens = [...]elt{
 	{token.COMMENT, "/* a comment */", special},
 	{token.COMMENT, "// a comment \n", special},
 	{token.COMMENT, "/*\r*/", special},
+	{token.COMMENT, "/**\r/*/", special}, // issue 11151
+	{token.COMMENT, "/**\r\r/*/", special},
 	{token.COMMENT, "//\r\n", special},
 
 	// Identifiers and basic type literals
@@ -270,7 +272,7 @@ func TestScan(t *testing.T) {
 		switch e.tok {
 		case token.COMMENT:
 			// no CRs in comments
-			elit = string(stripCR([]byte(e.lit)))
+			elit = string(stripCR([]byte(e.lit), e.lit[1] == '*'))
 			//-style comment literal doesn't contain newline
 			if elit[1] == '/' {
 				elit = elit[0 : len(elit)-1]
@@ -284,7 +286,7 @@ func TestScan(t *testing.T) {
 				// no CRs in raw string literals
 				elit = e.lit
 				if elit[0] == '`' {
-					elit = string(stripCR([]byte(elit)))
+					elit = string(stripCR([]byte(elit), false))
 				}
 			} else if e.tok.IsKeyword() {
 				elit = e.lit
@@ -309,6 +311,26 @@ func TestScan(t *testing.T) {
 	}
 }
 
+func TestStripCR(t *testing.T) {
+	for _, test := range []struct{ have, want string }{
+		{"//\n", "//\n"},
+		{"//\r\n", "//\n"},
+		{"//\r\r\r\n", "//\n"},
+		{"//\r*\r/\r\n", "//*/\n"},
+		{"/**/", "/**/"},
+		{"/*\r/*/", "/*/*/"},
+		{"/*\r*/", "/**/"},
+		{"/**\r/*/", "/**\r/*/"},
+		{"/*\r/\r*\r/*/", "/*/*\r/*/"},
+		{"/*\r\r\r\r*/", "/**/"},
+	} {
+		got := string(stripCR([]byte(test.have), len(test.have) >= 2 && test.have[1] == '*'))
+		if got != test.want {
+			t.Errorf("stripCR(%q) = %q; want %q", test.have, got, test.want)
+		}
+	}
+}
+
 func checkSemi(t *testing.T, line string, mode Mode) {
 	var S Scanner
 	file := fset.AddFile("TestSemis", fset.Base(), len(line))