go/printer: error out of Fprint when it would write a '//line' directive with a multiline file path

Line directives do not provide a way to escape newline characters, so
source file paths containing newlines must not be written in them.

Updates #60167.

Change-Id: I30f8b381cc7d1df6914c27591544edf424a4b634
Reviewed-on: https://go-review.googlesource.com/c/go/+/501578
Reviewed-by: Robert Griesemer <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: Bryan C. Mills

src/go/printer/printer.go

@@ -75,10 +75,11 @@ type printer struct {
 	// white space). If there's a difference and SourcePos is set in
 	// ConfigMode, //line directives are used in the output to restore
 	// original source positions for a reader.
-	pos     token.Position // current position in AST (source) space
-	out     token.Position // current position in output space
-	last    token.Position // value of pos after calling writeString
-	linePtr *int           // if set, record out.Line for the next token in *linePtr
+	pos          token.Position // current position in AST (source) space
+	out          token.Position // current position in output space
+	last         token.Position // value of pos after calling writeString
+	linePtr      *int           // if set, record out.Line for the next token in *linePtr
+	sourcePosErr error          // if non-nil, the first error emitting a //line directive
 
 	// The list of all source comments, in order of appearance.
 	comments        []*ast.CommentGroup // may be nil
@@ -196,6 +197,13 @@ func (p *printer) lineFor(pos token.Pos) int {
 // writeLineDirective writes a //line directive if necessary.
 func (p *printer) writeLineDirective(pos token.Position) {
 	if pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
+		if strings.ContainsAny(pos.Filename, "\r\n") {
+			if p.sourcePosErr == nil {
+				p.sourcePosErr = fmt.Errorf("go/printer: source filename contains unexpected newline character: %q", pos.Filename)
+			}
+			return
+		}
+
 		p.output = append(p.output, tabwriter.Escape) // protect '\n' in //line from tabwriter interpretation
 		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", pos.Filename, pos.Line)...)
 		p.output = append(p.output, tabwriter.Escape)
@@ -1169,7 +1177,7 @@ func (p *printer) printNode(node any) error {
 		goto unsupported
 	}
 
-	return nil
+	return p.sourcePosErr
 
 unsupported:
 	return fmt.Errorf("go/printer: unsupported node type %T", node)

src/go/printer/printer_test.go

@@ -797,3 +797,31 @@ func f() {
 		t.Fatalf("got %q, want %q", got, want)
 	}
 }
+
+func TestSourcePosNewline(t *testing.T) {
+	// We don't provide a syntax for escaping or unescaping characters in line
+	// directives (see https://go.dev/issue/24183#issuecomment-372449628).
+	// As a result, we cannot write a line directive with the correct path for a
+	// filename containing newlines. We should return an error rather than
+	// silently dropping or mangling it.
+
+	fname := "foo\nbar/bar.go"
+	src := `package bar`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, fname, src, parser.ParseComments|parser.AllErrors|parser.SkipObjectResolution)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cfg := &Config{
+		Mode:     SourcePos, // emit line comments
+		Tabwidth: 8,
+	}
+	var buf bytes.Buffer
+	if err := cfg.Fprint(&buf, fset, f); err == nil {
+		t.Errorf("Fprint did not error for source file path containing newline")
+	}
+	if buf.Len() != 0 {
+		t.Errorf("unexpected Fprint output:\n%s", buf.Bytes())
+	}
+}