go/parser: exit early if source file does not contain text

Partial fix for issue 3943.

R=r
CC=golang-dev
https://golang.org/cl/6458115

src/pkg/go/parser/interface.go

@@ -90,6 +90,15 @@ func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode)
 	var p parser
 	p.init(fset, filename, text, mode)
 	f := p.parseFile()
+	if f == nil {
+		// source is not a valid Go source file - satisfy
+		// ParseFile API and return a valid (but) empty
+		// *ast.File
+		f = &ast.File{
+			Name:  new(ast.Ident),
+			Scope: ast.NewScope(nil),
+		}
+	}
 
 	// sort errors
 	if p.mode&SpuriousErrors == 0 {

src/pkg/go/parser/parser.go

@@ -2285,6 +2285,12 @@ func (p *parser) parseFile() *ast.File {
 		defer un(trace(p, "File"))
 	}
 
+	// Don't bother parsing the rest if we had errors scanning the first token.
+	// Likely not a Go source file at all.
+	if p.errors.Len() != 0 {
+		return nil
+	}
+
 	// package clause
 	doc := p.leadComment
 	pos := p.expect(token.PACKAGE)
@@ -2296,13 +2302,16 @@ func (p *parser) parseFile() *ast.File {
 	}
 	p.expectSemi()
 
-	// Don't bother parsing the rest if we had errors already.
+	// Don't bother parsing the rest if we had errors parsing the package clause.
 	// Likely not a Go source file at all.
+	if p.errors.Len() != 0 {
+		return nil
+	}
 
 	p.openScope()
 	p.pkgScope = p.topScope
 	var decls []ast.Decl
-	if p.errors.Len() == 0 && p.mode&PackageClauseOnly == 0 {
+	if p.mode&PackageClauseOnly == 0 {
 		// import decls
 		for p.tok == token.IMPORT {
 			decls = append(decls, p.parseGenDecl(token.IMPORT, parseImportSpec))