go/parser: stop ParseFile after ten errors.

There wil be a panic if more than ten errors are encountered. ParseFile
will recover and return the ErrorList.

Fixes #3943.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/7307085

src/pkg/go/parser/error_test.go

@@ -145,6 +145,7 @@ func checkErrors(t *testing.T, filename string, input interface{}) {
 		t.Error(err)
 		return
 	}
+	found.RemoveMultiples()
 
 	// we are expecting the following errors
 	// (collect these after parsing a file so that it is found in the file set)

src/pkg/go/parser/interface.go

@@ -57,7 +57,8 @@ const (
 	ParseComments                                  // parse comments and add them to AST
 	Trace                                          // print a trace of parsed productions
 	DeclarationErrors                              // report declaration errors
-	SpuriousErrors                     // report all (not just the first) errors per line
+	SpuriousErrors                                 // same as AllErrors, for backward-compatibility
+	AllErrors         = SpuriousErrors             // report all (not just the first 10) errors per file
 )
 
 // ParseFile parses the source code of a single Go source file and returns
@@ -79,17 +80,20 @@ const (
 // representing the fragments of erroneous source code). Multiple errors
 // are returned via a scanner.ErrorList which is sorted by file position.
 //
-func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode) (*ast.File, error) {
+func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode) (f *ast.File, err error) {
 	// get source
 	text, err := readSource(filename, src)
 	if err != nil {
 		return nil, err
 	}
 
-	// parse source
 	var p parser
-	p.init(fset, filename, text, mode)
-	f := p.parseFile()
+	defer func() {
+		if e := recover(); e != nil {
+			_ = e.(bailout) // re-panics if it's not a bailout
+		}
+
+		// set result values
 		if f == nil {
 			// source is not a valid Go source file - satisfy
 			// ParseFile API and return a valid (but) empty
@@ -100,14 +104,15 @@ func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode)
 			}
 		}
 
-	// sort errors
-	if p.mode&SpuriousErrors == 0 {
-		p.errors.RemoveMultiples()
-	} else {
 		p.errors.Sort()
-	}
+		err = p.errors.Err()
+	}()
 
-	return f, p.errors.Err()
+	// parse source
+	p.init(fset, filename, text, mode)
+	f = p.parseFile()
+
+	return
 }
 
 // ParseDir calls ParseFile for the files in the directory specified by path and

src/pkg/go/parser/parser.go

@@ -340,7 +340,13 @@ func (p *parser) next() {
 	}
 }
 
+// A bailout panic is raised to indicate early termination.
+type bailout struct{}
+
 func (p *parser) error(pos token.Pos, msg string) {
+	if p.mode&SpuriousErrors == 0 && p.errors.Len() >= 10 {
+		panic(bailout{})
+	}
 	p.errors.Add(p.file.Position(pos), msg)
 }