oracle: disable parser bailout

This change sets ParserMode=AllErrors so that the parser is never
allowed to discard the AST and use a dummy one just because it saw too
many errors.

Also, change (*loader.Program).PathEnclosingInterval so that other
clients that forget to set this flag don't panic while calling
fset.File(f.Pos()).Base() on an ast.File f with no position info.

Change-Id: Ie544f169d367d2aa85426212b27063dc72e36fb1
Reviewed-on: https://go-review.googlesource.com/10290
Reviewed-by: Robert Griesemer <gri@golang.org>

go/loader/loader.go

@@ -313,6 +313,12 @@ func (conf *Config) addImport(path string, tests bool) {
 func (prog *Program) PathEnclosingInterval(start, end token.Pos) (pkg *PackageInfo, path []ast.Node, exact bool) {
 	for _, info := range prog.AllPackages {
 		for _, f := range info.Files {
+			if f.Pos() == token.NoPos {
+				// This can happen if the parser saw
+				// too many errors and bailed out.
+				// (Use parser.AllErrors to prevent that.)
+				continue
+			}
 			if !tokenFileContainsPos(prog.Fset.File(f.Pos()), start) {
 				continue
 			}

oracle/oracle.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/build"
+	"go/parser"
 	"go/token"
 	"io"
 	"path/filepath"
@@ -282,6 +283,9 @@ func allowErrors(lconf *loader.Config) {
 	ctxt.CgoEnabled = false
 	lconf.Build = &ctxt
 	lconf.AllowErrors = true
+	// AllErrors makes the parser always return an AST instead of
+	// bailing out after 10 errors and returning an empty ast.File.
+	lconf.ParserMode = parser.AllErrors
 	lconf.TypeChecker.Error = func(err error) {}
 }