1 files changed, 121 insertions, 0 deletions
diff --git a/gopls/internal/lsp/safetoken/safetoken_test.go b/gopls/internal/lsp/safetoken/safetoken_test.go
new file mode 100644
index 000000000..afd569472
--- /dev/null
+++ b/gopls/internal/lsp/safetoken/safetoken_test.go
@@ -0,0 +1,121 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package safetoken_test
+
+import (
+	"fmt"
+	"go/parser"
+	"go/token"
+	"go/types"
+	"os"
+	"testing"
+
+	"golang.org/x/tools/go/packages"
+	"golang.org/x/tools/gopls/internal/lsp/safetoken"
+	"golang.org/x/tools/internal/testenv"
+)
+
+func TestWorkaroundIssue57490(t *testing.T) {
+	// During error recovery the parser synthesizes various close
+	// tokens at EOF, causing the End position of incomplete
+	// syntax nodes, computed as Rbrace+len("}"), to be beyond EOF.
+	src := `package p; func f() { var x struct`
+	fset := token.NewFileSet()
+	file, _ := parser.ParseFile(fset, "a.go", src, 0)
+	tf := fset.File(file.Pos())
+
+	// Add another file to the FileSet.
+	file2, _ := parser.ParseFile(fset, "b.go", "package q", 0)
+
+	// This is the ambiguity of #57490...
+	if file.End() != file2.Pos() {
+		t.Errorf("file.End() %d != %d file2.Pos()", file.End(), file2.Pos())
+	}
+	// ...which causes these statements to panic.
+	if false {
+		tf.Offset(file.End())   // panic: invalid Pos value 36 (should be in [1, 35])
+		tf.Position(file.End()) // panic: invalid Pos value 36 (should be in [1, 35])
+	}
+
+	// The offset of the EOF position is the file size.
+	offset, err := safetoken.Offset(tf, file.End()-1)
+	if err != nil || offset != tf.Size() {
+		t.Errorf("Offset(EOF) = (%d, %v), want token.File.Size %d", offset, err, tf.Size())
+	}
+
+	// The offset of the file.End() position, 1 byte beyond EOF,
+	// is also the size of the file.
+	offset, err = safetoken.Offset(tf, file.End())
+	if err != nil || offset != tf.Size() {
+		t.Errorf("Offset(ast.File.End()) = (%d, %v), want token.File.Size %d", offset, err, tf.Size())
+	}
+
+	if got, want := safetoken.Position(tf, file.End()).String(), "a.go:1:35"; got != want {
+		t.Errorf("Position(ast.File.End()) = %s, want %s", got, want)
+	}
+
+	if got, want := safetoken.EndPosition(fset, file.End()).String(), "a.go:1:35"; got != want {
+		t.Errorf("EndPosition(ast.File.End()) = %s, want %s", got, want)
+	}
+
+	// Note that calling StartPosition on an end may yield the wrong file:
+	if got, want := safetoken.StartPosition(fset, file.End()).String(), "b.go:1:1"; got != want {
+		t.Errorf("StartPosition(ast.File.End()) = %s, want %s", got, want)
+	}
+}
+
+// To reduce the risk of panic, or bugs for which this package
+// provides a workaround, this test statically reports references to
+// forbidden methods of token.File or FileSet throughout gopls and
+// suggests alternatives.
+func TestGoplsSourceDoesNotCallTokenFileMethods(t *testing.T) {
+	testenv.NeedsGoPackages(t)
+
+	pkgs, err := packages.Load(&packages.Config{
+		Mode: packages.NeedName | packages.NeedModule | packages.NeedCompiledGoFiles | packages.NeedTypes | packages.NeedTypesInfo | packages.NeedSyntax | packages.NeedImports | packages.NeedDeps,
+	}, "go/token", "golang.org/x/tools/gopls/...")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var tokenPkg *packages.Package
+	for _, pkg := range pkgs {
+		if pkg.PkgPath == "go/token" {
+			tokenPkg = pkg
+			break
+		}
+	}
+	if tokenPkg == nil {
+		t.Fatal("missing package go/token")
+	}
+
+	File := tokenPkg.Types.Scope().Lookup("File")
+	FileSet := tokenPkg.Types.Scope().Lookup("FileSet")
+
+	alternative := make(map[types.Object]string)
+	setAlternative := func(recv types.Object, old, new string) {
+		oldMethod, _, _ := types.LookupFieldOrMethod(recv.Type(), true, recv.Pkg(), old)
+		alternative[oldMethod] = new
+	}
+	setAlternative(File, "Offset", "safetoken.Offset")
+	setAlternative(File, "Position", "safetoken.Position")
+	setAlternative(File, "PositionFor", "safetoken.Position")
+	setAlternative(FileSet, "Position", "safetoken.StartPosition or EndPosition")
+	setAlternative(FileSet, "PositionFor", "safetoken.StartPosition or EndPosition")
+
+	for _, pkg := range pkgs {
+		switch pkg.PkgPath {
+		case "go/token", "golang.org/x/tools/gopls/internal/lsp/safetoken":
+			continue // allow calls within these packages
+		}
+
+		for ident, obj := range pkg.TypesInfo.Uses {
+			if alt, ok := alternative[obj]; ok {
+				posn := safetoken.StartPosition(pkg.Fset, ident.Pos())
+				fmt.Fprintf(os.Stderr, "%s: forbidden use of %v; use %s instead.\n", posn, obj, alt)
+				t.Fail()
+			}
+		}
+	}
+}