vet: add range variable misuse detection

R=fullung, r, remyoudompheng, minux.ma, gri, rsc
CC=golang-dev
https://golang.org/cl/6494075

src/cmd/vet/Makefile

@@ -4,4 +4,4 @@
 
 test testshort:
 	go build
-	../../../test/errchk ./vet -printfuncs='Warn:1,Warnf:1' print.go
+	../../../test/errchk ./vet -printfuncs='Warn:1,Warnf:1' print.go rangeloop.go

src/cmd/vet/main.go

@@ -30,6 +30,7 @@ var (
 	vetPrintf          = flag.Bool("printf", false, "check printf-like invocations")
 	vetStructTags      = flag.Bool("structtags", false, "check that struct field tags have canonical format")
 	vetUntaggedLiteral = flag.Bool("composites", false, "check that composite literals used type-tagged elements")
+	vetRangeLoops      = flag.Bool("rangeloops", false, "check that range loop variables are used correctly")
 )
 
 // setExit sets the value for os.Exit when it is called, later.  It
@@ -60,7 +61,7 @@ func main() {
 	flag.Parse()
 
 	// If a check is named explicitly, turn off the 'all' flag.
-	if *vetMethods || *vetPrintf || *vetStructTags || *vetUntaggedLiteral {
+	if *vetMethods || *vetPrintf || *vetStructTags || *vetUntaggedLiteral || *vetRangeLoops {
 		*vetAll = false
 	}
 
@@ -197,6 +198,8 @@ func (f *File) Visit(node ast.Node) ast.Visitor {
 		f.walkMethodDecl(n)
 	case *ast.InterfaceType:
 		f.walkInterfaceType(n)
+	case *ast.RangeStmt:
+		f.walkRangeStmt(n)
 	}
 	return f
 }
@@ -206,6 +209,16 @@ func (f *File) walkCall(call *ast.CallExpr, name string) {
 	f.checkFmtPrintfCall(call, name)
 }
 
+// walkCallExpr walks a call expression.
+func (f *File) walkCallExpr(call *ast.CallExpr) {
+	switch x := call.Fun.(type) {
+	case *ast.Ident:
+		f.walkCall(call, x.Name)
+	case *ast.SelectorExpr:
+		f.walkCall(call, x.Sel.Name)
+	}
+}
+
 // walkCompositeLit walks a composite literal.
 func (f *File) walkCompositeLit(c *ast.CompositeLit) {
 	f.checkUntaggedLiteral(c)
@@ -242,12 +255,7 @@ func (f *File) walkInterfaceType(t *ast.InterfaceType) {
 	}
 }
 
-// walkCallExpr walks a call expression.
-func (f *File) walkCallExpr(call *ast.CallExpr) {
-	switch x := call.Fun.(type) {
-	case *ast.Ident:
-		f.walkCall(call, x.Name)
-	case *ast.SelectorExpr:
-		f.walkCall(call, x.Sel.Name)
-	}
+// walkRangeStmt walks a range statment.
+func (f *File) walkRangeStmt(n *ast.RangeStmt) {
+	checkRangeLoop(f, n)
 }

src/cmd/vet/rangeloop.go

@@ -0,0 +1,104 @@
+// Copyright 2012 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.
+
+/*
+This file contains the code to check range loop variables bound inside function
+literals that are deferred or launched in new goroutines. We only check
+instances where the defer or go statement is the last statement in the loop
+body, as otherwise we would need whole program analysis.
+
+For example:
+
+	for i, v := range s {
+		go func() {
+			println(i, v) // not what you might expect
+		}()
+	}
+
+See: http://golang.org/doc/go_faq.html#closures_and_goroutines
+*/
+
+package main
+
+import "go/ast"
+
+// checkRangeLoop walks the body of the provided range statement, checking if
+// its index or value variables are used unsafely inside goroutines or deferred
+// function literals.
+func checkRangeLoop(f *File, n *ast.RangeStmt) {
+	if !*vetRangeLoops && !*vetAll {
+		return
+	}
+	key, _ := n.Key.(*ast.Ident)
+	val, _ := n.Value.(*ast.Ident)
+	if key == nil && val == nil {
+		return
+	}
+	sl := n.Body.List
+	if len(sl) == 0 {
+		return
+	}
+	var last *ast.CallExpr
+	switch s := sl[len(sl)-1].(type) {
+	case *ast.GoStmt:
+		last = s.Call
+	case *ast.DeferStmt:
+		last = s.Call
+	default:
+		return
+	}
+	lit, ok := last.Fun.(*ast.FuncLit)
+	if !ok {
+		return
+	}
+	ast.Inspect(lit.Body, func(n ast.Node) bool {
+		if n, ok := n.(*ast.Ident); ok && n.Obj != nil && (n.Obj == key.Obj || n.Obj == val.Obj) {
+			f.Warn(n.Pos(), "range variable", n.Name, "enclosed by function")
+		}
+		return true
+	})
+}
+
+func BadRangeLoopsUsedInTests() {
+	var s []int
+	for i, v := range s {
+		go func() {
+			println(i) // ERROR "range variable i enclosed by function"
+			println(v) // ERROR "range variable v enclosed by function"
+		}()
+	}
+	for i, v := range s {
+		defer func() {
+			println(i) // ERROR "range variable i enclosed by function"
+			println(v) // ERROR "range variable v enclosed by function"
+		}()
+	}
+	for i := range s {
+		go func() {
+			println(i) // ERROR "range variable i enclosed by function"
+		}()
+	}
+	for _, v := range s {
+		go func() {
+			println(v) // ERROR "range variable v enclosed by function"
+		}()
+	}
+	for i, v := range s {
+		go func() {
+			println(i, v)
+		}()
+		println("unfortunately, we don't catch the error above because of this statement")
+	}
+	for i, v := range s {
+		go func(i, v int) {
+			println(i, v)
+		}(i, v)
+	}
+	for i, v := range s {
+		i, v := i, v
+		go func() {
+			println(i, v)
+		}()
+	}
+}