1 // Copyright 2012 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 /* 6 This file contains the code to check range loop variables bound inside function 7 literals that are deferred or launched in new goroutines. We only check 8 instances where the defer or go statement is the last statement in the loop 9 body, as otherwise we would need whole program analysis. 10 11 For example: 12 13 for i, v := range s { 14 go func() { 15 println(i, v) // not what you might expect 16 }() 17 } 18 19 See: https://golang.org/doc/go_faq.html#closures_and_goroutines 20 */ 21 22 package main 23 24 import "go/ast" 25 26 func init() { 27 register("rangeloops", 28 "check that loop variables are used correctly", 29 checkLoop, 30 rangeStmt, forStmt) 31 } 32 33 // checkLoop walks the body of the provided loop statement, checking whether 34 // its index or value variables are used unsafely inside goroutines or deferred 35 // function literals. 36 func checkLoop(f *File, node ast.Node) { 37 // Find the variables updated by the loop statement. 38 var vars []*ast.Ident 39 addVar := func(expr ast.Expr) { 40 if id, ok := expr.(*ast.Ident); ok { 41 vars = append(vars, id) 42 } 43 } 44 var body *ast.BlockStmt 45 switch n := node.(type) { 46 case *ast.RangeStmt: 47 body = n.Body 48 addVar(n.Key) 49 addVar(n.Value) 50 case *ast.ForStmt: 51 body = n.Body 52 switch post := n.Post.(type) { 53 case *ast.AssignStmt: 54 // e.g. for p = head; p != nil; p = p.next 55 for _, lhs := range post.Lhs { 56 addVar(lhs) 57 } 58 case *ast.IncDecStmt: 59 // e.g. for i := 0; i < n; i++ 60 addVar(post.X) 61 } 62 } 63 if vars == nil { 64 return 65 } 66 67 // Inspect a go or defer statement 68 // if it's the last one in the loop body. 69 // (We give up if there are following statements, 70 // because it's hard to prove go isn't followed by wait, 71 // or defer by return.) 72 if len(body.List) == 0 { 73 return 74 } 75 var last *ast.CallExpr 76 switch s := body.List[len(body.List)-1].(type) { 77 case *ast.GoStmt: 78 last = s.Call 79 case *ast.DeferStmt: 80 last = s.Call 81 default: 82 return 83 } 84 lit, ok := last.Fun.(*ast.FuncLit) 85 if !ok { 86 return 87 } 88 ast.Inspect(lit.Body, func(n ast.Node) bool { 89 id, ok := n.(*ast.Ident) 90 if !ok || id.Obj == nil { 91 return true 92 } 93 if f.pkg.types[id].Type == nil { 94 // Not referring to a variable (e.g. struct field name) 95 return true 96 } 97 for _, v := range vars { 98 if v.Obj == id.Obj { 99 f.Badf(id.Pos(), "loop variable %s captured by func literal", 100 id.Name) 101 } 102 } 103 return true 104 }) 105 } 106
