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/*
6This file contains the code to check range loop variables bound inside function
7literals that are deferred or launched in new goroutines. We only check
8instances where the defer or go statement is the last statement in the loop
9body, as otherwise we would need whole program analysis.
10
11For example:
12
13	for i, v := range s {
14		go func() {
15			println(i, v) // not what you might expect
16		}()
17	}
18
19See: https://golang.org/doc/go_faq.html#closures_and_goroutines
20*/
21
22package main
23
24import "go/ast"
25
26func 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.
36func 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