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// 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.
// Package loopclosure defines an Analyzer that checks for references to
// enclosing loop variables from within nested functions.
package loopclosure
import (
"go/ast"
"go/types"
"golang.org/x/tools/go/analysis"
"golang.org/x/tools/go/analysis/passes/inspect"
"golang.org/x/tools/go/ast/inspector"
"golang.org/x/tools/go/types/typeutil"
)
const Doc = `check references to loop variables from within nested functions
This analyzer checks for references to loop variables from within a
function literal inside the loop body. It checks only instances where
the function literal is called in a defer or go statement that 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: https://golang.org/doc/go_faq.html#closures_and_goroutines`
var Analyzer = &analysis.Analyzer{
Name: "loopclosure",
Doc: Doc,
Requires: []*analysis.Analyzer{inspect.Analyzer},
Run: run,
}
func run(pass *analysis.Pass) (interface{}, error) {
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
nodeFilter := []ast.Node{
(*ast.RangeStmt)(nil),
(*ast.ForStmt)(nil),
}
inspect.Preorder(nodeFilter, func(n ast.Node) {
// Find the variables updated by the loop statement.
var vars []*ast.Ident
addVar := func(expr ast.Expr) {
if id, ok := expr.(*ast.Ident); ok {
vars = append(vars, id)
}
}
var body *ast.BlockStmt
switch n := n.(type) {
case *ast.RangeStmt:
body = n.Body
addVar(n.Key)
addVar(n.Value)
case *ast.ForStmt:
body = n.Body
switch post := n.Post.(type) {
case *ast.AssignStmt:
// e.g. for p = head; p != nil; p = p.next
for _, lhs := range post.Lhs {
addVar(lhs)
}
case *ast.IncDecStmt:
// e.g. for i := 0; i < n; i++
addVar(post.X)
}
}
if vars == nil {
return
}
// Inspect a go or defer statement
// if it's the last one in the loop body.
// (We give up if there are following statements,
// because it's hard to prove go isn't followed by wait,
// or defer by return.)
if len(body.List) == 0 {
return
}
// The function invoked in the last return statement.
var fun ast.Expr
switch s := body.List[len(body.List)-1].(type) {
case *ast.GoStmt:
fun = s.Call.Fun
case *ast.DeferStmt:
fun = s.Call.Fun
case *ast.ExprStmt: // check for errgroup.Group.Go()
if call, ok := s.X.(*ast.CallExpr); ok {
fun = goInvokes(pass.TypesInfo, call)
}
}
lit, ok := fun.(*ast.FuncLit)
if !ok {
return
}
ast.Inspect(lit.Body, func(n ast.Node) bool {
id, ok := n.(*ast.Ident)
if !ok || id.Obj == nil {
return true
}
if pass.TypesInfo.Types[id].Type == nil {
// Not referring to a variable (e.g. struct field name)
return true
}
for _, v := range vars {
if v.Obj == id.Obj {
pass.ReportRangef(id, "loop variable %s captured by func literal",
id.Name)
}
}
return true
})
})
return nil, nil
}
// goInvokes returns a function expression that would be called asynchronously
// (but not awaited) in another goroutine as a consequence of the call.
// For example, given the g.Go call below, it returns the function literal expression.
//
// import "sync/errgroup"
// var g errgroup.Group
// g.Go(func() error { ... })
//
// Currently only "golang.org/x/sync/errgroup.Group()" is considered.
func goInvokes(info *types.Info, call *ast.CallExpr) ast.Expr {
f := typeutil.StaticCallee(info, call)
// Note: Currently only supports: golang.org/x/sync/errgroup.Go.
if f == nil || f.Name() != "Go" {
return nil
}
recv := f.Type().(*types.Signature).Recv()
if recv == nil {
return nil
}
rtype, ok := recv.Type().(*types.Pointer)
if !ok {
return nil
}
named, ok := rtype.Elem().(*types.Named)
if !ok {
return nil
}
if named.Obj().Name() != "Group" {
return nil
}
pkg := f.Pkg()
if pkg == nil {
return nil
}
if pkg.Path() != "golang.org/x/sync/errgroup" {
return nil
}
return call.Args[0]
}
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