| // Copyright 2023 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. |
| // |
| // # Analyzer loopclosure |
| // |
| // loopclosure: check references to loop variables from within nested functions |
| // |
| // This analyzer reports places where a function literal references the |
| // iteration variable of an enclosing loop, and the loop calls the function |
| // in such a way (e.g. with go or defer) that it may outlive the loop |
| // iteration and possibly observe the wrong value of the variable. |
| // |
| // Note: An iteration variable can only outlive a loop iteration in Go versions <=1.21. |
| // In Go 1.22 and later, the loop variable lifetimes changed to create a new |
| // iteration variable per loop iteration. (See go.dev/issue/60078.) |
| // |
| // In this example, all the deferred functions run after the loop has |
| // completed, so all observe the final value of v [<go1.22]. |
| // |
| // for _, v := range list { |
| // defer func() { |
| // use(v) // incorrect |
| // }() |
| // } |
| // |
| // One fix is to create a new variable for each iteration of the loop: |
| // |
| // for _, v := range list { |
| // v := v // new var per iteration |
| // defer func() { |
| // use(v) // ok |
| // }() |
| // } |
| // |
| // After Go version 1.22, the previous two for loops are equivalent |
| // and both are correct. |
| // |
| // The next example uses a go statement and has a similar problem [<go1.22]. |
| // In addition, it has a data race because the loop updates v |
| // concurrent with the goroutines accessing it. |
| // |
| // for _, v := range elem { |
| // go func() { |
| // use(v) // incorrect, and a data race |
| // }() |
| // } |
| // |
| // A fix is the same as before. The checker also reports problems |
| // in goroutines started by golang.org/x/sync/errgroup.Group. |
| // A hard-to-spot variant of this form is common in parallel tests: |
| // |
| // func Test(t *testing.T) { |
| // for _, test := range tests { |
| // t.Run(test.name, func(t *testing.T) { |
| // t.Parallel() |
| // use(test) // incorrect, and a data race |
| // }) |
| // } |
| // } |
| // |
| // The t.Parallel() call causes the rest of the function to execute |
| // concurrent with the loop [<go1.22]. |
| // |
| // The analyzer reports references only in the last statement, |
| // as it is not deep enough to understand the effects of subsequent |
| // statements that might render the reference benign. |
| // ("Last statement" is defined recursively in compound |
| // statements such as if, switch, and select.) |
| // |
| // See: https://golang.org/doc/go_faq.html#closures_and_goroutines |
| package loopclosure |