exportloopref

An analyzer that finds exporting pointers for loop variables. Pin them all!

As of Go 1.22, this problem no longer occurs and fixed by Go team, see here

What's this?

Sample problem code from: https://github.com/kyoh86/exportloopref/blob/main/testdata/src/simple/simple.go

package main

func main() {
	var intArray [4]*int
	var intSlice []*int
	var intRef *int
	var intStr struct{ x *int }

	println("loop expecting 10, 11, 12, 13")
	for i, p := range []int{10, 11, 12, 13} {
		printp(&p)                      // not a diagnostic
		intSlice = append(intSlice, &p) // want "exporting a pointer for the loop variable p"
		intArray[i] = &p                // want "exporting a pointer for the loop variable p"
		if i%2 == 0 {
			intRef = &p   // want "exporting a pointer for the loop variable p"
			intStr.x = &p // want "exporting a pointer for the loop variable p"
		}
		var vStr struct{ x *int }
		var vArray [4]*int
		var v *int
		if i%2 == 0 {
			v = &p         // not a diagnostic (x is local variable)
			vArray[1] = &p // not a diagnostic (x is local variable)
			vStr.x = &p
		}
		_ = v
	}

	println(`slice expecting "10, 11, 12, 13" but "13, 13, 13, 13"`)
	for _, p := range intSlice {
		printp(p)
	}
	println(`array expecting "10, 11, 12, 13" but "13, 13, 13, 13"`)
	for _, p := range intArray {
		printp(p)
	}
	println(`captured value expecting "12" but "13"`)
	printp(intRef)
}

func printp(p *int) {
	println(*p)
}

In Go, the p variable in the above loops is actually a single variable. So in many case (like the above), using it makes for us annoying bugs.

You can find them with exportloopref, and fix it.

package main

func main() {
	var intArray [4]*int
	var intSlice []*int
	var intRef *int
	var intStr struct{ x *int }

	println("loop expecting 10, 11, 12, 13")
	for i, p := range []int{10, 11, 12, 13} {
    p := p                          // FIX variable into the local variable
		printp(&p)
		intSlice = append(intSlice, &p) 
		intArray[i] = &p
		if i%2 == 0 {
			intRef = &p
			intStr.x = &p
		}
		var vStr struct{ x *int }
		var vArray [4]*int
		var v *int
		if i%2 == 0 {
			v = &p
			vArray[1] = &p
			vStr.x = &p
		}
		_ = v
	}

	println(`slice expecting "10, 11, 12, 13"`)
	for _, p := range intSlice {
		printp(p)
	}
	println(`array expecting "10, 11, 12, 13"`)
	for _, p := range intArray {
		printp(p)
	}
	println(`captured value expecting "12"`)
	printp(intRef)
}

func printp(p *int) {
	println(*p)
}

ref: https://github.com/kyoh86/exportloopref/blob/main/testdata/src/fixed/fixed.go

Sensing policy

I want to make exportloopref as accurately as possible. So some cases of lints will be false-negative.

e.g.

var s Foo
for _, p := range []int{10, 11, 12, 13} {
  s.Bar(&p) // If s stores the pointer, it will be bug.
}

If you want to report all of lints (with some false-positives), you should use looppointer.

Known false negatives

Case 1: pass the pointer to function to export.

Case 2: pass the pointer to local variable, and export it.

package main

type List []*int

func (l *List) AppendP(p *int) {
	*l = append(*l, p)
}

func main() {
	var slice []*int
	list := List{}

	println("loop expect exporting 10, 11, 12, 13")
	for _, v := range []int{10, 11, 12, 13} {
		list.AppendP(&v) // Case 1: wanted "exporting a pointer for the loop variable v", but cannot be found

		p := &v                  // p is the local variable
		slice = append(slice, p) // Case 2: wanted "exporting a pointer for the loop variable v", but cannot be found
	}

	println(`slice expecting "10, 11, 12, 13" but "13, 13, 13, 13"`)
	for _, p := range slice {
		printp(p)
	}
	println(`array expecting "10, 11, 12, 13" but "13, 13, 13, 13"`)
	for _, p := range ([]*int)(list) {
		printp(p)
	}
}

func printp(p *int) {
	println(*p)
}

Install

go:

$ go install github.com/kyoh86/exportloopref/cmd/exportloopref@latest

homebrew:

$ brew install kyoh86/tap/exportloopref

gordon:

$ gordon install kyoh86/exportloopref

Usage

exportloopref [-flag] [package]

Flags

Flag	Description
-V	print version and exit
-all	no effect (deprecated)
-c int	display offending line with this many lines of context (default -1)
-cpuprofile string	write CPU profile to this file
-debug string	debug flags, any subset of "fpstv"
-fix	apply all suggested fixes
-flags	print analyzer flags in JSON
-json	emit JSON output
-memprofile string	write memory profile to this file
-source	no effect (deprecated)
-tags string	no effect (deprecated)
-trace string	write trace log to this file
-v	no effect (deprecated)

LICENSE

This is distributed under the MIT License.