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capturedrefrace.go
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capturedrefrace.go
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// Copyright (c) 2023, 2024 Teddy Wing
//
// This file is part of Gocapturedrefrace.
//
// Gocapturedrefrace is free software: you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation, either version 3 of the
// License, or (at your option) any later version.
//
// Gocapturedrefrace is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Gocapturedrefrace. If not, see
// <https://www.gnu.org/licenses/>.
// Package capturedrefrace defines an Analyzer that checks for captured
// references in goroutine closures.
//
// # Analyzer capturedrefrace
//
// capturedrefrace: report captured variable references in goroutine
// closures.
//
// Goroutines that run function closures can capture reference variables from
// outer scopes which could lead to data races. This analyzer checks closures
// run by goroutines and reports uses of all variables declared in outer
// scopes, as well as arguments to the closure with a pointer type.
//
// For example:
//
// func (r *Record) CapturedReference() {
// capturedReference := 0
// spline := &Spline{Curvature: 5.0}
//
// go func(s *Spline) {
// capturedReference += 1 // closure captures the variable
// // 'capturedReference' in a goroutine, which could
// // lead to data races
//
// if capturedReference > 0 {
// r.reticulateSplines() // goroutine closure captures 'r'
// }
//
// s.Curvature = 3.0 // 's' is a pointer type which could
// // lead to data races
// }(spline)
// }
package capturedrefrace
import (
"go/ast"
"go/token"
"go/types"
"golang.org/x/tools/go/analysis"
"golang.org/x/tools/go/analysis/passes/inspect"
"golang.org/x/tools/go/ast/inspector"
)
var Analyzer = &analysis.Analyzer{
Name: "capturedrefrace",
Doc: "reports captured references in goroutine closures",
Run: run,
Requires: []*analysis.Analyzer{inspect.Analyzer},
}
func run(pass *analysis.Pass) (interface{}, error) {
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
nodeFilter := []ast.Node{
(*ast.GoStmt)(nil),
}
inspect.Preorder(
nodeFilter,
func(node ast.Node) {
// Find `go` statements.
goStmt, ok := node.(*ast.GoStmt)
if !ok {
return
}
var funcLit *ast.FuncLit
switch goStmtFunc := goStmt.Call.Fun.(type) {
case *ast.FuncLit:
// Look for a function literal after the `go` statement.
funcLit, ok = goStmt.Call.Fun.(*ast.FuncLit)
case *ast.Ident:
// Get a function literal stored in a local variable.
funcLit, ok = funcLitFromIdent(goStmtFunc)
default:
// The function being run by the `go` statement is an
// unsupported type.
return
}
if !ok {
return
}
// Inspect closure argument list.
for _, arg := range funcLit.Type.Params.List {
// Report reference arguments.
_, ok := arg.Type.(*ast.StarExpr)
if !ok {
continue
}
pass.Reportf(
arg.Pos(),
"reference %s in goroutine closure",
arg.Names[0],
)
}
checkClosure(pass, funcLit)
},
)
return nil, nil
}
// funcLitFromIdent takes a variable identifier and looks for a function
// literal assigned to it, returning the function literal.
func funcLitFromIdent(funcIdent *ast.Ident) (funcLit *ast.FuncLit, ok bool) {
// Find the assignment where the function literal was defined.
assignStmt, ok := funcIdent.Obj.Decl.(*ast.AssignStmt)
if !ok {
return nil, ok
}
funcVariableName := funcIdent.Name
// Find the index of funcVariableName in assignStmt.Lhs. We need this to
// find the closure in the corresponding assignStmt.Rhs list.
assignmentIndex := -1
for i, expr := range assignStmt.Lhs {
lhsIdent, ok := expr.(*ast.Ident)
if !ok {
continue
}
if lhsIdent.Name == funcVariableName {
assignmentIndex = i
break
}
}
if assignmentIndex == -1 ||
assignmentIndex > len(assignStmt.Rhs)-1 {
return nil, false
}
expr := assignStmt.Rhs[assignmentIndex]
funcLit, ok = expr.(*ast.FuncLit)
if !ok {
return nil, ok
}
return funcLit, true
}
// checkClosure reports variables used in funcLit that are captured from an
// outer scope.
func checkClosure(pass *analysis.Pass, funcLit *ast.FuncLit) {
// Get the closure's scope.
funcScope := pass.TypesInfo.Scopes[funcLit.Type]
// Build a list of assignments local to funcLit. These will be ignored as
// shadowed variables.
localVarDeclarations := findLocalVarDeclarations(funcLit)
ast.Inspect(
funcLit,
func(node ast.Node) bool {
ident, ok := node.(*ast.Ident)
if !ok {
return true
}
if ident.Obj == nil {
return true
}
// Ignore shadowed variables.
for _, declarationIdent := range localVarDeclarations {
if ident == declarationIdent {
return true
}
}
// Find out whether `ident` was defined in an outer scope.
//
// Starting in Go 1.22.0, function scopes in the 'go/types' package
// have changed:
//
// > The start position (Pos) of the lexical environment block
// > (Scope) that represents a function body has changed: it used
// > to start at the opening curly brace of the function body, but
// > now starts at the function's func token.
//
// (https://go.dev/doc/go1.22#go/types)
//
// This was caused by the following commit, which corrected
// behaviour for gopls:
//
// https://github.com/golang/go/commit/a27a525d1b4df74989ac9f6ad10394391fe3eb88
//
// As a result, we can no longer use `ident.NamePos` as the
// position for `funcScope.LookupParent`. Doing so causes false
// positives for function arguments, incorrectly finding them to be
// captured references from the outer scope.
//
// The commit message of a27a525d1b4df74989ac9f6ad10394391fe3eb88
// states:
//
// > set correct Var.scopePos for parameters/results
// >
// > Previously, its value was unset (NoPos), but the correct
// > value is a point after the signature (FuncType.End) and
// > before the body.
//
// We can restore the previous behaviour by using `token.NoPos`
// instead of `ident.NamePos`.
scope, scopeObj := funcScope.LookupParent(ident.Name, token.NoPos)
// Identifier is local to the closure.
if scope == nil && scopeObj == nil {
return true
}
// Ignore non-variable identifiers.
variable, ok := scopeObj.(*types.Var)
if !ok {
return true
}
// Ignore captured callable variables, like function arguments.
_, isVariableTypeSignature := variable.Type().(*types.Signature)
if isVariableTypeSignature {
return true
}
// Identifier was defined in a different scope.
if funcScope != scope {
pass.Reportf(
ident.Pos(),
"captured reference %s in goroutine closure",
ident,
)
}
return true
},
)
}
// findLocalVarDeclarations returns a list of all variable declarations in
// funcLit.
func findLocalVarDeclarations(
funcLit *ast.FuncLit,
) (declarations []*ast.Ident) {
declarations = []*ast.Ident{}
ast.Inspect(
funcLit,
func(node ast.Node) bool {
switch node := node.(type) {
case *ast.AssignStmt:
assignments := assignmentDefinitions(node)
declarations = append(declarations, assignments...)
case *ast.GenDecl:
decls := varDeclarations(node)
if decls != nil {
declarations = append(declarations, decls...)
}
}
return true
},
)
return declarations
}
// assignmentDefinitions returns the identifiers corresponding to variable
// assignments in assignStmt, or nil if assignStmt does not declare any
// variables.
func assignmentDefinitions(
assignStmt *ast.AssignStmt,
) (assignments []*ast.Ident) {
assignments = []*ast.Ident{}
if assignStmt.Tok != token.DEFINE {
return nil
}
for _, lhs := range assignStmt.Lhs {
ident, ok := lhs.(*ast.Ident)
if !ok {
continue
}
if ident == nil {
continue
}
assignments = append(assignments, ident)
}
return assignments
}
// varDeclarations returns the identifiers corresponding to variable
// declarations in decl, or nil if decl is not a variable declaration.
func varDeclarations(decl *ast.GenDecl) (declarations []*ast.Ident) {
if decl.Tok != token.VAR {
return nil
}
declarations = []*ast.Ident{}
for _, spec := range decl.Specs {
valueSpec, ok := spec.(*ast.ValueSpec)
if !ok {
return declarations
}
for _, ident := range valueSpec.Names {
declarations = append(declarations, ident)
}
}
return declarations
}