Your IP : 172.28.240.42
// Copyright 2016 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 lostcancel
import (
_ "embed"
"fmt"
"go/ast"
"go/types"
"golang.org/x/tools/go/analysis"
"golang.org/x/tools/go/analysis/passes/ctrlflow"
"golang.org/x/tools/go/analysis/passes/inspect"
"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
"golang.org/x/tools/go/ast/inspector"
"golang.org/x/tools/go/cfg"
)
//go:embed doc.go
var doc string
var Analyzer = &analysis.Analyzer{
Name: "lostcancel",
Doc: analysisutil.MustExtractDoc(doc, "lostcancel"),
URL: "https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/lostcancel",
Run: run,
Requires: []*analysis.Analyzer{
inspect.Analyzer,
ctrlflow.Analyzer,
},
}
const debug = false
var contextPackage = "context"
// checkLostCancel reports a failure to the call the cancel function
// returned by context.WithCancel, either because the variable was
// assigned to the blank identifier, or because there exists a
// control-flow path from the call to a return statement and that path
// does not "use" the cancel function. Any reference to the variable
// counts as a use, even within a nested function literal.
// If the variable's scope is larger than the function
// containing the assignment, we assume that other uses exist.
//
// checkLostCancel analyzes a single named or literal function.
func run(pass *analysis.Pass) (interface{}, error) {
// Fast path: bypass check if file doesn't use context.WithCancel.
if !analysisutil.Imports(pass.Pkg, contextPackage) {
return nil, nil
}
// Call runFunc for each Func{Decl,Lit}.
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
nodeTypes := []ast.Node{
(*ast.FuncLit)(nil),
(*ast.FuncDecl)(nil),
}
inspect.Preorder(nodeTypes, func(n ast.Node) {
runFunc(pass, n)
})
return nil, nil
}
func runFunc(pass *analysis.Pass, node ast.Node) {
// Find scope of function node
var funcScope *types.Scope
switch v := node.(type) {
case *ast.FuncLit:
funcScope = pass.TypesInfo.Scopes[v.Type]
case *ast.FuncDecl:
funcScope = pass.TypesInfo.Scopes[v.Type]
}
// Maps each cancel variable to its defining ValueSpec/AssignStmt.
cancelvars := make(map[*types.Var]ast.Node)
// TODO(adonovan): opt: refactor to make a single pass
// over the AST using inspect.WithStack and node types
// {FuncDecl,FuncLit,CallExpr,SelectorExpr}.
// Find the set of cancel vars to analyze.
stack := make([]ast.Node, 0, 32)
ast.Inspect(node, func(n ast.Node) bool {
switch n.(type) {
case *ast.FuncLit:
if len(stack) > 0 {
return false // don't stray into nested functions
}
case nil:
stack = stack[:len(stack)-1] // pop
return true
}
stack = append(stack, n) // push
// Look for [{AssignStmt,ValueSpec} CallExpr SelectorExpr]:
//
// ctx, cancel := context.WithCancel(...)
// ctx, cancel = context.WithCancel(...)
// var ctx, cancel = context.WithCancel(...)
//
if !isContextWithCancel(pass.TypesInfo, n) || !isCall(stack[len(stack)-2]) {
return true
}
var id *ast.Ident // id of cancel var
stmt := stack[len(stack)-3]
switch stmt := stmt.(type) {
case *ast.ValueSpec:
if len(stmt.Names) > 1 {
id = stmt.Names[1]
}
case *ast.AssignStmt:
if len(stmt.Lhs) > 1 {
id, _ = stmt.Lhs[1].(*ast.Ident)
}
}
if id != nil {
if id.Name == "_" {
pass.ReportRangef(id,
"the cancel function returned by context.%s should be called, not discarded, to avoid a context leak",
n.(*ast.SelectorExpr).Sel.Name)
} else if v, ok := pass.TypesInfo.Uses[id].(*types.Var); ok {
// If the cancel variable is defined outside function scope,
// do not analyze it.
if funcScope.Contains(v.Pos()) {
cancelvars[v] = stmt
}
} else if v, ok := pass.TypesInfo.Defs[id].(*types.Var); ok {
cancelvars[v] = stmt
}
}
return true
})
if len(cancelvars) == 0 {
return // no need to inspect CFG
}
// Obtain the CFG.
cfgs := pass.ResultOf[ctrlflow.Analyzer].(*ctrlflow.CFGs)
var g *cfg.CFG
var sig *types.Signature
switch node := node.(type) {
case *ast.FuncDecl:
sig, _ = pass.TypesInfo.Defs[node.Name].Type().(*types.Signature)
if node.Name.Name == "main" && sig.Recv() == nil && pass.Pkg.Name() == "main" {
// Returning from main.main terminates the process,
// so there's no need to cancel contexts.
return
}
g = cfgs.FuncDecl(node)
case *ast.FuncLit:
sig, _ = pass.TypesInfo.Types[node.Type].Type.(*types.Signature)
g = cfgs.FuncLit(node)
}
if sig == nil {
return // missing type information
}
// Print CFG.
if debug {
fmt.Println(g.Format(pass.Fset))
}
// Examine the CFG for each variable in turn.
// (It would be more efficient to analyze all cancelvars in a
// single pass over the AST, but seldom is there more than one.)
for v, stmt := range cancelvars {
if ret := lostCancelPath(pass, g, v, stmt, sig); ret != nil {
lineno := pass.Fset.Position(stmt.Pos()).Line
pass.ReportRangef(stmt, "the %s function is not used on all paths (possible context leak)", v.Name())
pass.ReportRangef(ret, "this return statement may be reached without using the %s var defined on line %d", v.Name(), lineno)
}
}
}
func isCall(n ast.Node) bool { _, ok := n.(*ast.CallExpr); return ok }
// isContextWithCancel reports whether n is one of the qualified identifiers
// context.With{Cancel,Timeout,Deadline}.
func isContextWithCancel(info *types.Info, n ast.Node) bool {
sel, ok := n.(*ast.SelectorExpr)
if !ok {
return false
}
switch sel.Sel.Name {
case "WithCancel", "WithTimeout", "WithDeadline":
default:
return false
}
if x, ok := sel.X.(*ast.Ident); ok {
if pkgname, ok := info.Uses[x].(*types.PkgName); ok {
return pkgname.Imported().Path() == contextPackage
}
// Import failed, so we can't check package path.
// Just check the local package name (heuristic).
return x.Name == "context"
}
return false
}
// lostCancelPath finds a path through the CFG, from stmt (which defines
// the 'cancel' variable v) to a return statement, that doesn't "use" v.
// If it finds one, it returns the return statement (which may be synthetic).
// sig is the function's type, if known.
func lostCancelPath(pass *analysis.Pass, g *cfg.CFG, v *types.Var, stmt ast.Node, sig *types.Signature) *ast.ReturnStmt {
vIsNamedResult := sig != nil && tupleContains(sig.Results(), v)
// uses reports whether stmts contain a "use" of variable v.
uses := func(pass *analysis.Pass, v *types.Var, stmts []ast.Node) bool {
found := false
for _, stmt := range stmts {
ast.Inspect(stmt, func(n ast.Node) bool {
switch n := n.(type) {
case *ast.Ident:
if pass.TypesInfo.Uses[n] == v {
found = true
}
case *ast.ReturnStmt:
// A naked return statement counts as a use
// of the named result variables.
if n.Results == nil && vIsNamedResult {
found = true
}
}
return !found
})
}
return found
}
// blockUses computes "uses" for each block, caching the result.
memo := make(map[*cfg.Block]bool)
blockUses := func(pass *analysis.Pass, v *types.Var, b *cfg.Block) bool {
res, ok := memo[b]
if !ok {
res = uses(pass, v, b.Nodes)
memo[b] = res
}
return res
}
// Find the var's defining block in the CFG,
// plus the rest of the statements of that block.
var defblock *cfg.Block
var rest []ast.Node
outer:
for _, b := range g.Blocks {
for i, n := range b.Nodes {
if n == stmt {
defblock = b
rest = b.Nodes[i+1:]
break outer
}
}
}
if defblock == nil {
panic("internal error: can't find defining block for cancel var")
}
// Is v "used" in the remainder of its defining block?
if uses(pass, v, rest) {
return nil
}
// Does the defining block return without using v?
if ret := defblock.Return(); ret != nil {
return ret
}
// Search the CFG depth-first for a path, from defblock to a
// return block, in which v is never "used".
seen := make(map[*cfg.Block]bool)
var search func(blocks []*cfg.Block) *ast.ReturnStmt
search = func(blocks []*cfg.Block) *ast.ReturnStmt {
for _, b := range blocks {
if seen[b] {
continue
}
seen[b] = true
// Prune the search if the block uses v.
if blockUses(pass, v, b) {
continue
}
// Found path to return statement?
if ret := b.Return(); ret != nil {
if debug {
fmt.Printf("found path to return in block %s\n", b)
}
return ret // found
}
// Recur
if ret := search(b.Succs); ret != nil {
if debug {
fmt.Printf(" from block %s\n", b)
}
return ret
}
}
return nil
}
return search(defblock.Succs)
}
func tupleContains(tuple *types.Tuple, v *types.Var) bool {
for i := 0; i < tuple.Len(); i++ {
if tuple.At(i) == v {
return true
}
}
return false
}