blob: d049a3e8880fd27f77c18fd0161ce361de4362db [file] [log] [blame]
// 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 main
import (
"cmd/vet/internal/cfg"
"fmt"
"go/ast"
"go/types"
"strconv"
)
func init() {
register("lostcancel",
"check for failure to call cancelation function returned by context.WithCancel",
checkLostCancel,
funcDecl, funcLit)
}
const debugLostCancel = 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.
//
// checkLostCancel analyzes a single named or literal function.
func checkLostCancel(f *File, node ast.Node) {
// Fast path: bypass check if file doesn't use context.WithCancel.
if !hasImport(f.file, contextPackage) {
return
}
// Maps each cancel variable to its defining ValueSpec/AssignStmt.
cancelvars := make(map[*types.Var]ast.Node)
// 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(f, n) && isCall(stack[len(stack)-2]) {
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 == "_" {
f.Badf(id.Pos(), "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 := f.pkg.uses[id].(*types.Var); ok {
cancelvars[v] = stmt
} else if v, ok := f.pkg.defs[id].(*types.Var); ok {
cancelvars[v] = stmt
}
}
}
return true
})
if len(cancelvars) == 0 {
return // no need to build CFG
}
// Tell the CFG builder which functions never return.
info := &types.Info{Uses: f.pkg.uses, Selections: f.pkg.selectors}
mayReturn := func(call *ast.CallExpr) bool {
name := callName(info, call)
return !noReturnFuncs[name]
}
// Build the CFG.
var g *cfg.CFG
var sig *types.Signature
switch node := node.(type) {
case *ast.FuncDecl:
sig, _ = f.pkg.defs[node.Name].Type().(*types.Signature)
g = cfg.New(node.Body, mayReturn)
case *ast.FuncLit:
sig, _ = f.pkg.types[node.Type].Type.(*types.Signature)
g = cfg.New(node.Body, mayReturn)
}
// Print CFG.
if debugLostCancel {
fmt.Println(g.Format(f.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(f, g, v, stmt, sig); ret != nil {
lineno := f.fset.Position(stmt.Pos()).Line
f.Badf(stmt.Pos(), "the %s function is not used on all paths (possible context leak)", v.Name())
f.Badf(ret.Pos(), "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 }
func hasImport(f *ast.File, path string) bool {
for _, imp := range f.Imports {
v, _ := strconv.Unquote(imp.Path.Value)
if v == path {
return true
}
}
return false
}
// isContextWithCancel reports whether n is one of the qualified identifiers
// context.With{Cancel,Timeout,Deadline}.
func isContextWithCancel(f *File, n ast.Node) bool {
if sel, ok := n.(*ast.SelectorExpr); ok {
switch sel.Sel.Name {
case "WithCancel", "WithTimeout", "WithDeadline":
if x, ok := sel.X.(*ast.Ident); ok {
if pkgname, ok := f.pkg.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(f *File, 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(f *File, 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 f.pkg.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(f *File, v *types.Var, b *cfg.Block) bool {
res, ok := memo[b]
if !ok {
res = uses(f, 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(f, 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] {
seen[b] = true
// Prune the search if the block uses v.
if blockUses(f, v, b) {
continue
}
// Found path to return statement?
if ret := b.Return(); ret != nil {
if debugLostCancel {
fmt.Printf("found path to return in block %s\n", b)
}
return ret // found
}
// Recur
if ret := search(b.Succs); ret != nil {
if debugLostCancel {
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
}
var noReturnFuncs = map[string]bool{
"(*testing.common).FailNow": true,
"(*testing.common).Fatal": true,
"(*testing.common).Fatalf": true,
"(*testing.common).Skip": true,
"(*testing.common).SkipNow": true,
"(*testing.common).Skipf": true,
"log.Fatal": true,
"log.Fatalf": true,
"log.Fatalln": true,
"os.Exit": true,
"panic": true,
"runtime.Goexit": true,
}
// callName returns the canonical name of the builtin, method, or
// function called by call, if known.
func callName(info *types.Info, call *ast.CallExpr) string {
switch fun := call.Fun.(type) {
case *ast.Ident:
// builtin, e.g. "panic"
if obj, ok := info.Uses[fun].(*types.Builtin); ok {
return obj.Name()
}
case *ast.SelectorExpr:
if sel, ok := info.Selections[fun]; ok && sel.Kind() == types.MethodVal {
// method call, e.g. "(*testing.common).Fatal"
meth := sel.Obj()
return fmt.Sprintf("(%s).%s",
meth.Type().(*types.Signature).Recv().Type(),
meth.Name())
}
if obj, ok := info.Uses[fun.Sel]; ok {
// qualified identifier, e.g. "os.Exit"
return fmt.Sprintf("%s.%s",
obj.Pkg().Path(),
obj.Name())
}
}
// function with no name, or defined in missing imported package
return ""
}