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// Copyright 2018 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 ctrlflow is an analysis that provides a syntactic
// control-flow graph (CFG) for the body of a function.
// It records whether a function cannot return.
// By itself, it does not report any diagnostics.
package ctrlflow
import (
"go/ast"
"go/types"
"log"
"reflect"
"golang.org/x/tools/go/analysis"
"golang.org/x/tools/go/analysis/passes/inspect"
"golang.org/x/tools/go/ast/inspector"
"golang.org/x/tools/go/cfg"
"golang.org/x/tools/go/types/typeutil"
)
var Analyzer = &analysis.Analyzer{
Name: "ctrlflow",
Doc: "build a control-flow graph",
Run: run,
ResultType: reflect.TypeOf(new(CFGs)),
FactTypes: []analysis.Fact{new(noReturn)},
Requires: []*analysis.Analyzer{inspect.Analyzer},
}
// noReturn is a fact indicating that a function does not return.
type noReturn struct{}
func (*noReturn) AFact() {}
func (*noReturn) String() string { return "noReturn" }
// A CFGs holds the control-flow graphs
// for all the functions of the current package.
type CFGs struct {
defs map[*ast.Ident]types.Object // from Pass.TypesInfo.Defs
funcDecls map[*types.Func]*declInfo
funcLits map[*ast.FuncLit]*litInfo
pass *analysis.Pass // transient; nil after construction
}
// CFGs has two maps: funcDecls for named functions and funcLits for
// unnamed ones. Unlike funcLits, the funcDecls map is not keyed by its
// syntax node, *ast.FuncDecl, because callMayReturn needs to do a
// look-up by *types.Func, and you can get from an *ast.FuncDecl to a
// *types.Func but not the other way.
type declInfo struct {
decl *ast.FuncDecl
cfg *cfg.CFG // iff decl.Body != nil
started bool // to break cycles
noReturn bool
}
type litInfo struct {
cfg *cfg.CFG
noReturn bool
}
// FuncDecl returns the control-flow graph for a named function.
// It returns nil if decl.Body==nil.
func (c *CFGs) FuncDecl(decl *ast.FuncDecl) *cfg.CFG {
if decl.Body == nil {
return nil
}
fn := c.defs[decl.Name].(*types.Func)
return c.funcDecls[fn].cfg
}
// FuncLit returns the control-flow graph for a literal function.
func (c *CFGs) FuncLit(lit *ast.FuncLit) *cfg.CFG {
return c.funcLits[lit].cfg
}
func run(pass *analysis.Pass) (interface{}, error) {
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
// Because CFG construction consumes and produces noReturn
// facts, CFGs for exported FuncDecls must be built before 'run'
// returns; we cannot construct them lazily.
// (We could build CFGs for FuncLits lazily,
// but the benefit is marginal.)
// Pass 1. Map types.Funcs to ast.FuncDecls in this package.
funcDecls := make(map[*types.Func]*declInfo) // functions and methods
funcLits := make(map[*ast.FuncLit]*litInfo)
var decls []*types.Func // keys(funcDecls), in order
var lits []*ast.FuncLit // keys(funcLits), in order
nodeFilter := []ast.Node{
(*ast.FuncDecl)(nil),
(*ast.FuncLit)(nil),
}
inspect.Preorder(nodeFilter, func(n ast.Node) {
switch n := n.(type) {
case *ast.FuncDecl:
// Type information may be incomplete.
if fn, ok := pass.TypesInfo.Defs[n.Name].(*types.Func); ok {
funcDecls[fn] = &declInfo{decl: n}
decls = append(decls, fn)
}
case *ast.FuncLit:
funcLits[n] = new(litInfo)
lits = append(lits, n)
}
})
c := &CFGs{
defs: pass.TypesInfo.Defs,
funcDecls: funcDecls,
funcLits: funcLits,
pass: pass,
}
// Pass 2. Build CFGs.
// Build CFGs for named functions.
// Cycles in the static call graph are broken
// arbitrarily but deterministically.
// We create noReturn facts as discovered.
for _, fn := range decls {
c.buildDecl(fn, funcDecls[fn])
}
// Build CFGs for literal functions.
// These aren't relevant to facts (since they aren't named)
// but are required for the CFGs.FuncLit API.
for _, lit := range lits {
li := funcLits[lit]
if li.cfg == nil {
li.cfg = cfg.New(lit.Body, c.callMayReturn)
if !hasReachableReturn(li.cfg) {
li.noReturn = true
}
}
}
// All CFGs are now built.
c.pass = nil
return c, nil
}
// di.cfg may be nil on return.
func (c *CFGs) buildDecl(fn *types.Func, di *declInfo) {
// buildDecl may call itself recursively for the same function,
// because cfg.New is passed the callMayReturn method, which
// builds the CFG of the callee, leading to recursion.
// The buildDecl call tree thus resembles the static call graph.
// We mark each node when we start working on it to break cycles.
if !di.started { // break cycle
di.started = true
if isIntrinsicNoReturn(fn) {
di.noReturn = true
}
if di.decl.Body != nil {
di.cfg = cfg.New(di.decl.Body, c.callMayReturn)
if !hasReachableReturn(di.cfg) {
di.noReturn = true
}
}
if di.noReturn {
c.pass.ExportObjectFact(fn, new(noReturn))
}
// debugging
if false {
log.Printf("CFG for %s:\n%s (noreturn=%t)\n", fn, di.cfg.Format(c.pass.Fset), di.noReturn)
}
}
}
// callMayReturn reports whether the called function may return.
// It is passed to the CFG builder.
func (c *CFGs) callMayReturn(call *ast.CallExpr) (r bool) {
if id, ok := call.Fun.(*ast.Ident); ok && c.pass.TypesInfo.Uses[id] == panicBuiltin {
return false // panic never returns
}
// Is this a static call? Also includes static functions
// parameterized by a type. Such functions may or may not
// return depending on the parameter type, but in some
// cases the answer is definite. We let ctrlflow figure
// that out.
fn := typeutil.StaticCallee(c.pass.TypesInfo, call)
if fn == nil {
return true // callee not statically known; be conservative
}
// Function or method declared in this package?
if di, ok := c.funcDecls[fn]; ok {
c.buildDecl(fn, di)
return !di.noReturn
}
// Not declared in this package.
// Is there a fact from another package?
return !c.pass.ImportObjectFact(fn, new(noReturn))
}
var panicBuiltin = types.Universe.Lookup("panic").(*types.Builtin)
func hasReachableReturn(g *cfg.CFG) bool {
for _, b := range g.Blocks {
if b.Live && b.Return() != nil {
return true
}
}
return false
}
// isIntrinsicNoReturn reports whether a function intrinsically never
// returns because it stops execution of the calling thread.
// It is the base case in the recursion.
func isIntrinsicNoReturn(fn *types.Func) bool {
// Add functions here as the need arises, but don't allocate memory.
path, name := fn.Pkg().Path(), fn.Name()
return path == "syscall" && (name == "Exit" || name == "ExitProcess" || name == "ExitThread") ||
path == "runtime" && name == "Goexit"
}