ctrlflow.go

Documentation: golang.org/x/tools/go/analysis/passes/ctrlflow

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package ctrlflow is an analysis that provides a syntactic
     6  // control-flow graph (CFG) for the body of a function.
     7  // It records whether a function cannot return.
     8  // By itself, it does not report any diagnostics.
     9  package ctrlflow
    10  
    11  import (
    12  	"go/ast"
    13  	"go/types"
    14  	"log"
    15  	"reflect"
    16  
    17  	"golang.org/x/tools/go/analysis"
    18  	"golang.org/x/tools/go/analysis/passes/inspect"
    19  	"golang.org/x/tools/go/ast/inspector"
    20  	"golang.org/x/tools/go/cfg"
    21  	"golang.org/x/tools/go/types/typeutil"
    22  )
    23  
    24  var Analyzer = &analysis.Analyzer{
    25  	Name:       "ctrlflow",
    26  	Doc:        "build a control-flow graph",
    27  	Run:        run,
    28  	ResultType: reflect.TypeOf(new(CFGs)),
    29  	FactTypes:  []analysis.Fact{new(noReturn)},
    30  	Requires:   []*analysis.Analyzer{inspect.Analyzer},
    31  }
    32  
    33  // noReturn is a fact indicating that a function does not return.
    34  type noReturn struct{}
    35  
    36  func (*noReturn) AFact() {}
    37  
    38  func (*noReturn) String() string { return "noReturn" }
    39  
    40  // A CFGs holds the control-flow graphs
    41  // for all the functions of the current package.
    42  type CFGs struct {
    43  	defs      map[*ast.Ident]types.Object // from Pass.TypesInfo.Defs
    44  	funcDecls map[*types.Func]*declInfo
    45  	funcLits  map[*ast.FuncLit]*litInfo
    46  	pass      *analysis.Pass // transient; nil after construction
    47  }
    48  
    49  // CFGs has two maps: funcDecls for named functions and funcLits for
    50  // unnamed ones. Unlike funcLits, the funcDecls map is not keyed by its
    51  // syntax node, *ast.FuncDecl, because callMayReturn needs to do a
    52  // look-up by *types.Func, and you can get from an *ast.FuncDecl to a
    53  // *types.Func but not the other way.
    54  
    55  type declInfo struct {
    56  	decl     *ast.FuncDecl
    57  	cfg      *cfg.CFG // iff decl.Body != nil
    58  	started  bool     // to break cycles
    59  	noReturn bool
    60  }
    61  
    62  type litInfo struct {
    63  	cfg      *cfg.CFG
    64  	noReturn bool
    65  }
    66  
    67  // FuncDecl returns the control-flow graph for a named function.
    68  // It returns nil if decl.Body==nil.
    69  func (c *CFGs) FuncDecl(decl *ast.FuncDecl) *cfg.CFG {
    70  	if decl.Body == nil {
    71  		return nil
    72  	}
    73  	fn := c.defs[decl.Name].(*types.Func)
    74  	return c.funcDecls[fn].cfg
    75  }
    76  
    77  // FuncLit returns the control-flow graph for a literal function.
    78  func (c *CFGs) FuncLit(lit *ast.FuncLit) *cfg.CFG {
    79  	return c.funcLits[lit].cfg
    80  }
    81  
    82  func run(pass *analysis.Pass) (interface{}, error) {
    83  	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
    84  
    85  	// Because CFG construction consumes and produces noReturn
    86  	// facts, CFGs for exported FuncDecls must be built before 'run'
    87  	// returns; we cannot construct them lazily.
    88  	// (We could build CFGs for FuncLits lazily,
    89  	// but the benefit is marginal.)
    90  
    91  	// Pass 1. Map types.Funcs to ast.FuncDecls in this package.
    92  	funcDecls := make(map[*types.Func]*declInfo) // functions and methods
    93  	funcLits := make(map[*ast.FuncLit]*litInfo)
    94  
    95  	var decls []*types.Func // keys(funcDecls), in order
    96  	var lits []*ast.FuncLit // keys(funcLits), in order
    97  
    98  	nodeFilter := []ast.Node{
    99  		(*ast.FuncDecl)(nil),
   100  		(*ast.FuncLit)(nil),
   101  	}
   102  	inspect.Preorder(nodeFilter, func(n ast.Node) {
   103  		switch n := n.(type) {
   104  		case *ast.FuncDecl:
   105  			// Type information may be incomplete.
   106  			if fn, ok := pass.TypesInfo.Defs[n.Name].(*types.Func); ok {
   107  				funcDecls[fn] = &declInfo{decl: n}
   108  				decls = append(decls, fn)
   109  			}
   110  		case *ast.FuncLit:
   111  			funcLits[n] = new(litInfo)
   112  			lits = append(lits, n)
   113  		}
   114  	})
   115  
   116  	c := &CFGs{
   117  		defs:      pass.TypesInfo.Defs,
   118  		funcDecls: funcDecls,
   119  		funcLits:  funcLits,
   120  		pass:      pass,
   121  	}
   122  
   123  	// Pass 2. Build CFGs.
   124  
   125  	// Build CFGs for named functions.
   126  	// Cycles in the static call graph are broken
   127  	// arbitrarily but deterministically.
   128  	// We create noReturn facts as discovered.
   129  	for _, fn := range decls {
   130  		c.buildDecl(fn, funcDecls[fn])
   131  	}
   132  
   133  	// Build CFGs for literal functions.
   134  	// These aren't relevant to facts (since they aren't named)
   135  	// but are required for the CFGs.FuncLit API.
   136  	for _, lit := range lits {
   137  		li := funcLits[lit]
   138  		if li.cfg == nil {
   139  			li.cfg = cfg.New(lit.Body, c.callMayReturn)
   140  			if !hasReachableReturn(li.cfg) {
   141  				li.noReturn = true
   142  			}
   143  		}
   144  	}
   145  
   146  	// All CFGs are now built.
   147  	c.pass = nil
   148  
   149  	return c, nil
   150  }
   151  
   152  // di.cfg may be nil on return.
   153  func (c *CFGs) buildDecl(fn *types.Func, di *declInfo) {
   154  	// buildDecl may call itself recursively for the same function,
   155  	// because cfg.New is passed the callMayReturn method, which
   156  	// builds the CFG of the callee, leading to recursion.
   157  	// The buildDecl call tree thus resembles the static call graph.
   158  	// We mark each node when we start working on it to break cycles.
   159  
   160  	if !di.started { // break cycle
   161  		di.started = true
   162  
   163  		if isIntrinsicNoReturn(fn) {
   164  			di.noReturn = true
   165  		}
   166  		if di.decl.Body != nil {
   167  			di.cfg = cfg.New(di.decl.Body, c.callMayReturn)
   168  			if !hasReachableReturn(di.cfg) {
   169  				di.noReturn = true
   170  			}
   171  		}
   172  		if di.noReturn {
   173  			c.pass.ExportObjectFact(fn, new(noReturn))
   174  		}
   175  
   176  		// debugging
   177  		if false {
   178  			log.Printf("CFG for %s:\n%s (noreturn=%t)\n", fn, di.cfg.Format(c.pass.Fset), di.noReturn)
   179  		}
   180  	}
   181  }
   182  
   183  // callMayReturn reports whether the called function may return.
   184  // It is passed to the CFG builder.
   185  func (c *CFGs) callMayReturn(call *ast.CallExpr) (r bool) {
   186  	if id, ok := call.Fun.(*ast.Ident); ok && c.pass.TypesInfo.Uses[id] == panicBuiltin {
   187  		return false // panic never returns
   188  	}
   189  
   190  	// Is this a static call? Also includes static functions
   191  	// parameterized by a type. Such functions may or may not
   192  	// return depending on the parameter type, but in some
   193  	// cases the answer is definite. We let ctrlflow figure
   194  	// that out.
   195  	fn := typeutil.StaticCallee(c.pass.TypesInfo, call)
   196  	if fn == nil {
   197  		return true // callee not statically known; be conservative
   198  	}
   199  
   200  	// Function or method declared in this package?
   201  	if di, ok := c.funcDecls[fn]; ok {
   202  		c.buildDecl(fn, di)
   203  		return !di.noReturn
   204  	}
   205  
   206  	// Not declared in this package.
   207  	// Is there a fact from another package?
   208  	return !c.pass.ImportObjectFact(fn, new(noReturn))
   209  }
   210  
   211  var panicBuiltin = types.Universe.Lookup("panic").(*types.Builtin)
   212  
   213  func hasReachableReturn(g *cfg.CFG) bool {
   214  	for _, b := range g.Blocks {
   215  		if b.Live && b.Return() != nil {
   216  			return true
   217  		}
   218  	}
   219  	return false
   220  }
   221  
   222  // isIntrinsicNoReturn reports whether a function intrinsically never
   223  // returns because it stops execution of the calling thread.
   224  // It is the base case in the recursion.
   225  func isIntrinsicNoReturn(fn *types.Func) bool {
   226  	// Add functions here as the need arises, but don't allocate memory.
   227  	path, name := fn.Pkg().Path(), fn.Name()
   228  	return path == "syscall" && (name == "Exit" || name == "ExitProcess" || name == "ExitThread") ||
   229  		path == "runtime" && name == "Goexit"
   230  }
   231
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