internal/runtime/exithook: make safe for concurrent os.Exit

Real programs can call os.Exit concurrently from multiple goroutines.
Make internal/runtime/exithook not crash in that case.

The throw on panic also now runs in the deferred context,
so that we will see the full stack trace that led to the panic.
That should give us more visibility into the flaky failures on
bugs golang#55167 and golang#56197 as well.

Fixes golang#67631.

Change-Id: Iefdf71b3a3b52a793ca88d89a9c270eb50ece094
Reviewed-on: https://go-review.googlesource.com/c/go/+/588235
Reviewed-by: Than McIntosh <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Russ Cox <[email protected]>

src/go/build/deps_test.go

@@ -58,7 +58,6 @@ var depsRules = `
 	  internal/nettrace,
 	  internal/platform,
 	  internal/profilerecord,
-	  internal/runtime/exithook,
 	  internal/trace/traceviewer/format,
 	  log/internal,
 	  math/bits,
@@ -79,7 +78,6 @@ var depsRules = `
 	internal/goexperiment,
 	internal/goos,
 	internal/profilerecord,
-	internal/runtime/exithook,
 	math/bits
 	< internal/bytealg
 	< internal/stringslite
@@ -88,6 +86,7 @@ var depsRules = `
 	< runtime/internal/sys
 	< internal/runtime/syscall
 	< internal/runtime/atomic
+	< internal/runtime/exithook
 	< runtime/internal/math
 	< runtime
 	< sync/atomic

src/internal/runtime/exithook/hooks.go

@@ -13,6 +13,11 @@
 // restricted dialects used for the trickier parts of the runtime.
 package exithook
 
+import (
+	"internal/runtime/atomic"
+	_ "unsafe" // for linkname
+)
+
 // A Hook is a function to be run at program termination
 // (when someone invokes os.Exit, or when main.main returns).
 // Hooks are run in reverse order of registration:
@@ -23,40 +28,56 @@ type Hook struct {
 }
 
 var (
+	locked  atomic.Int32
+	runGoid atomic.Uint64
 	hooks   []Hook
 	running bool
+
+	// runtime sets these for us
+	Gosched func()
+	Goid    func() uint64
+	Throw   func(string)
 )
 
 // Add adds a new exit hook.
 func Add(h Hook) {
+	for !locked.CompareAndSwap(0, 1) {
+		Gosched()
+	}
 	hooks = append(hooks, h)
+	locked.Store(0)
 }
 
 // Run runs the exit hooks.
-// It returns an error if Run is already running or
-// if one of the hooks panics.
-func Run(code int) (err error) {
-	if running {
-		return exitError("exit hook invoked exit")
+//
+// If an exit hook panics, Run will throw with the panic on the stack.
+// If an exit hook invokes exit in the same goroutine, the goroutine will throw.
+// If an exit hook invokes exit in another goroutine, that exit will block.
+func Run(code int) {
+	for !locked.CompareAndSwap(0, 1) {
+		if Goid() == runGoid.Load() {
+			Throw("exit hook invoked exit")
+		}
+		Gosched()
 	}
-	running = true
+	defer locked.Store(0)
+	runGoid.Store(Goid())
+	defer runGoid.Store(0)
 
 	defer func() {
-		if x := recover(); x != nil {
-			err = exitError("exit hook invoked panic")
+		if e := recover(); e != nil {
+			Throw("exit hook invoked panic")
 		}
 	}()
 
-	local := hooks
-	hooks = nil
-	for i := len(local) - 1; i >= 0; i-- {
-		h := local[i]
-		if code == 0 || h.RunOnFailure {
-			h.F()
+	for len(hooks) > 0 {
+		h := hooks[len(hooks)-1]
+		hooks = hooks[:len(hooks)-1]
+		if code != 0 && !h.RunOnFailure {
+			continue
 		}
+		h.F()
 	}
-	running = false
-	return nil
 }
 
 type exitError string

src/runtime/ehooks_test.go

@@ -28,32 +28,36 @@ func TestExitHooks(t *testing.T) {
 		scenarios := []struct {
 			mode     string
 			expected string
-			musthave string
+			musthave []string
 		}{
 			{
 				mode:     "simple",
 				expected: "bar foo",
-				musthave: "",
 			},
 			{
 				mode:     "goodexit",
 				expected: "orange apple",
-				musthave: "",
 			},
 			{
 				mode:     "badexit",
 				expected: "blub blix",
-				musthave: "",
 			},
 			{
-				mode:     "panics",
-				expected: "",
-				musthave: "fatal error: exit hook invoked panic",
+				mode: "panics",
+				musthave: []string{
+					"fatal error: exit hook invoked panic",
+					"main.testPanics",
+				},
+			},
+			{
+				mode: "callsexit",
+				musthave: []string{
+					"fatal error: exit hook invoked exit",
+				},
 			},
 			{
-				mode:     "callsexit",
+				mode:     "exit2",
 				expected: "",
-				musthave: "fatal error: exit hook invoked exit",
 			},
 		}
 
@@ -71,20 +75,18 @@ func TestExitHooks(t *testing.T) {
 			out, _ := cmd.CombinedOutput()
 			outs := strings.ReplaceAll(string(out), "\n", " ")
 			outs = strings.TrimSpace(outs)
-			if s.expected != "" {
-				if s.expected != outs {
-					t.Logf("raw output: %q", outs)
-					t.Errorf("failed%s mode %s: wanted %q got %q", bt,
-						s.mode, s.expected, outs)
-				}
-			} else if s.musthave != "" {
-				if !strings.Contains(outs, s.musthave) {
-					t.Logf("raw output: %q", outs)
-					t.Errorf("failed mode %s: output does not contain %q",
-						s.mode, s.musthave)
+			if s.expected != "" && s.expected != outs {
+				t.Fatalf("failed%s mode %s: wanted %q\noutput:\n%s", bt,
+					s.mode, s.expected, outs)
+			}
+			for _, need := range s.musthave {
+				if !strings.Contains(outs, need) {
+					t.Fatalf("failed mode %s: output does not contain %q\noutput:\n%s",
+						s.mode, need, outs)
 				}
-			} else {
-				panic("badly written scenario")
+			}
+			if s.expected == "" && s.musthave == nil && outs != "" {
+				t.Errorf("failed mode %s: wanted no output\noutput:\n%s", s.mode, outs)
 			}
 		}
 	}

src/runtime/proc.go

@@ -310,10 +310,14 @@ func os_beforeExit(exitCode int) {
 	}
 }
 
+func init() {
+	exithook.Gosched = Gosched
+	exithook.Goid = func() uint64 { return getg().goid }
+	exithook.Throw = throw
+}
+
 func runExitHooks(code int) {
-	if err := exithook.Run(code); err != nil {
-		throw(err.Error())
-	}
+	exithook.Run(code)
 }
 
 // start forcegc helper goroutine

src/runtime/testdata/testexithooks/testexithooks.go

@@ -6,8 +6,9 @@ package main
 
 import (
 	"flag"
-	"os"
 	"internal/runtime/exithook"
+	"os"
+	"time"
 	_ "unsafe"
 )
 
@@ -26,6 +27,8 @@ func main() {
 		testPanics()
 	case "callsexit":
 		testHookCallsExit()
+	case "exit2":
+		testExit2()
 	default:
 		panic("unknown mode")
 	}
@@ -81,3 +84,12 @@ func testHookCallsExit() {
 	exithook.Add(exithook.Hook{F: f3, RunOnFailure: true})
 	os.Exit(1)
 }
+
+func testExit2() {
+	f1 := func() { time.Sleep(100 * time.Millisecond) }
+	exithook.Add(exithook.Hook{F: f1})
+	for range 10 {
+		go os.Exit(0)
+	}
+	os.Exit(0)
+}