container/dispatcher: optional stub wait4 signal association

This synchronises the wait4 return after the toplevel signal call in lowlastcap_signaled_cancel_forward_error.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/dispatcher_test.go

@@ -148,7 +148,8 @@ func checkSimple(t *testing.T, fname string, testCases []simpleTestCase) {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Helper()
 
-			k := &kstub{stub.New(t, func(s *stub.Stub[syscallDispatcher]) syscallDispatcher { return &kstub{s} }, tc.want)}
+			wait4signal := make(chan struct{})
+			k := &kstub{wait4signal, stub.New(t, func(s *stub.Stub[syscallDispatcher]) syscallDispatcher { return &kstub{wait4signal, s} }, tc.want)}
 			defer stub.HandleExit(t)
 			if err := tc.f(k); !reflect.DeepEqual(err, tc.wantErr) {
 				t.Errorf("%s: error = %v, want %v", fname, err, tc.wantErr)
@@ -185,8 +186,8 @@ func checkOpBehaviour(t *testing.T, testCases []opBehaviourTestCase) {
 				t.Helper()
 
 				state := &setupState{Params: tc.params}
-				k := &kstub{stub.New(t,
-					func(s *stub.Stub[syscallDispatcher]) syscallDispatcher { return &kstub{s} },
+				k := &kstub{nil, stub.New(t,
+					func(s *stub.Stub[syscallDispatcher]) syscallDispatcher { return &kstub{nil, s} },
 					stub.Expect{Calls: slices.Concat(tc.early, []stub.Call{{Name: stub.CallSeparator}}, tc.apply)},
 				)}
 				defer stub.HandleExit(t)
@@ -296,7 +297,18 @@ func (nameDentry) IsDir() bool                { panic("unreachable") }
 func (nameDentry) Type() fs.FileMode          { panic("unreachable") }
 func (nameDentry) Info() (fs.FileInfo, error) { panic("unreachable") }
 
-type kstub struct{ *stub.Stub[syscallDispatcher] }
+const (
+	// magicWait4Signal must be used in a single pair of signal and wait4 calls across two goroutines
+	// originating from the same toplevel kstub.
+	// To enable this behaviour this value must be the last element of the args field in the wait4 call
+	// and the ret value of the signal call.
+	magicWait4Signal = 0xdef
+)
+
+type kstub struct {
+	wait4signal chan struct{}
+	*stub.Stub[syscallDispatcher]
+}
 
 func (k *kstub) new(f func(k syscallDispatcher)) { k.Helper(); k.New(f) }
 
@@ -463,7 +475,14 @@ func (k *kstub) start(c *exec.Cmd) error {
 
 func (k *kstub) signal(c *exec.Cmd, sig os.Signal) error {
 	k.Helper()
-	return k.Expects("signal").Error(
+	expect := k.Expects("signal")
+	if v, ok := expect.Ret.(int); ok && v == magicWait4Signal {
+		if k.wait4signal == nil {
+			panic("kstub not initialised for wait4 simulation")
+		}
+		defer func() { close(k.wait4signal) }()
+	}
+	return expect.Error(
 		stub.CheckArg(k.Stub, "c.Path", c.Path, 0),
 		stub.CheckArgReflect(k.Stub, "c.Args", c.Args, 1),
 		stub.CheckArgReflect(k.Stub, "c.Env", c.Env, 2),
@@ -648,9 +667,17 @@ func (k *kstub) unmount(target string, flags int) (err error) {
 func (k *kstub) wait4(pid int, wstatus *syscall.WaitStatus, options int, rusage *syscall.Rusage) (wpid int, err error) {
 	k.Helper()
 	expect := k.Expects("wait4")
-	// special case to prevent leaking the wait4 goroutine when testing initEntrypoint
-	if v, ok := expect.Args[4].(int); ok && v == stub.PanicExit {
-		panic(stub.PanicExit)
+	if v, ok := expect.Args[4].(int); ok {
+		switch v {
+		case stub.PanicExit: // special case to prevent leaking the wait4 goroutine while testing initEntrypoint
+			panic(stub.PanicExit)
+
+		case magicWait4Signal: // block until corresponding signal call
+			if k.wait4signal == nil {
+				panic("kstub not initialised for wait4 simulation")
+			}
+			<-k.wait4signal
+		}
 	}
 
 	wpid = expect.Ret.(int)