1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
|
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this file,
* You can obtain one at http://mozilla.org/MPL/2.0/. */
#include "FinalizationWitnessService.h"
#include "nsString.h"
#include "jsapi.h"
#include "js/CallNonGenericMethod.h"
#include "js/Object.h" // JS::GetClass, JS::GetReservedSlot
#include "js/PropertyAndElement.h" // JS_DefineFunctions
#include "js/PropertySpec.h"
#include "nsIThread.h"
#include "mozilla/Services.h"
#include "nsIObserverService.h"
#include "nsThreadUtils.h"
// Implementation of nsIFinalizationWitnessService
static bool gShuttingDown = false;
namespace mozilla {
namespace {
/**
* An event meant to be dispatched to the main thread upon finalization
* of a FinalizationWitness, unless method |forget()| has been called.
*
* Held as private data by each instance of FinalizationWitness.
* Important note: we maintain the invariant that these private data
* slots are already addrefed.
*/
class FinalizationEvent final : public Runnable {
public:
FinalizationEvent(const char* aTopic, const char16_t* aValue)
: Runnable("FinalizationEvent"), mTopic(aTopic), mValue(aValue) {}
NS_IMETHOD Run() override {
nsCOMPtr<nsIObserverService> observerService =
mozilla::services::GetObserverService();
if (!observerService) {
// This is either too early or, more likely, too late for notifications.
// Bail out.
return NS_ERROR_NOT_AVAILABLE;
}
(void)observerService->NotifyObservers(nullptr, mTopic.get(), mValue.get());
return NS_OK;
}
private:
/**
* The topic on which to broadcast the notification of finalization.
*
* Deallocated on the main thread.
*/
const nsCString mTopic;
/**
* The result of converting the exception to a string.
*
* Deallocated on the main thread.
*/
const nsString mValue;
};
enum { WITNESS_SLOT_EVENT, WITNESS_INSTANCES_SLOTS };
/**
* Extract the FinalizationEvent from an instance of FinalizationWitness
* and clear the slot containing the FinalizationEvent.
*/
already_AddRefed<FinalizationEvent> ExtractFinalizationEvent(
JSObject* objSelf) {
JS::Value slotEvent = JS::GetReservedSlot(objSelf, WITNESS_SLOT_EVENT);
if (slotEvent.isUndefined()) {
// Forget() has been called
return nullptr;
}
JS_SetReservedSlot(objSelf, WITNESS_SLOT_EVENT, JS::UndefinedValue());
return dont_AddRef(static_cast<FinalizationEvent*>(slotEvent.toPrivate()));
}
/**
* Finalizer for instances of FinalizationWitness.
*
* Unless method Forget() has been called, the finalizer displays an error
* message.
*/
void Finalize(JS::GCContext* gcx, JSObject* objSelf) {
RefPtr<FinalizationEvent> event = ExtractFinalizationEvent(objSelf);
if (event == nullptr || gShuttingDown) {
// NB: event will be null if Forget() has been called
return;
}
// Notify observers. Since we are executed during garbage-collection,
// we need to dispatch the notification to the main thread.
nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
if (mainThread) {
mainThread->Dispatch(event.forget(), NS_DISPATCH_NORMAL);
}
// We may fail at dispatching to the main thread if we arrive too late
// during shutdown. In that case, there is not much we can do.
}
static const JSClassOps sWitnessClassOps = {
nullptr /* addProperty */,
nullptr /* delProperty */,
nullptr /* enumerate */,
nullptr /* newEnumerate */,
nullptr /* resolve */,
nullptr /* mayResolve */,
Finalize /* finalize */
};
static const JSClass sWitnessClass = {
"FinalizationWitness",
JSCLASS_HAS_RESERVED_SLOTS(WITNESS_INSTANCES_SLOTS) |
JSCLASS_FOREGROUND_FINALIZE,
&sWitnessClassOps};
bool IsWitness(JS::Handle<JS::Value> v) {
return v.isObject() && JS::GetClass(&v.toObject()) == &sWitnessClass;
}
/**
* JS method |forget()|
*
* === JS documentation
*
* Neutralize the witness. Once this method is called, the witness will
* never report any error.
*/
bool ForgetImpl(JSContext* cx, const JS::CallArgs& args) {
if (args.length() != 0) {
JS_ReportErrorASCII(cx, "forget() takes no arguments");
return false;
}
JS::Rooted<JS::Value> valSelf(cx, args.thisv());
JS::Rooted<JSObject*> objSelf(cx, &valSelf.toObject());
RefPtr<FinalizationEvent> event = ExtractFinalizationEvent(objSelf);
if (event == nullptr) {
JS_ReportErrorASCII(cx, "forget() called twice");
return false;
}
args.rval().setUndefined();
return true;
}
bool Forget(JSContext* cx, unsigned argc, JS::Value* vp) {
JS::CallArgs args = CallArgsFromVp(argc, vp);
return JS::CallNonGenericMethod<IsWitness, ForgetImpl>(cx, args);
}
static const JSFunctionSpec sWitnessClassFunctions[] = {
JS_FN("forget", Forget, 0, JSPROP_READONLY | JSPROP_PERMANENT), JS_FS_END};
} // namespace
NS_IMPL_ISUPPORTS(FinalizationWitnessService, nsIFinalizationWitnessService,
nsIObserver)
/**
* Create a new Finalization Witness.
*
* A finalization witness is an object whose sole role is to notify
* observers when it is gc-ed. Once the witness is created, call its
* method |forget()| to prevent the observers from being notified.
*
* @param aTopic The notification topic.
* @param aValue The notification value. Converted to a string.
*
* @constructor
*/
NS_IMETHODIMP
FinalizationWitnessService::Make(const char* aTopic, const char16_t* aValue,
JSContext* aCx,
JS::MutableHandle<JS::Value> aRetval) {
JS::Rooted<JSObject*> objResult(aCx, JS_NewObject(aCx, &sWitnessClass));
if (!objResult) {
return NS_ERROR_OUT_OF_MEMORY;
}
if (!JS_DefineFunctions(aCx, objResult, sWitnessClassFunctions)) {
return NS_ERROR_FAILURE;
}
RefPtr<FinalizationEvent> event = new FinalizationEvent(aTopic, aValue);
// Transfer ownership of the addrefed |event| to |objResult|.
JS_SetReservedSlot(objResult, WITNESS_SLOT_EVENT,
JS::PrivateValue(event.forget().take()));
aRetval.setObject(*objResult);
return NS_OK;
}
NS_IMETHODIMP
FinalizationWitnessService::Observe(nsISupports* aSubject, const char* aTopic,
const char16_t* aValue) {
MOZ_ASSERT(strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0);
gShuttingDown = true;
nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
if (obs) {
obs->RemoveObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID);
}
return NS_OK;
}
nsresult FinalizationWitnessService::Init() {
nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
if (!obs) {
return NS_ERROR_FAILURE;
}
return obs->AddObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID, false);
}
} // namespace mozilla
|
