libbinder_ndk: service notifications

Hey - this API looks shiny. You probably want to use
waitForService instead - because 99% of the time, guaranteeing the
code works is more important than controlling the boot order. For
the cases we really really really need to be impatient, here we
go! :)

Bug: 232878913
Test: libbinder_ndk_unit_test
Change-Id: Iae090a8cc48e968faec1d75cf304e2995b9553e0

4 files changed, 166 insertions, 0 deletions

diff --git a/libs/binder/ndk/include_platform/android/binder_manager.h b/libs/binder/ndk/include_platform/android/binder_manager.h
index c23427012a..1f1defef52 100644
--- a/libs/binder/ndk/include_platform/android/binder_manager.h
+++ b/libs/binder/ndk/include_platform/android/binder_manager.h
@@ -108,6 +108,67 @@ __attribute__((warn_unused_result)) AIBinder* AServiceManager_waitForService(con
         __INTRODUCED_IN(31);
 
 /**
+ * Function to call when a service is registered. The instance is passed as well as
+ * ownership of the binder named 'registered'.
+ *
+ * WARNING: a lock is held when this method is called in order to prevent races with
+ * AServiceManager_NotificationRegistration_delete. Do not make synchronous binder calls when
+ * implementing this method to avoid deadlocks.
+ *
+ * \param instance instance name of service registered
+ * \param registered ownership-passed instance of service registered
+ * \param cookie data passed during registration for notifications
+ */
+typedef void (*AServiceManager_onRegister)(const char* instance, AIBinder* registered,
+                                           void* cookie);
+
+/**
+ * Represents a registration to servicemanager which can be cleared anytime.
+ */
+struct AServiceManager_NotificationRegistration;
+
+/**
+ * Get notifications when a service is registered. If the service is already registered,
+ * you will immediately get a notification.
+ *
+ * WARNING: it is strongly recommended to use AServiceManager_waitForService API instead.
+ * That API will wait synchronously, which is what you usually want in cases, including
+ * using some feature or during boot up. There is a history of bugs where waiting for
+ * notifications like this races with service startup. Also, when this API is used, a service
+ * bug will result in silent failure (rather than a debuggable deadlock). Furthermore, there
+ * is a history of this API being used to know when a service is up as a proxy for whethre
+ * that service should be started. This should only be used if you are intending to get
+ * ahold of the service as a client. For lazy services, whether a service is registered
+ * should not be used as a proxy for when it should be registered, which is only known
+ * by the real client.
+ *
+ * WARNING: if you use this API, you must also ensure that you check missing services are
+ * started and crash otherwise. If service failures are ignored, the system rots.
+ *
+ * \param instance name of service to wait for notifications about
+ * \param onRegister callback for when service is registered
+ * \param cookie data associated with this callback
+ *
+ * \return the token for this registration. Deleting this token will unregister.
+ */
+__attribute__((warn_unused_result)) AServiceManager_NotificationRegistration*
+AServiceManager_registerForServiceNotifications(const char* instance,
+                                                AServiceManager_onRegister onRegister, void* cookie)
+        __INTRODUCED_IN(34);
+
+/**
+ * Unregister for notifications and delete the object.
+ *
+ * After this method is called, the callback is guaranteed to no longer be invoked. This will block
+ * until any in-progress onRegister callbacks have completed. It is therefore safe to immediately
+ * destroy the void* cookie that was registered when this method returns.
+ *
+ * \param notification object to dismiss
+ */
+void AServiceManager_NotificationRegistration_delete(
+        AServiceManager_NotificationRegistration* notification) __INTRODUCED_IN(34);
+
+/**
  * Check if a service is declared (e.g. VINTF manifest).
  *
  * \param instance identifier of the service.
diff --git a/libs/binder/ndk/libbinder_ndk.map.txt b/libs/binder/ndk/libbinder_ndk.map.txt
index 32ca5649dd..5c7005ceb7 100644
--- a/libs/binder/ndk/libbinder_ndk.map.txt
+++ b/libs/binder/ndk/libbinder_ndk.map.txt
@@ -155,6 +155,8 @@ LIBBINDER_NDK33 { # introduced=33
 LIBBINDER_NDK34 { # introduced=UpsideDownCake
   global:
     AServiceManager_getUpdatableApexName; # systemapi
+    AServiceManager_registerForServiceNotifications; # systemapi llndk
+    AServiceManager_NotificationRegistration_delete; # systemapi llndk
 };
 
 LIBBINDER_NDK_PLATFORM {
diff --git a/libs/binder/ndk/service_manager.cpp b/libs/binder/ndk/service_manager.cpp
index a12d0e9e8d..e107c83d14 100644
--- a/libs/binder/ndk/service_manager.cpp
+++ b/libs/binder/ndk/service_manager.cpp
@@ -28,6 +28,7 @@ using ::android::IBinder;
 using ::android::IServiceManager;
 using ::android::sp;
 using ::android::status_t;
+using ::android::statusToString;
 using ::android::String16;
 using ::android::String8;
 
@@ -86,6 +87,67 @@ AIBinder* AServiceManager_waitForService(const char* instance) {
     AIBinder_incStrong(ret.get());
     return ret.get();
 }
+typedef void (*AServiceManager_onRegister)(const char* instance, AIBinder* registered,
+                                           void* cookie);
+
+struct AServiceManager_NotificationRegistration
+    : public IServiceManager::LocalRegistrationCallback {
+    std::mutex m;
+    const char* instance = nullptr;
+    void* cookie = nullptr;
+    AServiceManager_onRegister onRegister = nullptr;
+
+    virtual void onServiceRegistration(const String16& smInstance, const sp<IBinder>& binder) {
+        std::lock_guard<std::mutex> l(m);
+        if (onRegister == nullptr) return;
+
+        CHECK_EQ(String8(smInstance), instance);
+
+        sp<AIBinder> ret = ABpBinder::lookupOrCreateFromBinder(binder);
+        AIBinder_incStrong(ret.get());
+
+        onRegister(instance, ret.get(), cookie);
+    }
+
+    void clear() {
+        std::lock_guard<std::mutex> l(m);
+        instance = nullptr;
+        cookie = nullptr;
+        onRegister = nullptr;
+    }
+};
+
+__attribute__((warn_unused_result)) AServiceManager_NotificationRegistration*
+AServiceManager_registerForServiceNotifications(const char* instance,
+                                                AServiceManager_onRegister onRegister,
+                                                void* cookie) {
+    CHECK_NE(instance, nullptr);
+    CHECK_NE(onRegister, nullptr) << instance;
+    // cookie can be nullptr
+
+    auto cb = sp<AServiceManager_NotificationRegistration>::make();
+    cb->instance = instance;
+    cb->onRegister = onRegister;
+    cb->cookie = cookie;
+
+    sp<IServiceManager> sm = defaultServiceManager();
+    if (status_t res = sm->registerForNotifications(String16(instance), cb); res != STATUS_OK) {
+        LOG(ERROR) << "Failed to register for service notifications for " << instance << ": "
+                   << statusToString(res);
+        return nullptr;
+    }
+
+    cb->incStrong(nullptr);
+    return cb.get();
+}
+
+void AServiceManager_NotificationRegistration_delete(
+        AServiceManager_NotificationRegistration* notification) {
+    CHECK_NE(notification, nullptr);
+    notification->clear();
+    notification->decStrong(nullptr);
+}
+
 bool AServiceManager_isDeclared(const char* instance) {
     if (instance == nullptr) {
         return false;
diff --git a/libs/binder/ndk/tests/libbinder_ndk_unit_test.cpp b/libs/binder/ndk/tests/libbinder_ndk_unit_test.cpp
index e221e4c2bb..9d5ef6805a 100644
--- a/libs/binder/ndk/tests/libbinder_ndk_unit_test.cpp
+++ b/libs/binder/ndk/tests/libbinder_ndk_unit_test.cpp
@@ -254,6 +254,47 @@ TEST(NdkBinder, CheckServiceThatDoesExist) {
     AIBinder_decStrong(binder);
 }
 
+struct ServiceData {
+    std::string instance;
+    ndk::SpAIBinder binder;
+
+    static void fillOnRegister(const char* instance, AIBinder* binder, void* cookie) {
+        ServiceData* d = reinterpret_cast<ServiceData*>(cookie);
+        d->instance = instance;
+        d->binder = ndk::SpAIBinder(binder);
+    }
+};
+
+TEST(NdkBinder, RegisterForServiceNotificationsNonExisting) {
+    ServiceData data;
+    auto* notif = AServiceManager_registerForServiceNotifications(
+            "DOES_NOT_EXIST", ServiceData::fillOnRegister, (void*)&data);
+    ASSERT_NE(notif, nullptr);
+
+    sleep(1);  // give us a chance to fail
+    AServiceManager_NotificationRegistration_delete(notif);
+
+    // checking after deleting to avoid needing a mutex over the data - otherwise
+    // in an environment w/ multiple threads, you would need to guard access
+    EXPECT_EQ(data.instance, "");
+    EXPECT_EQ(data.binder, nullptr);
+}
+
+TEST(NdkBinder, RegisterForServiceNotificationsExisting) {
+    ServiceData data;
+    auto* notif = AServiceManager_registerForServiceNotifications(
+            kExistingNonNdkService, ServiceData::fillOnRegister, (void*)&data);
+    ASSERT_NE(notif, nullptr);
+
+    sleep(1);  // give us a chance to fail
+    AServiceManager_NotificationRegistration_delete(notif);
+
+    // checking after deleting to avoid needing a mutex over the data - otherwise
+    // in an environment w/ multiple threads, you would need to guard access
+    EXPECT_EQ(data.instance, kExistingNonNdkService);
+    EXPECT_EQ(data.binder, ndk::SpAIBinder(AServiceManager_checkService(kExistingNonNdkService)));
+}
+
 TEST(NdkBinder, UnimplementedDump) {
     sp<IFoo> foo = IFoo::getService(IFoo::kSomeInstanceName);
     ASSERT_NE(foo, nullptr);