Dynamically stop lazy services

Services can choose to register with the new LazyServiceRegistrar.
ServiceManager perpetually checks the reference counts of services
registered in this way. If ServiceManager detects that a service no
longer has any clients, it will notify the LazyServiceRegistrar, which
will attempt to shut down the service.

Bug: 143108344
Test: aidl_lazy_test
Change-Id: Ic01981b767ab4402e7aecdf1cdf9ed64df1f5af4

9 files changed, 580 insertions, 8 deletions

diff --git a/cmds/servicemanager/ServiceManager.cpp b/cmds/servicemanager/ServiceManager.cpp
index 141171bf68..4a92ed862d 100644
--- a/cmds/servicemanager/ServiceManager.cpp
+++ b/cmds/servicemanager/ServiceManager.cpp
@@ -18,6 +18,9 @@
 
 #include <android-base/logging.h>
 #include <android-base/properties.h>
+#include <binder/BpBinder.h>
+#include <binder/IPCThreadState.h>
+#include <binder/ProcessState.h>
 #include <binder/Stability.h>
 #include <cutils/android_filesystem_config.h>
 #include <cutils/multiuser.h>
@@ -108,10 +111,11 @@ sp<IBinder> ServiceManager::tryGetService(const std::string& name, bool startIfN
     auto ctx = mAccess->getCallingContext();
 
     sp<IBinder> out;
+    Service* service = nullptr;
     if (auto it = mNameToService.find(name); it != mNameToService.end()) {
-        const Service& service = it->second;
+        service = &(it->second);
 
-        if (!service.allowIsolated) {
+        if (!service->allowIsolated) {
             uid_t appid = multiuser_get_app_id(ctx.uid);
             bool isIsolated = appid >= AID_ISOLATED_START && appid <= AID_ISOLATED_END;
 
@@ -119,7 +123,7 @@ sp<IBinder> ServiceManager::tryGetService(const std::string& name, bool startIfN
                 return nullptr;
             }
         }
-        out = service.binder;
+        out = service->binder;
     }
 
     if (!mAccess->canFind(ctx, name)) {
@@ -130,6 +134,12 @@ sp<IBinder> ServiceManager::tryGetService(const std::string& name, bool startIfN
         tryStartService(name);
     }
 
+    if (out) {
+        // Setting this guarantee each time we hand out a binder ensures that the client-checking
+        // loop knows about the event even if the client immediately drops the service
+        service->guaranteeClient = true;
+    }
+
     return out;
 }
 
@@ -182,15 +192,17 @@ Status ServiceManager::addService(const std::string& name, const sp<IBinder>& bi
         return Status::fromExceptionCode(Status::EX_ILLEGAL_STATE);
     }
 
-    mNameToService[name] = Service {
+    auto entry = mNameToService.emplace(name, Service {
         .binder = binder,
         .allowIsolated = allowIsolated,
         .dumpPriority = dumpPriority,
-    };
+        .debugPid = ctx.debugPid,
+    });
 
     auto it = mNameToCallback.find(name);
     if (it != mNameToCallback.end()) {
         for (const sp<IServiceCallback>& cb : it->second) {
+            entry.first->second.guaranteeClient = true;
             // permission checked in registerForNotifications
             cb->onRegistration(name, binder);
         }
@@ -330,6 +342,10 @@ void ServiceManager::binderDied(const wp<IBinder>& who) {
     for (auto it = mNameToCallback.begin(); it != mNameToCallback.end();) {
         removeCallback(who, &it, nullptr /*found*/);
     }
+
+    for (auto it = mNameToClientCallback.begin(); it != mNameToClientCallback.end();) {
+        removeClientCallback(who, &it);
+    }
 }
 
 void ServiceManager::tryStartService(const std::string& name) {
@@ -341,4 +357,183 @@ void ServiceManager::tryStartService(const std::string& name) {
     }).detach();
 }
 
-}  // namespace android
+Status ServiceManager::registerClientCallback(const std::string& name, const sp<IBinder>& service,
+                                              const sp<IClientCallback>& cb) {
+    if (cb == nullptr) {
+        return Status::fromExceptionCode(Status::EX_NULL_POINTER);
+    }
+
+    auto ctx = mAccess->getCallingContext();
+    if (!mAccess->canAdd(ctx, name)) {
+        return Status::fromExceptionCode(Status::EX_SECURITY);
+    }
+
+    auto serviceIt = mNameToService.find(name);
+    if (serviceIt == mNameToService.end()) {
+        LOG(ERROR) << "Could not add callback for nonexistent service: " << name;
+        return Status::fromExceptionCode(Status::EX_ILLEGAL_ARGUMENT);
+    }
+
+    if (serviceIt->second.debugPid != IPCThreadState::self()->getCallingPid()) {
+        LOG(WARNING) << "Only a server can register for client callbacks (for " << name << ")";
+        return Status::fromExceptionCode(Status::EX_UNSUPPORTED_OPERATION);
+    }
+
+    if (serviceIt->second.binder != service) {
+        LOG(WARNING) << "Tried to register client callback for " << name
+            << " but a different service is registered under this name.";
+        return Status::fromExceptionCode(Status::EX_ILLEGAL_ARGUMENT);
+    }
+
+    if (OK != IInterface::asBinder(cb)->linkToDeath(this)) {
+        LOG(ERROR) << "Could not linkToDeath when adding client callback for " << name;
+        return Status::fromExceptionCode(Status::EX_ILLEGAL_STATE);
+    }
+
+    mNameToClientCallback[name].push_back(cb);
+
+    return Status::ok();
+}
+
+void ServiceManager::removeClientCallback(const wp<IBinder>& who,
+                                          ClientCallbackMap::iterator* it) {
+    std::vector<sp<IClientCallback>>& listeners = (*it)->second;
+
+    for (auto lit = listeners.begin(); lit != listeners.end();) {
+        if (IInterface::asBinder(*lit) == who) {
+            lit = listeners.erase(lit);
+        } else {
+            ++lit;
+        }
+    }
+
+    if (listeners.empty()) {
+        *it = mNameToClientCallback.erase(*it);
+    } else {
+        (*it)++;
+    }
+}
+
+ssize_t ServiceManager::Service::getNodeStrongRefCount() {
+    sp<BpBinder> bpBinder = binder->remoteBinder();
+    if (bpBinder == nullptr) return -1;
+
+    return ProcessState::self()->getStrongRefCountForNodeByHandle(bpBinder->handle());
+}
+
+void ServiceManager::handleClientCallbacks() {
+    for (const auto& [name, service] : mNameToService) {
+        handleServiceClientCallback(name);
+    }
+}
+
+ssize_t ServiceManager::handleServiceClientCallback(const std::string& serviceName) {
+    auto serviceIt = mNameToService.find(serviceName);
+    if (serviceIt == mNameToService.end() || mNameToClientCallback.count(serviceName) < 1) {
+        return -1;
+    }
+
+    Service& service = serviceIt->second;
+    ssize_t count = service.getNodeStrongRefCount();
+
+    // binder driver doesn't support this feature
+    if (count == -1) return count;
+
+    bool hasClients = count > 1; // this process holds a strong count
+
+    if (service.guaranteeClient) {
+        // we have no record of this client
+        if (!service.hasClients && !hasClients) {
+            sendClientCallbackNotifications(serviceName, true);
+        }
+
+        // guarantee is temporary
+        service.guaranteeClient = false;
+    }
+
+    if (hasClients && !service.hasClients) {
+        // client was retrieved in some other way
+        sendClientCallbackNotifications(serviceName, true);
+    }
+
+    // there are no more clients, but the callback has not been called yet
+    if (!hasClients && service.hasClients) {
+        sendClientCallbackNotifications(serviceName, false);
+    }
+
+    return count;
+}
+
+void ServiceManager::sendClientCallbackNotifications(const std::string& serviceName, bool hasClients) {
+    auto serviceIt = mNameToService.find(serviceName);
+    if (serviceIt == mNameToService.end()) {
+        LOG(WARNING) << "sendClientCallbackNotifications could not find service " << serviceName;
+        return;
+    }
+    Service& service = serviceIt->second;
+
+    CHECK(hasClients != service.hasClients) << "Record shows: " << service.hasClients
+        << " so we can't tell clients again that we have client: " << hasClients;
+
+    LOG(INFO) << "Notifying " << serviceName << " they have clients: " << hasClients;
+
+    auto ccIt = mNameToClientCallback.find(serviceName);
+    CHECK(ccIt != mNameToClientCallback.end())
+        << "sendClientCallbackNotifications could not find callbacks for service ";
+
+    for (const auto& callback : ccIt->second) {
+        callback->onClients(service.binder, hasClients);
+    }
+
+    service.hasClients = hasClients;
+}
+
+Status ServiceManager::tryUnregisterService(const std::string& name, const sp<IBinder>& binder) {
+    if (binder == nullptr) {
+        return Status::fromExceptionCode(Status::EX_NULL_POINTER);
+    }
+
+    auto ctx = mAccess->getCallingContext();
+    if (!mAccess->canAdd(ctx, name)) {
+        return Status::fromExceptionCode(Status::EX_SECURITY);
+    }
+
+    auto serviceIt = mNameToService.find(name);
+    if (serviceIt == mNameToService.end()) {
+        LOG(WARNING) << "Tried to unregister " << name
+            << ", but that service wasn't registered to begin with.";
+        return Status::fromExceptionCode(Status::EX_ILLEGAL_STATE);
+    }
+
+    if (serviceIt->second.debugPid != IPCThreadState::self()->getCallingPid()) {
+        LOG(WARNING) << "Only a server can unregister itself (for " << name << ")";
+        return Status::fromExceptionCode(Status::EX_UNSUPPORTED_OPERATION);
+    }
+
+    sp<IBinder> storedBinder = serviceIt->second.binder;
+
+    if (binder != storedBinder) {
+        LOG(WARNING) << "Tried to unregister " << name
+            << ", but a different service is registered under this name.";
+        return Status::fromExceptionCode(Status::EX_ILLEGAL_STATE);
+    }
+
+    int clients = handleServiceClientCallback(name);
+
+    // clients < 0: feature not implemented or other error. Assume clients.
+    // Otherwise:
+    // - kernel driver will hold onto one refcount (during this transaction)
+    // - servicemanager has a refcount (guaranteed by this transaction)
+    // So, if clients > 2, then at least one other service on the system must hold a refcount.
+    if (clients < 0 || clients > 2) {
+        // client callbacks are either disabled or there are other clients
+        LOG(INFO) << "Tried to unregister " << name << " but there are clients: " << clients;
+        return Status::fromExceptionCode(Status::EX_ILLEGAL_STATE);
+    }
+
+    mNameToService.erase(name);
+
+    return Status::ok();
+}
+
+}  // namespace android
\ No newline at end of file
diff --git a/cmds/servicemanager/ServiceManager.h b/cmds/servicemanager/ServiceManager.h
index 7dcdaa4661..d3c89ee2ad 100644
--- a/cmds/servicemanager/ServiceManager.h
+++ b/cmds/servicemanager/ServiceManager.h
@@ -17,12 +17,14 @@
 #pragma once
 
 #include <android/os/BnServiceManager.h>
+#include <android/os/IClientCallback.h>
 #include <android/os/IServiceCallback.h>
 
 #include "Access.h"
 
 namespace android {
 
+using os::IClientCallback;
 using os::IServiceCallback;
 
 class ServiceManager : public os::BnServiceManager, public IBinder::DeathRecipient {
@@ -40,9 +42,13 @@ public:
                                             const sp<IServiceCallback>& callback) override;
     binder::Status unregisterForNotifications(const std::string& name,
                                               const sp<IServiceCallback>& callback) override;
-    binder::Status isDeclared(const std::string& name, bool* outReturn) override;
 
+    binder::Status isDeclared(const std::string& name, bool* outReturn) override;
+    binder::Status registerClientCallback(const std::string& name, const sp<IBinder>& service,
+                                          const sp<IClientCallback>& cb) override;
+    binder::Status tryUnregisterService(const std::string& name, const sp<IBinder>& binder) override;
     void binderDied(const wp<IBinder>& who) override;
+    void handleClientCallbacks();
 
 protected:
     virtual void tryStartService(const std::string& name);
@@ -52,9 +58,16 @@ private:
         sp<IBinder> binder; // not null
         bool allowIsolated;
         int32_t dumpPriority;
+        bool hasClients = false; // notifications sent on true -> false.
+        bool guaranteeClient = false; // forces the client check to true
+        pid_t debugPid = 0; // the process in which this service runs
+
+        // the number of clients of the service, including servicemanager itself
+        ssize_t getNodeStrongRefCount();
     };
 
     using CallbackMap = std::map<std::string, std::vector<sp<IServiceCallback>>>;
+    using ClientCallbackMap = std::map<std::string, std::vector<sp<IClientCallback>>>;
     using ServiceMap = std::map<std::string, Service>;
 
     // removes a callback from mNameToCallback, removing it if the vector is empty
@@ -62,10 +75,18 @@ private:
     void removeCallback(const wp<IBinder>& who,
                         CallbackMap::iterator* it,
                         bool* found);
+    ssize_t handleServiceClientCallback(const std::string& serviceName);
+     // Also updates mHasClients (of what the last callback was)
+    void sendClientCallbackNotifications(const std::string& serviceName, bool hasClients);
+    // removes a callback from mNameToClientCallback, deleting the entry if the vector is empty
+    // this updates the iterator to the next location
+    void removeClientCallback(const wp<IBinder>& who, ClientCallbackMap::iterator* it);
+
     sp<IBinder> tryGetService(const std::string& name, bool startIfNotFound);
 
     CallbackMap mNameToCallback;
     ServiceMap mNameToService;
+    ClientCallbackMap mNameToClientCallback;
 
     std::unique_ptr<Access> mAccess;
 };
diff --git a/cmds/servicemanager/main.cpp b/cmds/servicemanager/main.cpp
index 4b12fc6e72..2618906261 100644
--- a/cmds/servicemanager/main.cpp
+++ b/cmds/servicemanager/main.cpp
@@ -18,18 +18,101 @@
 #include <binder/IPCThreadState.h>
 #include <binder/ProcessState.h>
 #include <binder/Status.h>
+#include <sys/timerfd.h>
+#include <utils/Looper.h>
 #include <utils/StrongPointer.h>
 
 #include "Access.h"
 #include "ServiceManager.h"
 
 using ::android::Access;
+using ::android::sp;
+using ::android::Looper;
+using ::android::LooperCallback;
+using ::android::ProcessState;
 using ::android::IPCThreadState;
 using ::android::ProcessState;
 using ::android::ServiceManager;
 using ::android::os::IServiceManager;
 using ::android::sp;
 
+class BinderCallback : public LooperCallback {
+public:
+    static sp<BinderCallback> setupTo(const sp<Looper>& looper) {
+        sp<BinderCallback> cb = new BinderCallback;
+
+        int binder_fd = -1;
+        IPCThreadState::self()->setupPolling(&binder_fd);
+        LOG_ALWAYS_FATAL_IF(binder_fd < 0, "Failed to setupPolling: %d", binder_fd);
+
+        // Flush after setupPolling(), to make sure the binder driver
+        // knows about this thread handling commands.
+        IPCThreadState::self()->flushCommands();
+
+        int ret = looper->addFd(binder_fd,
+                                Looper::POLL_CALLBACK,
+                                Looper::EVENT_INPUT,
+                                cb,
+                                nullptr /*data*/);
+        LOG_ALWAYS_FATAL_IF(ret != 1, "Failed to add binder FD to Looper");
+
+        return cb;
+    }
+
+    int handleEvent(int /* fd */, int /* events */, void* /* data */) override {
+        IPCThreadState::self()->handlePolledCommands();
+        return 1;  // Continue receiving callbacks.
+    }
+};
+
+// LooperCallback for IClientCallback
+class ClientCallbackCallback : public LooperCallback {
+public:
+    static sp<ClientCallbackCallback> setupTo(const sp<Looper>& looper, const sp<ServiceManager>& manager) {
+        sp<ClientCallbackCallback> cb = new ClientCallbackCallback(manager);
+
+        int fdTimer = timerfd_create(CLOCK_MONOTONIC, 0 /*flags*/);
+        LOG_ALWAYS_FATAL_IF(fdTimer < 0, "Failed to timerfd_create: fd: %d err: %d", fdTimer, errno);
+
+        itimerspec timespec {
+            .it_interval = {
+                .tv_sec = 5,
+                .tv_nsec = 0,
+            },
+            .it_value = {
+                .tv_sec = 5,
+                .tv_nsec = 0,
+            },
+        };
+
+        int timeRes = timerfd_settime(fdTimer, 0 /*flags*/, &timespec, nullptr);
+        LOG_ALWAYS_FATAL_IF(timeRes < 0, "Failed to timerfd_settime: res: %d err: %d", timeRes, errno);
+
+        int addRes = looper->addFd(fdTimer,
+                                   Looper::POLL_CALLBACK,
+                                   Looper::EVENT_INPUT,
+                                   cb,
+                                   nullptr);
+        LOG_ALWAYS_FATAL_IF(addRes != 1, "Failed to add client callback FD to Looper");
+
+        return cb;
+    }
+
+    int handleEvent(int fd, int /*events*/, void* /*data*/) override {
+        uint64_t expirations;
+        int ret = read(fd, &expirations, sizeof(expirations));
+        if (ret != sizeof(expirations)) {
+            ALOGE("Read failed to callback FD: ret: %d err: %d", ret, errno);
+        }
+
+        mManager->handleClientCallbacks();
+        return 1;  // Continue receiving callbacks.
+    }
+private:
+    ClientCallbackCallback(const sp<ServiceManager>& manager) : mManager(manager) {}
+    sp<ServiceManager> mManager;
+};
+
 int main(int argc, char** argv) {
     if (argc > 2) {
         LOG(FATAL) << "usage: " << argv[0] << " [binder driver]";
@@ -49,7 +132,14 @@ int main(int argc, char** argv) {
     IPCThreadState::self()->setTheContextObject(manager);
     ps->becomeContextManager(nullptr, nullptr);
 
-    IPCThreadState::self()->joinThreadPool();
+    sp<Looper> looper = Looper::prepare(false /*allowNonCallbacks*/);
+
+    BinderCallback::setupTo(looper);
+    ClientCallbackCallback::setupTo(looper, manager);
+
+    while(true) {
+        looper->pollAll(-1);
+    }
 
     // should not be reached
     return EXIT_FAILURE;
diff --git a/libs/binder/Android.bp b/libs/binder/Android.bp
index 7ee4882b6a..079dd82ecd 100644
--- a/libs/binder/Android.bp
+++ b/libs/binder/Android.bp
@@ -90,6 +90,7 @@ cc_library {
         "IResultReceiver.cpp",
         "IServiceManager.cpp",
         "IShellCallback.cpp",
+        "LazyServiceRegistrar.cpp",
         "MemoryBase.cpp",
         "MemoryDealer.cpp",
         "MemoryHeapBase.cpp",
@@ -160,6 +161,7 @@ filegroup {
     name: "libbinder_aidl",
     srcs: [
         "aidl/android/content/pm/IPackageManagerNative.aidl",
+        "aidl/android/os/IClientCallback.aidl",
         "aidl/android/os/IServiceCallback.aidl",
         "aidl/android/os/IServiceManager.aidl",
     ],
diff --git a/libs/binder/IServiceManager.cpp b/libs/binder/IServiceManager.cpp
index bac8b6604b..5ca9156fb2 100644
--- a/libs/binder/IServiceManager.cpp
+++ b/libs/binder/IServiceManager.cpp
@@ -271,6 +271,8 @@ sp<IBinder> ServiceManagerShim::waitForService(const String16& name16)
             std::unique_lock<std::mutex> lock(mMutex);
             mBinder = binder;
             lock.unlock();
+            // Flushing here helps ensure the service's ref count remains accurate
+            IPCThreadState::self()->flushCommands();
             mCv.notify_one();
             return Status::ok();
         }
diff --git a/libs/binder/LazyServiceRegistrar.cpp b/libs/binder/LazyServiceRegistrar.cpp
new file mode 100644
index 0000000000..dc9482c536
--- /dev/null
+++ b/libs/binder/LazyServiceRegistrar.cpp
@@ -0,0 +1,167 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#define LOG_TAG "AidlLazyServiceRegistrar"
+
+#include <binder/LazyServiceRegistrar.h>
+#include <binder/IPCThreadState.h>
+#include <binder/IServiceManager.h>
+#include <android/os/BnClientCallback.h>
+#include <android/os/IServiceManager.h>
+#include <utils/Log.h>
+
+namespace android {
+namespace binder {
+namespace internal {
+
+using AidlServiceManager = android::os::IServiceManager;
+
+class ClientCounterCallback : public ::android::os::BnClientCallback {
+public:
+    ClientCounterCallback() : mNumConnectedServices(0) {}
+
+    bool registerService(const sp<IBinder>& service, const std::string& name,
+                         bool allowIsolated, int dumpFlags);
+
+protected:
+    Status onClients(const sp<IBinder>& service, bool clients) override;
+
+private:
+    /**
+     * Unregisters all services that we can. If we can't unregister all, re-register other
+     * services.
+     */
+    void tryShutdown();
+
+    /**
+     * Counter of the number of services that currently have at least one client.
+     */
+    size_t mNumConnectedServices;
+
+    struct Service {
+        sp<IBinder> service;
+        std::string name;
+        bool allowIsolated;
+        int dumpFlags;
+    };
+    /**
+     * Number of services that have been registered.
+     */
+    std::vector<Service> mRegisteredServices;
+};
+
+bool ClientCounterCallback::registerService(const sp<IBinder>& service, const std::string& name,
+                                            bool allowIsolated, int dumpFlags) {
+    auto manager = interface_cast<AidlServiceManager>(
+                    ProcessState::self()->getContextObject(nullptr));
+
+    ALOGI("Registering service %s", name.c_str());
+
+    if (!manager->addService(name.c_str(), service, allowIsolated, dumpFlags).isOk()) {
+        ALOGE("Failed to register service %s", name.c_str());
+        return false;
+    }
+
+    if (!manager->registerClientCallback(name, service, this).isOk())
+    {
+      ALOGE("Failed to add client callback for service %s", name.c_str());
+      return false;
+    }
+
+    mRegisteredServices.push_back({service, name, allowIsolated, dumpFlags});
+
+    return true;
+}
+
+/**
+ * onClients is oneway, so no need to worry about multi-threading. Note that this means multiple
+ * invocations could occur on different threads however.
+ */
+Status ClientCounterCallback::onClients(const sp<IBinder>& service, bool clients) {
+    if (clients) {
+        mNumConnectedServices++;
+    } else {
+        mNumConnectedServices--;
+    }
+
+    ALOGI("Process has %zu (of %zu available) client(s) in use after notification %s has clients: %d",
+          mNumConnectedServices, mRegisteredServices.size(),
+          String8(service->getInterfaceDescriptor()).string(), clients);
+
+    if (mNumConnectedServices == 0) {
+        tryShutdown();
+    }
+
+    return Status::ok();
+}
+
+void ClientCounterCallback::tryShutdown() {
+    ALOGI("Trying to shut down the service. No clients in use for any service in process.");
+
+    // This makes the same assumption as IServiceManager.cpp. Could dedupe if used in more places.
+    auto manager = interface_cast<AidlServiceManager>(
+            ProcessState::self()->getContextObject(nullptr));
+
+    auto unRegisterIt = mRegisteredServices.begin();
+    for (; unRegisterIt != mRegisteredServices.end(); ++unRegisterIt) {
+        auto& entry = (*unRegisterIt);
+
+        bool success = manager->tryUnregisterService(entry.name, entry.service).isOk();
+
+        if (!success) {
+            ALOGI("Failed to unregister service %s", entry.name.c_str());
+            break;
+        }
+    }
+
+    if (unRegisterIt == mRegisteredServices.end()) {
+        ALOGI("Unregistered all clients and exiting");
+        exit(EXIT_SUCCESS);
+    }
+
+    for (auto reRegisterIt = mRegisteredServices.begin(); reRegisterIt != unRegisterIt;
+         reRegisterIt++) {
+        auto& entry = (*reRegisterIt);
+
+        // re-register entry
+        if (!registerService(entry.service, entry.name, entry.allowIsolated, entry.dumpFlags)) {
+            // Must restart. Otherwise, clients will never be able to get a hold of this service.
+            ALOGE("Bad state: could not re-register services");
+        }
+    }
+}
+
+}  // namespace internal
+
+LazyServiceRegistrar::LazyServiceRegistrar() {
+    mClientCC = std::make_shared<internal::ClientCounterCallback>();
+}
+
+LazyServiceRegistrar& LazyServiceRegistrar::getInstance() {
+    static auto registrarInstance = new LazyServiceRegistrar();
+    return *registrarInstance;
+}
+
+status_t LazyServiceRegistrar::registerService(const sp<IBinder>& service, const std::string& name,
+                                               bool allowIsolated, int dumpFlags) {
+    if (!mClientCC->registerService(service, name, allowIsolated, dumpFlags)) {
+        return UNKNOWN_ERROR;
+    }
+    return OK;
+}
+
+}  // namespace hardware
+}  // namespace android
\ No newline at end of file
diff --git a/libs/binder/aidl/android/os/IClientCallback.aidl b/libs/binder/aidl/android/os/IClientCallback.aidl
new file mode 100644
index 0000000000..36d7ee61fb
--- /dev/null
+++ b/libs/binder/aidl/android/os/IClientCallback.aidl
@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.os;
+
+/**
+ * @hide
+ */
+oneway interface IClientCallback {
+    /**
+     * This is called when there is a transition between having >= 1 clients and having 0 clients
+     * (or vice versa).
+     *
+     * Upon receiving hasClients false, if the process decides to exit, it is recommended to try to
+     * unregister using IServiceManager's tryUnregister before quitting in case another client
+     * associates.
+     *
+     * @param registered binder 'server' registered with IServiceManager's registerClientCallback
+     * @param hasClients whether there are currently clients
+     *     true - when there are >= 1 clients. This must be called as soon as IServiceManager::get
+     *         is called (no race).
+     *     false - when there are 0 clients. This may be delayed if it is thought that another
+     *         may be used again soon.
+     */
+    void onClients(IBinder registered, boolean hasClients);
+}
diff --git a/libs/binder/aidl/android/os/IServiceManager.aidl b/libs/binder/aidl/android/os/IServiceManager.aidl
index b965881e7f..ff154603e4 100644
--- a/libs/binder/aidl/android/os/IServiceManager.aidl
+++ b/libs/binder/aidl/android/os/IServiceManager.aidl
@@ -16,6 +16,7 @@
 
 package android.os;
 
+import android.os.IClientCallback;
 import android.os.IServiceCallback;
 
 /**
@@ -96,4 +97,15 @@ interface IServiceManager {
      * manifest.
      */
     boolean isDeclared(@utf8InCpp String name);
+
+    /**
+     * Request a callback when the number of clients of the service changes.
+     * Used by LazyServiceRegistrar to dynamically stop services that have no clients.
+     */
+    void registerClientCallback(@utf8InCpp String name, IBinder service, IClientCallback callback);
+
+    /**
+     * Attempt to unregister and remove a service. Will fail if the service is still in use.
+     */
+    void tryUnregisterService(@utf8InCpp String name, IBinder service);
 }
diff --git a/libs/binder/include/binder/LazyServiceRegistrar.h b/libs/binder/include/binder/LazyServiceRegistrar.h
new file mode 100644
index 0000000000..efdecc4eca
--- /dev/null
+++ b/libs/binder/include/binder/LazyServiceRegistrar.h
@@ -0,0 +1,44 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <binder/IServiceManager.h>
+#include <binder/Status.h>
+#include <utils/StrongPointer.h>
+
+namespace android {
+namespace binder {
+namespace internal {
+class ClientCounterCallback;
+}  // namespace internal
+
+/** Exits when all services registered through this object have 0 clients */
+class LazyServiceRegistrar {
+   public:
+     static LazyServiceRegistrar& getInstance();
+     status_t registerService(const sp<IBinder>& service,
+                              const std::string& name = "default",
+                              bool allowIsolated = false,
+                              int dumpFlags = IServiceManager::DUMP_FLAG_PRIORITY_DEFAULT);
+
+   private:
+     std::shared_ptr<internal::ClientCounterCallback> mClientCC;
+     LazyServiceRegistrar();
+};
+
+}  // namespace binder
+}  // namespace android
\ No newline at end of file