service/am: Implement EnterFatalSection and LeaveFatalSection

These functions act in tandem similar to how a lock or mutex require a
balanced lock()/unlock() sequence.

EnterFatalSection simply increments a counter for how many times it has
been called, while LeaveFatalSection ensures that a previous call to
EnterFatalSection has occured. If a previous call has occurred (the
counter is not zero), then the counter gets decremented as one would
expect. If a previous call has not occurred (the counter is zero), then
an error code is returned.

src/core/hle/service/am/am.cpp

@@ -239,8 +239,8 @@ ISelfController::ISelfController(std::shared_ptr<NVFlinger::NVFlinger> nvflinger
         {0, nullptr, "Exit"},
         {1, &ISelfController::LockExit, "LockExit"},
         {2, &ISelfController::UnlockExit, "UnlockExit"},
-        {3, nullptr, "EnterFatalSection"},
+        {3, &ISelfController::EnterFatalSection, "EnterFatalSection"},
-        {4, nullptr, "LeaveFatalSection"},
+        {4, &ISelfController::LeaveFatalSection, "LeaveFatalSection"},
         {9, &ISelfController::GetLibraryAppletLaunchableEvent, "GetLibraryAppletLaunchableEvent"},
         {10, &ISelfController::SetScreenShotPermission, "SetScreenShotPermission"},
         {11, &ISelfController::SetOperationModeChangedNotification, "SetOperationModeChangedNotification"},
@@ -299,6 +299,30 @@ void ISelfController::UnlockExit(Kernel::HLERequestContext& ctx) {
     rb.Push(RESULT_SUCCESS);
 }
 
+void ISelfController::EnterFatalSection(Kernel::HLERequestContext& ctx) {
+    ++num_fatal_sections_entered;
+    LOG_DEBUG(Service_AM, "called. Num fatal sections entered: {}", num_fatal_sections_entered);
+
+    IPC::ResponseBuilder rb{ctx, 2};
+    rb.Push(RESULT_SUCCESS);
+}
+
+void ISelfController::LeaveFatalSection(Kernel::HLERequestContext& ctx) {
+    LOG_DEBUG(Service_AM, "called.");
+
+    // Entry and exit of fatal sections must be balanced.
+    if (num_fatal_sections_entered == 0) {
+        IPC::ResponseBuilder rb{ctx, 2};
+        rb.Push(ResultCode{ErrorModule::AM, 512});
+        return;
+    }
+
+    --num_fatal_sections_entered;
+
+    IPC::ResponseBuilder rb{ctx, 2};
+    rb.Push(RESULT_SUCCESS);
+}
+
 void ISelfController::GetLibraryAppletLaunchableEvent(Kernel::HLERequestContext& ctx) {
     LOG_WARNING(Service_AM, "(STUBBED) called");
 

src/core/hle/service/am/am.h

@@ -119,6 +119,8 @@ public:
 private:
     void LockExit(Kernel::HLERequestContext& ctx);
     void UnlockExit(Kernel::HLERequestContext& ctx);
+    void EnterFatalSection(Kernel::HLERequestContext& ctx);
+    void LeaveFatalSection(Kernel::HLERequestContext& ctx);
     void GetLibraryAppletLaunchableEvent(Kernel::HLERequestContext& ctx);
     void SetScreenShotPermission(Kernel::HLERequestContext& ctx);
     void SetOperationModeChangedNotification(Kernel::HLERequestContext& ctx);
@@ -135,6 +137,7 @@ private:
     std::shared_ptr<NVFlinger::NVFlinger> nvflinger;
     Kernel::EventPair launchable_event;
     u32 idle_time_detection_extension = 0;
+    u64 num_fatal_sections_entered = 0;
 };
 
 class ICommonStateGetter final : public ServiceFramework<ICommonStateGetter> {