kernel/readable_event: Add member function for enforcing a strict reset contract

svcResetSignal relies on the event instance to have already been
signaled before attempting to reset it. If this isn't the case, then an
error code has to be returned.

src/core/hle/kernel/readable_event.cpp

@@ -4,10 +4,10 @@
 
 #include <algorithm>
 #include "common/assert.h"
+#include "core/hle/kernel/errors.h"
 #include "core/hle/kernel/object.h"
 #include "core/hle/kernel/readable_event.h"
 #include "core/hle/kernel/thread.h"
-#include "core/hle/kernel/writable_event.h"
 
 namespace Kernel {
 
@@ -34,6 +34,16 @@ void ReadableEvent::Clear() {
     signaled = false;
 }
 
+ResultCode ReadableEvent::Reset() {
+    if (!signaled) {
+        return ERR_INVALID_STATE;
+    }
+
+    Clear();
+
+    return RESULT_SUCCESS;
+}
+
 void ReadableEvent::WakeupAllWaitingThreads() {
     WaitObject::WakeupAllWaitingThreads();
 

src/core/hle/kernel/readable_event.h

@@ -7,6 +7,8 @@
 #include "core/hle/kernel/object.h"
 #include "core/hle/kernel/wait_object.h"
 
+union ResultCode;
+
 namespace Kernel {
 
 class KernelCore;
@@ -39,8 +41,17 @@ public:
 
     void WakeupAllWaitingThreads() override;
 
+    /// Unconditionally clears the readable event's state.
     void Clear();
 
+    /// Clears the readable event's state if and only if it
+    /// has already been signaled.
+    ///
+    /// @pre The event must be in a signaled state. If this event
+    ///      is in an unsignaled state and this function is called,
+    ///      then ERR_INVALID_STATE will be returned.
+    ResultCode Reset();
+
 private:
     explicit ReadableEvent(KernelCore& kernel);