Fix for issue #6872 - Mutex lock has possibility to fail at runtime (returning status flag)

Add assertions for statuses returned by osMutexAcquire().
Add void Mutex::lock(void) member function - lock function which does not return status and waits for the mutex forever.
Make osStatus Mutex::lock(uint32_t millisec=osWaitForever) member function deprecated in favour of bool Mutex::trylock_for(uint32_t millisec).

rtos/Mutex.cpp

@@ -50,11 +50,25 @@ void Mutex::constructor(const char *name)
     MBED_ASSERT(_id);
 }
 
+void Mutex::lock(void) {
+    osStatus status = osMutexAcquire(_id, osWaitForever);
+    if (osOK == status) {
+        _count++;
+    }
+
+    MBED_ASSERT(status == osOK);
+}
+
 osStatus Mutex::lock(uint32_t millisec) {
     osStatus status = osMutexAcquire(_id, millisec);
     if (osOK == status) {
         _count++;
     }
+
+    MBED_ASSERT(status == osOK ||
+                (status == osErrorResource && millisec == 0) ||
+                (status == osErrorTimeout && millisec != osWaitForever));
+
     return status;
 }
 
@@ -63,12 +77,14 @@ bool Mutex::trylock() {
 }
 
 bool Mutex::trylock_for(uint32_t millisec) {
-    osStatus status = lock(millisec);
+    osStatus status = osMutexAcquire(_id, millisec);
     if (status == osOK) {
         return true;
     }
 
-    MBED_ASSERT(status == osErrorTimeout || status == osErrorResource);
+    MBED_ASSERT(status == osOK ||
+                (status == osErrorResource && millisec == 0) ||
+                (status == osErrorTimeout && millisec != osWaitForever));
 
     return false;
 }

rtos/Mutex.h

@@ -78,7 +78,18 @@ public:
     */
     Mutex(const char *name);
 
-    /** Wait until a Mutex becomes available.
+    /**
+      Wait until a Mutex becomes available.
+
+      @note You cannot call this function from ISR context.
+     */
+    void lock(void);
+
+    /**
+      For backwards compatibility.
+      @deprecated Do not use this function. This function has been replaced with trylock_for and lock(void) functions.
+
+      Wait until a Mutex becomes available.
       @param   millisec  timeout value or 0 in case of no time-out. (default: osWaitForever)
       @return  status code that indicates the execution status of the function:
                @a osOK the mutex has been obtained.
@@ -89,6 +100,7 @@ public:
 
       @note You cannot call this function from ISR context.
      */
+    MBED_DEPRECATED_SINCE("mbed-os-5.10.0", "Replaced with trylock_for and lock(void) functions")
     osStatus lock(uint32_t millisec=osWaitForever);
 
     /** Try to lock the mutex, and return immediately