Add Mutex::trylock_until and trylock_for

Given the 64-bit timebase, add trylock_until to Mutex.

Naming is based on a combination of Mutex::trylock, Thread::wait_until,
and C++11 timed_mutex::try_lock_until.

pthreads and C11 use "timedlock", but that's not a good fit against our
existing trylock() and lock(timeout) - they have only absolute-time
waits, not relative.

To increase the similarity to C++11, add trylock_for - same parameters
as lock, but with the bool return value of trylock and trylock_until.

Add an assertion when convering status codes to booleans to check that
there are no non-timeout errors.

rtos/Mutex.cpp

@@ -20,6 +20,7 @@
  * SOFTWARE.
  */
 #include "rtos/Mutex.h"
+#include "rtos/Kernel.h"
 
 #include <string.h>
 #include "mbed_error.h"
@@ -58,11 +59,30 @@ osStatus Mutex::lock(uint32_t millisec) {
 }
 
 bool Mutex::trylock() {
-    if (osMutexAcquire(_id, 0) == osOK) {
-        _count++;
+    return trylock_for(0);
+}
+
+bool Mutex::trylock_for(uint32_t millisec) {
+    osStatus status = lock(millisec);
+    if (status == osOK) {
         return true;
+    }
+
+    MBED_ASSERT(status == osErrorTimeout || status == osErrorResource);
+
+    return false;
+}
+
+bool Mutex::trylock_until(uint64_t millisec) {
+    uint64_t now = Kernel::get_ms_count();
+
+    if (now >= millisec) {
+        return trylock();
+    } else if (millisec - now >= osWaitForever) {
+        // API permits early return
+        return trylock_for(osWaitForever - 1);
     } else {
-        return false;
+        return trylock_for(millisec - now);
     }
 }
 

rtos/Mutex.h

@@ -78,11 +78,36 @@ public:
 
     /** Try to lock the mutex, and return immediately
       @return true if the mutex was acquired, false otherwise.
+      @note equivalent to trylock_for(0)
 
-      @note This function cannot be called from ISR context.
+      @note You cannot call this function from ISR context.
      */
     bool trylock();
 
+    /** Try to lock the mutex for a specified time
+      @param   millisec  timeout value or 0 in case of no time-out.
+      @return true if the mutex was acquired, false otherwise.
+      @note the underlying RTOS may have a limit to the maximum wait time
+            due to internal 32-bit computations, but this is guaranteed to work if the
+            wait is <= 0x7fffffff milliseconds (~24 days). If the limit is exceeded,
+            the lock attempt will time out earlier than specified.
+
+      @note You cannot call this function from ISR context.
+     */
+    bool trylock_for(uint32_t millisec);
+
+    /** Try to lock the mutex until specified time
+      @param   millisec  absolute timeout time, referenced to Kernel::get_ms_count()
+      @return true if the mutex was acquired, false otherwise.
+      @note the underlying RTOS may have a limit to the maximum wait time
+            due to internal 32-bit computations, but this is guaranteed to work if the
+            wait is <= 0x7fffffff milliseconds (~24 days). If the limit is exceeded,
+            the lock attempt will time out earlier than specified.
+
+      @note You cannot call this function from ISR context.
+     */
+    bool trylock_until(uint64_t millisec);
+
     /** Unlock the mutex that has previously been locked by the same thread
       @return status code that indicates the execution status of the function:
               @a osOK the mutex has been released.
@@ -90,7 +115,7 @@ public:
               @a osErrorResource the mutex was not locked or the current thread wasn't the owner.
               @a osErrorISR this function cannot be called from the interrupt service routine.
 
-      @note This function cannot be called from ISR context.
+      @note You cannot call this function from ISR context.
      */
     osStatus unlock();