mbed_error_puts: Prime STDERR_FILENO before writing

Prime the console outside the critical section, improving the chances of
nice initialisation.

platform/mbed_board.c

@@ -74,6 +74,28 @@ void mbed_error_vprintf(const char *format, va_list arg)
 
 void mbed_error_puts(const char *str)
 {
+    // Writing the string to the console in a critical section is
+    // potentially beneficial - for example in UARTSerial it
+    // forces the "unbuffered" mode that makes sure all characters
+    // go out now. If we made the call not in a critical section,
+    // it would go to the software buffer and we would be reliant
+    // on platform.stdio-flush-at-exit forcing a fsync before
+    // entering mbed_die().
+    //
+    // But this may be the very first write to the console, and hence
+    // require it to be initialized - doing this in a critical
+    // section could be problematic. So we prime it outside the
+    // critical section with a zero-length write - this forces
+    // the initialization.
+    //
+    // It's still possible that we were in a critical section
+    // or interrupt on entry anyway (eg if this is an error coming
+    // from inside RTX), so in other areas of the system we suppress
+    // things like mutex creation asserts and RTX traps while
+    // an error is in progress, so that console initialization
+    // may work.
+    write(STDERR_FILENO, str, 0);
+
     core_util_critical_section_enter();
 #if MBED_CONF_PLATFORM_STDIO_CONVERT_NEWLINES || MBED_CONF_PLATFORM_STDIO_CONVERT_TTY_NEWLINES
     char stdio_out_prev = '\0';

platform/mbed_interface.h

@@ -127,7 +127,9 @@ MBED_NORETURN void mbed_die(void);
 /** Print out an error message.  This is typically called when
  * handling a crash.
  *
- * @note Synchronization level: Interrupt safe
+ * @note Synchronization level: Interrupt safe, as long as the
+ *       FileHandle::write of the stderr device is. See mbed_error_puts
+ *       for more information.
  * @note This uses an internal 128-byte buffer to format the string,
  *       so the output may be truncated. If you need to write a potentially
  *       long string, use mbed_error_puts.
@@ -145,7 +147,9 @@ void mbed_error_printf(const char *format, ...) MBED_PRINTF(1, 2);
 /** Print out an error message.  Similar to mbed_error_printf
  * but uses a va_list.
  *
- * @note Synchronization level: Interrupt safe
+ * @note Synchronization level: Interrupt safe, as long as the
+ *       FileHandle::write of the stderr device is. See mbed_error_puts
+ *       for more information.
  *
  * @param format    C string that contains data stream to be printed.
  * @param arg       Variable arguments list
@@ -160,7 +164,13 @@ void mbed_error_vprintf(const char *format, va_list arg) MBED_PRINTF(1, 0);
  * length. Unlike standard puts, but like standard fputs, this does not
  * append a '\n' character.
  *
- * @note Synchronization level: Interrupt safe
+ * @note Synchronization level: Interrupt safe, as long as the
+ *       FileHandle::write of the stderr device is. The default
+ *       serial console is safe, either buffered or not. If the
+ *       console has not previously been initialized, an attempt
+ *       to use this from interrupt may during console initialization.
+ *       Special handling of `mbed_error` relaxes various system traps
+ *       to increase the chance of initialization working.
  *
  * @param str    C string that contains data stream to be printed.
  *