py: Add config option to print warnings/errors to stderr.

This adds a new configuration option to print runtime warnings and errors to
stderr. On Unix, CPython prints warnings and unhandled exceptions to stderr,
so the unix port here is configured to use this option.

The unix port already printed unhandled exceptions on the main thread to
stderr. This patch fixes unhandled exceptions on other threads and warnings
(issue #2838) not printing on stderr.

Additionally, a couple tests needed to be fixed to handle this new behavior.
This is done by also capturing stderr when running tests.

ports/unix/mpconfigport.h

@@ -145,8 +145,11 @@
 // names in exception messages (may require more RAM).
 #define MICROPY_ERROR_REPORTING     (MICROPY_ERROR_REPORTING_DETAILED)
 #define MICROPY_WARNINGS            (1)
+#define MICROPY_ERROR_PRINTER       (&mp_stderr_print)
 #define MICROPY_PY_STR_BYTES_CMP_WARN (1)
 
+extern const struct _mp_print_t mp_stderr_print;
+
 // Define to 1 to use undertested inefficient GC helper implementation
 // (if more efficient arch-specific one is not available).
 #ifndef MICROPY_GCREGS_SETJMP

py/modthread.c

@@ -192,10 +192,10 @@ STATIC void *thread_entry(void *args_in) {
             // swallow exception silently
         } else {
             // print exception out
-            mp_printf(&mp_plat_print, "Unhandled exception in thread started by ");
-            mp_obj_print_helper(&mp_plat_print, args->fun, PRINT_REPR);
-            mp_printf(&mp_plat_print, "\n");
-            mp_obj_print_exception(&mp_plat_print, MP_OBJ_FROM_PTR(exc));
+            mp_printf(MICROPY_ERROR_PRINTER, "Unhandled exception in thread started by ");
+            mp_obj_print_helper(MICROPY_ERROR_PRINTER, args->fun, PRINT_REPR);
+            mp_printf(MICROPY_ERROR_PRINTER, "\n");
+            mp_obj_print_exception(MICROPY_ERROR_PRINTER, MP_OBJ_FROM_PTR(exc));
         }
     }
 

py/mpconfig.h

@@ -533,6 +533,11 @@ typedef long long mp_longint_impl_t;
 #define MICROPY_WARNINGS (0)
 #endif
 
+// This macro is used when printing runtime warnings and errors
+#ifndef MICROPY_ERROR_PRINTER
+#define MICROPY_ERROR_PRINTER (&mp_plat_print)
+#endif
+
 // Float and complex implementation
 #define MICROPY_FLOAT_IMPL_NONE (0)
 #define MICROPY_FLOAT_IMPL_FLOAT (1)

py/warning.c

@@ -35,9 +35,9 @@
 void mp_warning(const char *msg, ...) {
     va_list args;
     va_start(args, msg);
-    mp_print_str(&mp_plat_print, "Warning: ");
-    mp_vprintf(&mp_plat_print, msg, args);
-    mp_print_str(&mp_plat_print, "\n");
+    mp_print_str(MICROPY_ERROR_PRINTER, "Warning: ");
+    mp_vprintf(MICROPY_ERROR_PRINTER, msg, args);
+    mp_print_str(MICROPY_ERROR_PRINTER, "\n");
     va_end(args);
 }
 

tests/run-tests

@@ -103,7 +103,7 @@ def run_micropython(pyb, args, test_file, is_special=False):
                         os.close(master)
                         os.close(slave)
                 else:
-                    output_mupy = subprocess.check_output(args + [test_file])
+                    output_mupy = subprocess.check_output(args + [test_file], stderr=subprocess.STDOUT)
             except subprocess.CalledProcessError:
                 return b'CRASH'
 
@@ -124,7 +124,7 @@ def run_micropython(pyb, args, test_file, is_special=False):
 
             # run the actual test
             try:
-                output_mupy = subprocess.check_output(cmdlist)
+                output_mupy = subprocess.check_output(cmdlist, stderr=subprocess.STDOUT)
             except subprocess.CalledProcessError:
                 output_mupy = b'CRASH'