sys/main: process all events before power off

This fixes an odd behavior where the City hub turns itself back on as
soon as it powers off. This is probably a hardware issue, e.g. an
induced voltage somewhere is enough to turn it back on.

Fixes: pybricks/support#385

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 # Changelog
 
+
+## [Unreleased]
+
+### Fixed
+- Fixed City hub not always powering off on shutdown ([support#385])
+
 ## [3.1.0a2] - 2021-07-06
 
 ### Added
@@ -79,6 +85,7 @@ Prerelease changes are documented at [support#48].
 [support#352]: https://github.com/pybricks/support/issues/352
 [support#361]: https://github.com/pybricks/support/issues/361
 [support#379]: https://github.com/pybricks/support/issues/379
+[support#385]: https://github.com/pybricks/support/issues/385
 
 [Unreleased]: https://github.com/pybricks/pybricks-micropython/compare/v3.1.0a2...HEAD
 [3.1.0a2]: https://github.com/pybricks/pybricks-micropython/compare/v3.0.0a1...v3.1.0a2

lib/pbio/sys/main.c

@@ -42,8 +42,14 @@ void pbsys_main(pbsys_main_t main) {
         pbsys_user_program_unprepare();
     }
 
+    // The power could be held on due to someone pressing the center button
+    // or USB being plugged in, so we have this loop to keep pumping events
+    // to turn off most of the peripherals and keep the battery charger running.
     for (;;) {
+        // We must handle all pending events before turning the power off the
+        // first time, otherwise the city hub turns itself back on sometimes.
+        while (pbio_do_one_event()) {
+        }
         pbdrv_reset_power_off();
-        pbio_do_one_event();
     }
 }