py/gc: Allow gc_free from inside a gc_sweep finalizer.

Do this by tracking being inside gc collection with a
separate flag, GC_COLLECT_FLAG. In gc_free(),
ignore this flag when determining if the heap is locked.

* For finalisers calling gc_free() when heap is otherwise unlocked,
  this allows memory to be immediately freed (potentially
  avoiding a MemoryError).

* Hard IRQs still can't call gc_free(), as heap will be locked via
  gc_lock().

* If finalisers are disabled then all of this code can be compiled
  out to save some code size.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

1 files changed, 13 insertions, 0 deletions

diff --git a/py/mpstate.h b/py/mpstate.h
index 54eca596da..51f290b55b 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -77,6 +77,18 @@ typedef struct _mp_sched_item_t {
     mp_obj_t arg;
 } mp_sched_item_t;
 
+// gc_lock_depth field is a combination of the GC_COLLECT_FLAG
+// bit and a lock depth shifted GC_LOCK_DEPTH_SHIFT bits left.
+#if MICROPY_ENABLE_FINALISER
+#define GC_COLLECT_FLAG 1
+#define GC_LOCK_DEPTH_SHIFT 1
+#else
+// If finalisers are disabled then this check doesn't matter, as gc_lock()
+// is called anywhere else that heap can't be changed. So save some code size.
+#define GC_COLLECT_FLAG 0
+#define GC_LOCK_DEPTH_SHIFT 0
+#endif
+
 // This structure holds information about a single contiguous area of
 // memory reserved for the memory manager.
 typedef struct _mp_state_mem_area_t {
@@ -268,6 +280,7 @@ typedef struct _mp_state_thread_t {
     #endif
 
     // Locking of the GC is done per thread.
+    // See GC_LOCK_DEPTH_SHIFT for an explanation of this field.
     uint16_t gc_lock_depth;
 
     ////////////////////////////////////////////////////////////