unix/coverage: Add extra GC coverage test for ATB gap byte.

The assertion that is added here (to gc.c) fails when running this new test
if ALLOC_TABLE_GAP_BYTE is set to 0.

Signed-off-by: Jeff Epler <jepler@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/py/gc.c b/py/gc.c
index 99eacfbe6..3c106fd67 100644
--- a/py/gc.c
+++ b/py/gc.c
@@ -297,6 +297,9 @@ STATIC void gc_mark_subtree(size_t block)
             n_blocks += 1;
         } while (ATB_GET_KIND(area, block + n_blocks) == AT_TAIL);
 
+        // check that the consecutive blocks didn't overflow past the end of the area
+        assert(area->gc_pool_start + (block + n_blocks) * BYTES_PER_BLOCK <= area->gc_pool_end);
+
         // check this block's children
         void **ptrs = (void **)PTR_FROM_BLOCK(area, block);
         for (size_t i = n_blocks * BYTES_PER_BLOCK / sizeof(void *); i > 0; i--, ptrs++) {