py/gc: Split out running finalizers to a separate pass.

Currently a finalizer may run and access memory which has already been
freed. (This happens mostly during gc_sweep_all() but could happen during
any garbage collection pass.)

Includes some speed improvement tweaks to skip empty FTB blocks. These help
compensate for the inherent slowdown of having to walk the heap twice.

Signed-off-by: Angus Gratton <[email protected]>

Author: projectgus

py/gc.c

@@ -477,29 +477,20 @@ static void gc_deal_with_stack_overflow(void) {
     }
 }
 
-static void gc_sweep(void) {
-    #if MICROPY_PY_GC_COLLECT_RETVAL
-    MP_STATE_MEM(gc_collected) = 0;
-    #endif
-    // free unmarked heads and their tails
-    int free_tail = 0;
-    #if MICROPY_GC_SPLIT_HEAP_AUTO
-    mp_state_mem_area_t *prev_area = NULL;
-    #endif
-    for (mp_state_mem_area_t *area = &MP_STATE_MEM(area); area != NULL; area = NEXT_AREA(area)) {
-        size_t end_block = area->gc_alloc_table_byte_len * BLOCKS_PER_ATB;
-        if (area->gc_last_used_block < end_block) {
-            end_block = area->gc_last_used_block + 1;
-        }
-
-        size_t last_used_block = 0;
-
-        for (size_t block = 0; block < end_block; block++) {
-            MICROPY_GC_HOOK_LOOP(block);
-            switch (ATB_GET_KIND(area, block)) {
-                case AT_HEAD:
-                    #if MICROPY_ENABLE_FINALISER
-                    if (FTB_GET(area, block)) {
+// Run finalisers for all to-be-freed blocks
+static void gc_sweep_run_finalisers(void) {
+    #if MICROPY_ENABLE_FINALISER
+    for (const mp_state_mem_area_t *area = &MP_STATE_MEM(area); area != NULL; area = NEXT_AREA(area)) {
+        assert(area->gc_last_used_block <= area->gc_alloc_table_byte_len * BLOCKS_PER_ATB);
+        // Small speed optimisation: skip over empty FTB blocks
+        size_t ftb_end = area->gc_last_used_block / BLOCKS_PER_FTB; // index is inclusive
+        for (size_t ftb_idx = 0; ftb_idx <= ftb_end; ftb_idx++) {
+            byte ftb = area->gc_finaliser_table_start[ftb_idx];
+            size_t block = ftb_idx * BLOCKS_PER_FTB;
+            while (ftb) {
+                MICROPY_GC_HOOK_LOOP(block);
+                if (ftb & 1) { // FTB_GET(area, block) shortcut
+                    if (ATB_GET_KIND(area, block) == AT_HEAD) {
                         mp_obj_base_t *obj = (mp_obj_base_t *)PTR_FROM_BLOCK(area, block);
                         if (obj->type != NULL) {
                             // if the object has a type then see if it has a __del__ method
@@ -519,7 +510,35 @@ static void gc_sweep(void) {
                         // clear finaliser flag
                         FTB_CLEAR(area, block);
                     }
-                    #endif
+                }
+                ftb >>= 1;
+                block++;
+            }
+        }
+    }
+    #endif // MICROPY_ENABLE_FINALISER
+}
+
+static void gc_sweep(void) {
+    #if MICROPY_PY_GC_COLLECT_RETVAL
+    MP_STATE_MEM(gc_collected) = 0;
+    #endif
+    // free unmarked heads and their tails
+    int free_tail = 0;
+    #if MICROPY_GC_SPLIT_HEAP_AUTO
+    mp_state_mem_area_t *prev_area = NULL;
+    #endif
+
+    gc_sweep_run_finalisers();
+
+    for (mp_state_mem_area_t *area = &MP_STATE_MEM(area); area != NULL; area = NEXT_AREA(area)) {
+        size_t last_used_block = 0;
+        assert(area->gc_last_used_block <= area->gc_alloc_table_byte_len * BLOCKS_PER_ATB);
+
+        for (size_t block = 0; block <= area->gc_last_used_block; block++) {
+            MICROPY_GC_HOOK_LOOP(block);
+            switch (ATB_GET_KIND(area, block)) {
+                case AT_HEAD:
                     free_tail = 1;
                     DEBUG_printf("gc_sweep(%p)\n", (void *)PTR_FROM_BLOCK(area, block));
                     #if MICROPY_PY_GC_COLLECT_RETVAL