Fail to get_free_region in thread_final_regions due to low memory for mark array might lead to AV. #74401

While our logic guarantee a free region is available in the free region list before thread_final_regions, in case we are running out of memory to commit for the mark array, the call to init_table_for_region called in get_free_region may fail and therefore return nullptr.

To reproduce the issue, we can run GCPerfSim with these parameters:

GCPerfSim.dll -tc 6 -tagb 100.0 -tlgb 0.2 -lohar 0 -pohar 0 -sohsi 10 -lohsi 0 -pohsi 0 -sohsr 100-4000 -lohsr 102400-204800 -pohsr 100-4000 -sohpi 10 -lohpi 0 -sohfi 0 -lohfi 0 -pohfi 0 -allocType reference -testKind time

and these environment variables:

COMPlus_GCHeapCount=6
COMPlus_GCHeapHardLimit=1C200000
COMPlus_GCServer=1

On this commit 5f80c24 with a simple change to replace this line in sufficient_space_regions

return check_against_hard_limit (end_space_required);

to

return true;

The change was made to make this easier to repro only and is not an intended product change.

To make this issue easier to identify, we can add a check after get_free_region in thread_final_regions as follow. Change

start_region = get_free_region (gen_idx);

to

start_region = get_free_region (gen_idx);
if (start_region == nullptr) { FATAL_GC_ERROR(); }

The process will fail at the spot. Without the check, the nullptr might be threaded into the generation, and will lead to bad consequences (such as an AV).