block alloc: add validations around cache_entry lifecyle

Add an option (controlled by an environment variable) perform extra
validations on mem_pool allocated cache entries. When set:

  1) Invalidate cache_entry memory when discarding cache_entry.

  2) When discarding index_state struct, verify that all cache_entries
     were allocated from expected mem_pool.

  3) When discarding mem_pools, invalidate mem_pool memory.

This should provide extra checks that mem_pools and their allocated
cache_entries are being used as expected.

Signed-off-by: Jameson Miller <jamill@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mem-pool.c

@@ -50,7 +50,7 @@ void mem_pool_init(struct mem_pool **mem_pool, size_t initial_size)
 	*mem_pool = pool;
 }
 
-void mem_pool_discard(struct mem_pool *mem_pool)
+void mem_pool_discard(struct mem_pool *mem_pool, int invalidate_memory)
 {
 	struct mp_block *block, *block_to_free;
 
@@ -59,6 +59,10 @@ void mem_pool_discard(struct mem_pool *mem_pool)
 	{
 		block_to_free = block;
 		block = block->next_block;
+
+		if (invalidate_memory)
+			memset(block_to_free->space, 0xDD, ((char *)block_to_free->end) - ((char *)block_to_free->space));
+
 		free(block_to_free);
 	}