Problem with `min_block_size`

[jwdevel]

The calculation for memory_pool::min_block_size seems wrong in some cases.

Here is a failing test I added to test/memory_pool.cpp:

#include "container.hpp"
#include "smart_ptr.hpp"

// ... 

TEST_CASE("memory_pool min_block_size", "[pool]") {

	using pool_type = memory_pool<>;

	struct MyObj {
		float value;
		char  data[100];
	};
	CAPTURE(alignof(MyObj));

	auto nodesize = allocate_shared_node_size<MyObj, pool_type>::value;
	CAPTURE(nodesize);

	// Make a pool with the smallest block size possible.
	auto minblock = pool_type::min_block_size(nodesize, 1);
	CAPTURE(minblock);
	pool_type pool(nodesize, minblock);

	auto ptr = allocate_shared<MyObj>(pool);
}

This fails with:

[foonathan::memory] Assertion failure in function insert_impl (.../foonathan-memory/src/detail/free_list.cpp:536): Assertion "no_nodes > 0" failed.

.../foonathan-memory/test/memory_pool.cpp:71: FAILED:
due to a fatal error condition:
  alignof(MyObj) := 4
  nodesize := 136
  minblock := 184
  SIGABRT - Abort (abnormal termination) signal

This seems to be due to ordered_free_memory_list::insert_impl using actual_size = node_size + 2 * fence_size(), but min_block_size does not do the same calculation. Specifically, ordered_free_memory_list::fence_size() gives node_size_ as the fence size. I don't fully understand this, but it definitely seems to be on purpose.

I'm happy to work on a fix for this, but I wonder if you have a recommended approach.
My initial idea is to somehow expose the "actual_size" computation, so it is accessible via memory_pool::pool_type::XXX, but interested in other ideas.

[Jonathan Müller]

The calculation for memory_pool::min_block_size seems wrong in some cases.

Good catch, thanks. This was probably exposed by #105.

This seems to be due to ordered_free_memory_list::insert_impl using actual_size = node_size + 2 * fence_size(), but min_block_size does not do the same calculation. Specifically, ordered_free_memory_list::fence_size() gives node_size_ as the fence size. I don't fully understand this, but it definitely seems to be on purpose.

This is on purpose to catch buffer overflow in debug mode yes.

I'm happy to work on a fix for this, but I wonder if you have a recommended approach.
My initial idea is to somehow expose the "actual_size" computation, so it is accessible via memory_pool::pool_type::XXX, but interested in other ideas.

Adding a constexpr std::size_t actual_node_size() to the pool type seems reasonable, yes. Can you do a PR?

[jwdevel]

Sure thing, will take a stab a it.

[Jonathan Müller]

Should be fixed now. For simplicity (and to get rid of lots of potential bugs, if there are any), I've just removed debug fences from the pools. If needed, I can add them back later.