Do not extend the pool on each allocation

Reported in Boost mailing list:
https://lists.boost.org/Archives/boost/2023/06/254750.php

Commit 951ca57 changed malloc_need_resize() to use set_next_size(),
which sets start_size in addition to setting next_size. this causes
repeated use of purge_memory() to allocate 2x size after every use.

Fixes #54

include/boost/pool/pool.hpp

@@ -295,6 +295,18 @@ class pool: protected simple_segregated_storage < typename UserAllocator::size_t
     //! Called if malloc/ordered_malloc needs to resize the free list.
     void * malloc_need_resize(); //! Called if malloc needs to resize the free list.
     void * ordered_malloc_need_resize();  //! Called if ordered_malloc needs to resize the free list.
+    void advance_next(size_type partition_size)
+    {
+      BOOST_USING_STD_MIN();
+      size_type nnext_size;
+      if(!max_size)
+        nnext_size = next_size << 1;
+      else if(next_size*partition_size/requested_size < max_size)
+        nnext_size = min BOOST_PREVENT_MACRO_SUBSTITUTION(next_size << 1, max_size * requested_size / partition_size);
+      else
+        return;
+      next_size = min BOOST_PREVENT_MACRO_SUBSTITUTION(nnext_size, max_chunks());
+    }
 
   protected:
     details::PODptr<size_type> list; //!< List structure holding ordered blocks.
@@ -717,11 +729,7 @@ void * pool<UserAllocator>::malloc_need_resize()
   }
   const details::PODptr<size_type> node(ptr, POD_size);
 
-  BOOST_USING_STD_MIN();
-  if(!max_size)
-    set_next_size(next_size << 1);
-  else if( next_size*partition_size/requested_size < max_size)
-    set_next_size(min BOOST_PREVENT_MACRO_SUBSTITUTION(next_size << 1, max_size * requested_size / partition_size));
+  advance_next(partition_size);
 
   //  initialize it,
   store().add_block(node.begin(), node.element_size(), partition_size);
@@ -757,11 +765,7 @@ void * pool<UserAllocator>::ordered_malloc_need_resize()
   }
   const details::PODptr<size_type> node(ptr, POD_size);
 
-  BOOST_USING_STD_MIN();
-  if(!max_size)
-    set_next_size(next_size << 1);
-  else if( next_size*partition_size/requested_size < max_size)
-    set_next_size(min BOOST_PREVENT_MACRO_SUBSTITUTION(next_size << 1, max_size * requested_size / partition_size));
+  advance_next(partition_size);
 
   //  initialize it,
   //  (we can use "add_block" here because we know that
@@ -851,11 +855,7 @@ void * pool<UserAllocator>::ordered_malloc(const size_type n)
     store().add_ordered_block(node.begin() + num_chunks * partition_size,
         node.element_size() - num_chunks * partition_size, partition_size);
 
-  BOOST_USING_STD_MIN();
-  if(!max_size)
-    set_next_size(next_size << 1);
-  else if( next_size*partition_size/requested_size < max_size)
-    set_next_size(min BOOST_PREVENT_MACRO_SUBSTITUTION(next_size << 1, max_size * requested_size / partition_size));
+  advance_next(partition_size);
 
   //  insert it into the list,
   //   handle border case.

test/Jamfile.v2

@@ -37,6 +37,7 @@ test-suite pool :
     [ run test_bug_2696.cpp ]
     [ run test_bug_5526.cpp : : : <library>/boost/smart_ptr//boost_smart_ptr [ requires cxx11_noexcept ] ]
     [ run test_bug_6701.cpp ]
+    [ run test_bug_54.cpp ]
     [ run test_threading.cpp : : : <threading>multi <library>/boost/thread//boost_thread <library>/boost/random//boost_random ]
     [ compile test_poisoned_macros.cpp ]
     ;

test/test_bug_54.cpp

@@ -0,0 +1,26 @@
+/* Copyright (C) 2023 Orgad Shaneh
+*
+* Use, modification and distribution is subject to the
+* Boost Software License, Version 1.0. (See accompanying
+* file LICENSE_1_0.txt or http://www.boost.org/LICENSE_1_0.txt)
+*/
+
+// Test of bug #54 (https://github.com/boostorg/pool/issues/54)
+
+#include <boost/pool/pool.hpp>
+#include <boost/limits.hpp>
+
+int main()
+{
+  boost::pool<> pool(8, 32, 64);
+  // On 32 next_size reaches max_size.
+  // One more round to asserts that it remains 64 (max_size)
+  for (int i = 0; i <= 33; ++i) {
+	size_t expected = (i == 0) ? 32 : 64;
+	BOOST_ASSERT(pool.get_next_size() == expected);
+	void *ptr = pool.malloc();
+	BOOST_ASSERT(ptr);
+  }
+
+  return 0;
+}