[Firebird-devel] Emergency Fix to Memory Manager

[Jim S. <ja...@ne...>]

I strong advise that the following be applied to Firebird 2.0.

In alloc.h, class Firebird::MemoryPool:

        /// Returns the type associated with the allocated memory.
        static SSHORT blk_type(const blk* mem) {
            return ((MemoryBlock*)((char *)mem -
    MEM_ALIGN(sizeof(MemoryBlock))))->type;
        }
       
        /// Returns the pool the memory was allocated from.
        static MemoryPool* blk_pool(const blk* mem) {
            return ((MemoryBlock*)((char *)mem -
    MEM_ALIGN(sizeof(MemoryBlock))))->pool;
        }


This means an object not inheriting from blk will force a compilation 
error.  This requires that the line

    struct blk;

be included before the "namespace Firebird" in alloc.h.

In fb_blk.h:

    struct blk
    {
        virtual void doesNothing() {};
    };

This forces "blk" to be a virtual class.  The implicit type conversion 
from a virtual subclass to a non-virtual superclass changes the pointer 
on at MSVC to skip over the virtual function vector pointer.

It is also necessary to change any class that has blk_type or blk_pool 
references from a declaration like

    class Database : pool_alloc<type_dbb>

to

    class Database : public pool_alloc<type_dbb>

so "blk" is accessible.

This doesn't fix the problem, but it will cause a compiler exception in 
aggregious cases.   The proper fix is to move "blk_type" and "blk_pool" 
back into "blk" where they belong, then hunt down and fix the cases 
where implicit assumptions have been made about memory manager 
implementation.

-- 

Jim Starkey
Netfrastructure, Inc.
978 526-1376