[phobos] Looks like more problems with the array append patches

[Steve Schveighoffer]

Through debugging Ellery Newcomer's test case (see http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=108685), I have found another bug in the array append patch.

See if you can spot it, I literally had to stare at this code for about 40 minutes, confused by its behavior, before I finally saw the problem.  The function below searches the LRU cache for a blockinfo (base pointer, size, and attributes) containing an interior pointer:

struct BlkInfo
{
    void*  base;
    size_t size;
    uint   attr;
}

BlkInfo *__getBlkInfo(void *interior)
{
        // try to do a smart lookup, using __nextBlkIdx as the "head"
        BlkInfo *ptr = __blkcache.ptr;
        for(int i = __nextBlkIdx; i >= 0; --i)
        {
            if(ptr[i].base <= interior && (interior - ptr[i].base) < ptr.size)
                return ptr + i;
        }

        for(int i = N_CACHE_BLOCKS - 1; i > __nextBlkIdx; --i)
        {
            if(ptr[i].base <= interior && (interior - ptr[i].base) < ptr.size)
                return ptr + i;
        }
        return null; // not in cache.
}

to give you a hint, here is the erroneous behavior that puzzled me:

given a pointer 0x38bd0, the function returned a blockinfo that started at 0x38bc0 with size 16.

If you can't figure it out, here is the answer: http://www.dsource.org/projects/druntime/changeset/282

I'm waiting to hear back from Ellery that the new code fixes the problem, and I also pinged David for his issue with AAs.  If all is OK, I recommend a patch release.  I'm not sure the state of the AAs in trunk, since Walter has changed them significantly -- we could be fixing one bug and introducing many more.  Given how simple this fix is, it might be good to release just this fix in order to have a stable version that doesn't do memory stomping.

-Steve



----- Original Message ----
> From: Walter Bright <walter at digitalmars.com>
> To: Discuss the phobos library for D <phobos at puremagic.com>
> Sent: Fri, April 2, 2010 1:29:18 PM
> Subject: [phobos] Looks like more problems with the array append patches
> 
> 

-------- Original Message --------
Subject: Memory Corruption with 
> AAs
Date: Fri, 2 Apr 2010 17:15:36 +0000 (UTC)
From: dsimcha <
> ymailto="mailto:dsimcha at yahoo.com" 
> href="mailto:dsimcha at yahoo.com">dsimcha at yahoo.com>
Organization: XXX 
> News Server
Newsgroups: digitalmars.D

Has anyone else still been 
> noticing difficult to reproduce memory corruption
issues in the presence of 
> associative arrays with 2.042?  They seem to happen
very infrequently 
> and non-deterministically.  I can only reproduce them in the
context of 
> a large program.  However, they don't occur in 2.040 (the release
before 
> the array stomping patch), and they are clearly a result of 
> memory
corruption, as contents of arrays change from what I expect them to be 
> to
completely random-looking values inside a loop that does a lot of 
> memory
management and uses AAs heavily but doesn't modify the 
> values.
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