From: Seebs <usenet-nospam@...>
Date: 2008-06-30T15:47:19+09:00
Subject: Re: mysterious memory corruption, very confused

On 2008-06-30, Roger Pack <rogerpack2005@gmail.com> wrote:
>> I get random data corruption when trying to execute queries.
> valgrind might tell you if memory is being tramped.

It is.  There's a loop of
	VALUE x;
	char **foo = malloc(buncha char *);

	for (big list of things) {
		x = rb_obj_as_string(y);
		foo[i] = GetStringValue(x);
	}

The idiom of using rb_obj_as_string, and then using the value, is common in
the Ruby source.  It works.  ... It works *as long as you don't allocate
anything more before you're done with it*.  What ends up happening is that,
if enough of the objects in question need a new string allocated by
rb_obj_as_string, sooner or later you end up invoking the garbage collector.
Now, since there's only one x, the garbage collector assumes the current
rb_obj_as_string() return is in use, *and the others aren't*.  So it might,
if it wants the space, free one... And then the memory gets reused.

With "big list" being about 12 items, about half of which needed new strings
allocated, this ended up blowing up about once every thousand or two thousand
runs.  Unfortunately for me, I had about 80,000 data points.  :)

I submitted a more detailed bug report to the ruby-pg project, and I've
adopted a workaround (possibly very inefficient) involving an array of VALUE
objects and rb_gc_{un}register_address.  It's ugly but it eliminates the bug.

-- 
Copyright 2008, all wrongs reversed.  Peter Seebach / usenet-nospam@seebs.net
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