Re: Bug#593141: Bug#653582: ruby-hpricot: FTBFS on ia64: ruby crashes while running tests
I took a look at this a few weeks ago.
The problem is the code in the cont.c file which implements continuations.
A thread saves its own stack and its thread context itself while it is
running. The ruby programmers believe that that the saved info can be
used by another thread to switch over. They are wrong!
This is simply ill-formed code; wrong usage of the
makecontext()/swapcontext() functions. It is a miracle that it works
on other architectures - on sparc it did after doing some dirty tricks.
The problem causes crashes which are almost impossible to understand
with the debugger.
The code was written in 2007 and made a lot of trouble until now; when
you take a look to cont.c, you are see some really weird code
fragments - dirty hacks to workaround some fundamental design flaws.
I'm not satisfied with the code quality of the ruby project at all; I
don't understand how it could be included in a ruby version that is
for production use.
So the continuations and perhaps some related threading features are
broken - very advanced and new features in Ruby.
I think a fix is feasable for platforms which use
makecontext()/swapcontext() - as Linux is (rather than Windows Win32
functions).
What can be implememnted is that a thread switches to another
context/stack; the initial context/stack is saved after that. The
thread switches back to the initial context/stack finally.
This means:
- The performance becomes worse due to additional context switches. I
think it isn't that bad; the ruby code copies over huge portions of
memory in its implementation all the time; the entire original code is
a huge performance penalty (if it wouldn't crash). I don't believe
that the additional context switches makes it noticeable worse.
- The patch would be a real patch bomb. It would remove a lot of mess
for (not working) ia64 workarounds and the most recent sparc patches
of Debian as well. The patch replaces a lot of code of cont.c.
- The patch would change the implementation for all Debian archs, not
only ia64.
I appreciate comments on that.
For now I'd prefer the 'wheezy-ignore' rather than removing the ia64
ruby package.
Stephan
If you want to read something pleasing about ia64 for a change, you
can take a look at bug#659186 or #582774.
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