Linux threads support

[Mark Kettenis <kettenis at wins dot uva dot nl>]

After concluding that the current Linux threads support is pretty much
broken beyond repair, I have been working on a reimplementation, which
consists of two layers:

1. A Linux LWP layer.  This presents a group of Linux processes (which
   are assumed to share a single address space) to GDB as a
   multi-threaded process.  It should be useful on its right, i.e. for
   programs that don't use the LinuxThreads library, but the clone()
   system call directly.

2. A generic libthread_db layer (contains only a few Linux-specific
   hooks).  This contains much code from Michael Snyder's lin-thread.c
   module, but since most of the Linux-specific stuff is factored out
   into the Linux LWP layer, it should be possible to make it usable
   for Solaris without much problems.

The old LinuxThreads support contained linux-thread.c isn't there
anymore.  It shouldn't be too difficult to reimplement it, but I'm not
eager to do so.  Supporting one threads debugging module is bad enough.

Anyway, I'm now at a point where I think it functions better than the
current threads support, and I really need to get it more widely
tested.  I see two possibilities:

1. Post a patch and hope that people take the trouble of applying it.

2. Put the new code directly in the CVS (possible in a seperate
   branch).

I think option 1 is doomed to fail.  For option 2 I'd like some
opinions (especially from the maintainers of the various Linux ports):

* Should the stuff be checked in on a seperate branch?

* How important is support for older Linux systems (pre-libthread_db,
  i.e. pre glibc-2.1.3) give the fact that GDB has never had any
  satisfactory threads support for these systems, and older versions
  of the LinuxThreads library contain many critical bugs?

* Who needs to review this stuff?  Michael Snyder is listed as the
  threads maintainer and did the origional integration/implementation
  of the Linux threads support.  However, he seems to be pretty busy,
  and he once told me he isn't too familiar with the peculiarities of
  the Linux stuff.

Mark