[Dbix-class] mpm_winnt threads and threading in general

[Brandon Black]

Thought I'd start a thread here about DBIx::Class and threads, and
specifically the issue raised by Ryan Parr on the Catalyst mailing
list about mpm_winnt threads compatibility.  Apparently under such
scenarios, we can treat the case just like we treat things if someone
was manually using ithreads in their perl code and using DBIx::Class.

The issue here is that DBIx::Class[::Storage::DBI] really is already
thread-safe (for that matter it was already fork-safe before the fork
changes too).  It's DBI.pm's lack of fork/thread-safety that we're
working around there (which is why the standard ithreads advice about
how to make thread-safe modules without explicitly "use threads"
doesn't apply).

We cannot see thread changes without "use threads" to get at
"threads->tid" and handle it like the fork stuff.  But if the calling
code isn't using threads, we shouldn't be doing "use threads"
(especially if the perl in question doesn't even support threads).

Basically, we want the connection routines in Storage::DBI to make use
of the thread id returned by "threads->tid", but if and only if we're
running under a threaded perl (can use Config.pm to see that), and if
and only if the application that's using us already did "use threads"
(because we shouldn't "use threads" if they didn't).  I'm guessing we
can see that by checking %INC for "threads"?  Make both checks in a
BEGIN block and decide whether to "require threads;" and set some flag
for the connection code to later know that it can safely check thread
id's?

Thoughts on any of this from anyone that knows more about ithreads?

-- Brandon