One relevant difference may be the change to the fork semantics in Solaris 10: fork() now means the same as fork1(). Before, fork() meant the same as forkall(), unless the applications was linked with -lpthread, in which case fork() also meant fork1(). See fork(2).

So I'd be curious if a) the test case passes on Solaris 8 if fork1 is being used, and b) whether it passes if you make sure -lpthread is being used. If so, I'd rather fix this issue by changing the linkage for Solaris 8 (or declaring that system as unsupported altogether), instead of fiddling yet again with the fork handling. In Python, posix.fork definitely needs to mean "POSIX fork".

If it's something else (i.e. the thread ID changes in Solaris 8 whether fork1 or forkall is used), then the patch is fine in principle. However, I would always reset the thread ID. In your patch, it is not clear why you apply this resetting to Solaris and AIX, but not other systems.