Resolution of the library-path mess

[Eric S. Raymond]

Hal Murray <hmurray at megapathdsl.net>:
> >> Do we need a way to check that we are using the right library?
> 
> >> I think that means we need the version string or time stamp in both the 
> >> library and the code that uses the library with a simple sanity check at 
> the
> >> top of the uses code.
> 
> > It's not normal practice to do this in Python. GPSD never has.
> 
> > I think the reason is that when you do have such a mismatch, the Python
> > stack trace it generally produces is likely to be more informative than
> > a version-mismatch message.
> 
> Given the confusion we have had in this area, it seemed like a simple and low 
> cost way to avoid what might turn into a long chase.  Sure, if you get a 
> messy backtrace you can probably figure out that you have the wrong library.  
> But what if it doesn't crash and just gives the wrong answer?

It is rather unusual to be able to address that effectively with a
version check.  I say this as a person who has written more than his
share of service libraries and clients for them.

Yes, in theory one would think this is a common case (error discovered without
API change).  But thinking back I can only remember one time *I* ever did this -
that was when I discovered that the error-bounds computation in GPSD had been
slightly wrong for years because of a duplicated variable in a nested loop.

I bumped the library version, sure, but it didn't matter.  As with our
Python libraries, libgpsd is never updated separately from its client code.

I deduce that my experience is representative from the fact that Python
library version checks are not routine in other peoples' code.  They're
not nonexistent - GTK clients commonly do this - so people do know it
is possible.

So, think through the cases. If a library is always shipped coupled with
its client code, you can't get the kind of skew a version check would
notice.  And...how is a third-party client to know when it should bump
the floor to check for?
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>

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