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Subject: Re: [boost] [serialization] automation of class version sanity checking
From: Robert Ramey (ramey_at_[hidden])
Date: 2009-03-04 12:34:35

A worthy idea. I would have to think about it some more though. Why not
re-submit this as a TRAK item.

Robert Ramey

troy d. straszheim wrote:
> Hey (Robert),
> Use case. It is possible (and does happen) that a user tries to
> deserialize version N+1 of a class C with software that only
> understands versions up to N. What typically happens is that C
> appears to deserialize OK, but it has eaten too many (or few) bytes from
> the
> archive. This means that the next class that gets loaded from the
> archive will fail in some unpredictable way.
> To deal with this we've started adding the following to all of our
> classes:
> struct C
> {
> double x; // this was present in version 0
> double y; // this arrived with version 1
> template <typename Archive>
> void
> serialize(Archive& ar, const unsigned int version);
> };
> template <typename Archive>
> void
> C::serialize(Archive& ar, const unsigned int version)
> {
> if (version > 1)
> throw unsupported_version(version, 1);
> if (version == 0)
> ar & x;
> if (version == 1)
> ar & y;
> }
> ( Note the two arguments to unsupported_version ).
> This has the pleasant side effect of often catching errors in *other*
> classes. Eg if I'm trying to deserialize C and that exception is
> thrown, you can generate a message "trying to read version 3243856293
> of class C, but I only know about version 0-1". Now I know that
> something has gone quite wrong, for instance someone has removed a member
> from
> my enclosing class without correctly incrementing and handling the
> version there, and what I've been given as 'version' is garbage. If
> the message is instead "trying to read version 4 of class C but I only
> know about
> versions 0-3", it looks like I simply need to update my software,
> nothing more sinister. This greatly simplifies these situations,
> which otherwise could spiral into a long, painful back-and-forth over
> a segfault in a serialization routine involving questions about what
> software the user has, where the data came from, what software was
> used to write it, etc.
> So:
> 1. I can't think of a situation where having file_version greater
> than BOOST_CLASS_VERSION isn't a catastrophic error.
> 1a. It'd be nice to configurably auto-check for this kind of
> versioning error.
> 2. As far as I can tell, these checks could be done cleanly inside
> the library, in serialize_adl() (?).
> 3. We don't actually use that unsupported_version exception, which
> takes no arguments and is thrown from serialization of variant (I
> wrote that and now find it confusing). I think the variant error should
> be
> specific to variant (or just totally generic), and unsupported_version
> should be used for the case currently under consideration... and to
> enable the 'pleasant side effect' above, I think the exception should
> carry the greatest version known to the code and the attempted-to-read
> version.
> Sanity check please, say so if you want me to try implementing...
> -t
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