Raise Dalli::UnmarshalError on ArgumentError "dump format error (user class)"

[olleolleolle]

This PR makes Rails recognize the unmarshalling error and can deal with it normally.

Before this change, such an ArgumentError would bubble up and raise in Rails.

Details

In Marshal, a "user class" is a class which extends a "core class" (String, Hash...).

When a "user class" is dumped by Marshal.dump, it becomes a binary string. Marshal.load, expects that the inheritance of this class looks the same as it did when the binary string was created.

If the classes do not match, due to some change in the code, an ArgumentError is raised. (The specific ArgumentError is raised in marshal.c.)

Marshal's concept of "user class" is explained in: "Caching With MessagePack" by Chris Salzberg at RubyKaigi 2022

Context for Rails

This Rails commit, which is in Rails 7.1, rails/rails@a6be798, expects Dalli to reraise such errors, so that Rails can decide to regenerate that cache entry.

Between 7.0 and 7.1 Rails changed the inheritance hierarchy in its String-extending classes OutputBuffer and SafeBuffer, which triggered this error in production for me.

Further

In #1009 I made the regular expression-making code a little easier to read, using Regexp.union.

But, later, in #1011, I made a wider change, reraising Dalli::UnmarshalError for all errors raised during serializer.load. It's a simplification of the code, of the solution, but takes more steps than this tiny change.

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EDIT: I posted a feature request in bugs.ruby-lang.org for having error classes for these ArgumentError instances, instead. (Perhaps they can become 1 error, (class UnmarshalingError < ArgumentError), or something.)

[kaspth]

Can confirm there was a bunch of refactoring done on the SafeBuffer and OutputBuffer (iirc to leverage Ruby's native String#<<). I'm surprised the error ended up being thrown in production now because of those changes, but this fix looks good to me!

[petergoldstein]

@olleolleolle As #1011 has been merged, I'm assuming this can be closed, correct?

[olleolleolle]

Thanks, and confirming, yes! Closing.