Ruby » Ruby master

Feature #20669

Updated by ioquatix (Samuel Williams) 4 days ago

Make it possible to differentiate general non-marshal errors from failures to decode Marshal data using a specific exception class. 

 ## Background 

 There are a variety of error conditions that can cause Marshal to fail, including both corrupt data, and between versions of Ruby/Marshal binary formats, e.g. 

 ``` ```ruby 
 > Marshal.load("foobar") 
 <internal:marshal>:34:in `load': incompatible marshal file format (can't be read) (TypeError) 

 > Marshal.load(Marshal.dump(Object.new).slice(0, 10)) 
 <internal:marshal>:34:in `load': marshal data too short (ArgumentError) 

 > MyThing = Struct.new(:name, :age) 
 => MyThing 
 > Marshal.dump(MyThing.new("Alice", 20)) 
 => "\x04\bS:\fMyThing\a:\tnameI\"\nAlice\x06:\x06ET:\bagei\x19" 

 (in a separate session) 
 > Marshal.load "\x04\bS:\fMyThing\a:\tnameI\"\nAlice\x06:\x06ET:\bagei\x19" 
 <internal:marshal>:34:in `load': undefined class/module MyThing (ArgumentError) 
 ``` 

 When data is corrupt, or incompatible, Marshal may raise either `ArgumentError` or `TypeError` with various messages. Not all errors are from `marshal.c`. 

 Generally, `TypeError` and `ArgumentError` are used to indicate some semantic problem with the program, rather than a runtime issue with the data. For example, `ArgumentError` may be used to indicate less than the required number of arguments passed to a method, and `TypeError` may indicate than the wrong data type was given as an argument. 

 Dalli is a library for accessing memcached servers. By default it uses Marshal to store serialised Ruby objects. However, when updating major versions of Ruby, or application code, `Marshal.load` may start raising `TypeError` or `ArgumentError`. This class of error needs to be handled differently from "normal" `TypeError` and `ArgumentError`. As such, the only way to differentiate right now is to pattern match the exception message, [which is done here](https://github.com/petergoldstein/dalli/blob/1d4cbfc78e6470ad57a883801a783bf30890c400/lib/dalli/protocol/value_serializer.rb#L36-L72): 

 ```ruby 
       # TODO: Some of these error messages need to be validated.    It's not obvious 
       # that all of them are actually generated by the invoked code 
       # in current systems 
       # rubocop:disable Layout/LineLength 
       TYPE_ERR_REGEXP = %r{needs to have method `_load'|exception class/object expected|instance of IO needed|incompatible marshal file format}.freeze 
       ARGUMENT_ERR_REGEXP = /undefined class|marshal data too short/.freeze 
       NAME_ERR_STR = 'uninitialized constant' 
       # rubocop:enable Layout/LineLength 

       def retrieve(value, bitflags) 
         serialized = (bitflags & FLAG_SERIALIZED) != 0 
         serialized ? serializer.load(value) : value 
       rescue TypeError => e 
         filter_type_error(e) 
       rescue ArgumentError => e 
         filter_argument_error(e) 
       rescue NameError => e 
         filter_name_error(e) 
       end 

       def filter_type_error(err) 
         raise err unless TYPE_ERR_REGEXP.match?(err.message) 

         raise UnmarshalError, "Unable to unmarshal value: #{err.message}" 
       end 

       def filter_argument_error(err) 
         raise err unless ARGUMENT_ERR_REGEXP.match?(err.message) 

         raise UnmarshalError, "Unable to unmarshal value: #{err.message}" 
       end 

       def filter_name_error(err) 
         raise err unless err.message.include?(NAME_ERR_STR) 

         raise UnmarshalError, "Unable to unmarshal value: #{err.message}" 
       end 
 ``` 

 ## Proposal 

 Ideally, the above code could use a specific (one or more) error class for detecting failed `Marshal.load`. 

 ```ruby 
       def retrieve(value, bitflags) 
         serialized = (bitflags & FLAG_SERIALIZED) != 0 
         serialized ? serializer.load(value) : value 
       rescue Marshal::LoadError => error 
         raise UnmarshalError, "Unable to unmarshal value!" 
       end 
 ``` 

 One difficulty is retaining backwards compatibility. We may be able to create a sub-class of `class LoadError < ArgumentError` but it fundamentally seems wrong to me (should really be a `RuntimeError`). 

 ## See also 

 - [Dalli PR where another error message would be added](https://github.com/petergoldstein/dalli/pull/1008) 
 - https://github.com/search?q=Marshal.load+ArgumentError&type=code - Git code search showing some variations of the problem (`rescue ArgumentError` and similar).

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