Description
The current implementations of writeExternal and readExternal are inconsistent in the way they serialize and deserialize data. On the one hand, writeExternal first serializes each data field into its individual byte array that is then written to writeExternal's ObjectOutput. On the other hand, readExternal deserializes the data as if it all has been written directly to the ObjectOutput without any intermediate FieldOut -> DataOutputStream -> ByteArrayOutputStream -> byte[] steps.
Such approach may works in some cases (for example, when using standard Java serialization) and some others, unfortunately, it does not (for example, https://issues.apache.org/jira/browse/IGNITE-1001).
Specifically, the current implementation incorrectly assumes that on-the-wire representation produced by the DataOutputStream is the same as the one that would have been produced by the ObjectOutput had it been used directly, and is indeed the one that ObjectInput (passed into readExternal) knows how to read. However, different serialization libraries may use different approaches to serializing data. For example, DataOutput.writeUTF() implementations vary widely in how they serialize the length of the string, as well as its characters. So the current approach of writing a UTF string using DataOutputStream's implementation and then reading it using some other serializer's DataInput implementation is never guaranteed to work reliably.
The good news is that the fix is really simple: read the entire byte array in readExternal method and wrap it into a DataInputStream instance, which is then used to deserialize the rest of Config's hierarchy.
I have a fix. It's literally 4 lines of code.