I have an index that was created in 4.5.1 using a custom Codec (actually just the Lucene45Codec codec with the getPostingsFormatForField method overridden to return a MemoryPostingsFormat for a particular field). I attempted to upgrade using IndexUpgrader to a 4.8.1 index, and it failed with a CorruptIndexException. Digging a little deeper, it seems like MemoryPostingsFormat did not use a header (as supplied by org.apache.lucene.codecs.CodecUtil) in 4.5, but now does in 4.8, so the on-disk formats are incompatible.
I know MemoryPostingsFormat is experimental, subject to change, etc, but has anyone given any thought to allowing an upgrade in scenarios like this? I think for my existing index it's too late, since the name "Memory" is already written into the segment, and the PostingsFormat returned by NamedSPILoader for "Memory" will be one that only allows reading 4.8 "Memory" postings. Maybe in the future it would be nice to do something as simple as having the PostingsFormats in the lucene-codecs package have a version in the name (i.e. Memory45, Memory48) so that at least I could do the work of porting the 4.5 code to 4.8 enough to allow an upgrade if needed? Or maybe there's some API that I'm unaware of that I can hook into the loading of old format segments and return an alternate postings format? (looking at the code nothing like this seems to exist, but wanted to double check)
