Synchronize statistic enumeration values between statistics.h and java API

[bentorfs]

(Copied from #2171 ).
Problem: The enumerations of tickers and histogram types for retrieving statistics are not in sync between the C++ source and the java API:

Java version:
https://github.com/facebook/rocksdb/blob/master/java/src/main/java/org/rocksdb/TickerType.java
https://github.com/facebook/rocksdb/blob/master/java/src/main/java/org/rocksdb/HistogramType.java
C++ version:
https://github.com/facebook/rocksdb/blob/master/include/rocksdb/statistics.h

The java Enumeration is missing a lot of declarations, and their values do not correspond to the (implicit) order of values in the C++ enum.

Impact: for example, retrieving the BLOOM_FILTER_USEFUL statistic through the java API, actually returns the data for BLOCK_CACHE_FILTER_MISS.

[adamretter]

@bentorfs This looks good to me in that it fixes the issue.

However, as a more robust solution we should not rely on implicit enumeration values. For other enums that are similar in C++ and Java we have used an explicit mapping in portal.h, for an example see: https://github.com/facebook/rocksdb/blob/master/java/rocksjni/portal.h#L243. I think it would be good to do the same thing here really.

[dflemstr]

@adamretter @bentorfs It would be really great to see this merged, since it improves the current situation, and then we can talk about a nicer way of keeping things in sync later.

I'm not convinced that the status code example solves the problem because it relies on there not being anybody who tampers with the Java enum. I guess that is less likely than somebody unknowingly tampering with the C++ enum though.

[facebook-github-bot]

@bentorfs updated the pull request - view changes

[bentorfs]

Rebased to master

[sagar0]

I recently had to fix something like this in #2429. I wasn't aware of this PR while working on #2429, and got to know about this only today when it got bumped to the top due to today's comments and updates.

I do acknowledge @adamretter 's concern that there could be a more robust solution, but until someone implements it, this looks good to me. #UpForGrabs.

[facebook-github-bot]

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