NIFI-4944: Guard against race condition in Snappy for PutHiveStreaming

[mattyb149]

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[MikeThomsen]

+1 LGTM. Unit tests ran and the solution looks like a sensible and effective one.

[mcgilman]

@mattyb149 Previously the loading of Snappy happened during onTrigger. This could lead to the race condition described in the JIRA/PR. Moving this to a static initializer should alleviate the issue. Additionally, the act of service discovery (Java SPI) will load the class. Because of this, I actually don't believe the synchronization here is necessary. Thoughts?

[moonkev]

Hi

Seeing this same issue with the PutHDFS processor and came across this. Curious with this solution, if the same condition will arise? Won't the static initializer here be called in each instance class loader which makes use of this processor? I was reading here and it seems that each processor itself will be loaded again, thus causing snappy's static initializer to be called, which in turn will again attempt to load its native library.

Thanks
Kevin

[mcgilman]

@moonkev I think there a couple different scenario's here. This is admittedly pretty complicated, but my understanding is as follows:

• In a case like PutHDFS, snappy compression is supported through the native hadoop library. In this case, the native snappy library is not loaded. Issues seen here are due to multiple classloaders attempting to load "hadoop" native library.
• In a case like CompressContent, snappy support is supplied by the native snappy library loaded indirectly through snappy-java. In more recent version, they effectively bypass the native loading issues by copying the native library to a temp directory and loading it with a UUID appended to the name. This allows the native library to be loaded multiple times (since the name is different each time).
• In a case like PutHiveStreaming, snappy support is also supplied through snappy-java. The difference is that its an old version of snappy-java that does not perform the UUID trick. Force upgrading in this case is tricky since it is a transitive dependency (PutHiveStreaming -> Hive -> avro -> snappy). The legacy snappy-java addresses the native library issue by directly injecting the class that loads the native library into the root classloader. The ultimate issue here was that multiple processors were attempting to do this loading at the same time (due to the loading happening in onTrigger).

@mattyb149 Please chime in here if any of my understanding isn't correct.

Hope this helps

[mcgilman]

Thanks @mattyb149! This has been merged to master.

[moonkev]

I was aware that PutHiveStreaming used snappy-java vs native snappy, but was unaware of the methods that snappy-java protect against loading in multiple class loaders. Many thanks for the detailed explanation @mcgilman!