Fix NullPointerException crash in PipeFlowFluids

[anonyco]

FIXES a bug where accesing lastSentDirection.nbtValue throws a NullPointerException when placing a wooden fluid pipe next to a Magneticraft Infinite Water block and then breaking wooden fluid pipe. To the best of my Java ability I couldn't figure out why :(

[AlexIIL]

Thanks for the bug report - sadly my lack of javadocs didn't help much, but Section.lastSentDirection is used solely as a way to avoid sending excess packets to the client - (I.E. only send a packet to the client if anything actually changed). As such it doesn't even need to be saved!

So the real fix for this is to delete these two lines: https://github.com/BuildCraft/BuildCraft/blob/8.0.x/common/buildcraft/transport/pipe/flow/PipeFlowFluids.java#L815 and https://github.com/BuildCraft/BuildCraft/blob/8.0.x/common/buildcraft/transport/pipe/flow/PipeFlowFluids.java#L826

[anonyco]

I have fixed my PR so that it now removes those two lines as you suggested.

Also, after looking at it with a fresh set of eyes, I realized the reason why it was throwing an error:

• In Java, enums are not integers as they are in C/C++, rather they are full-fledged objects
• The current (prior to my PR) declaration of Dir lastSentDirection; I believe is analogous to Dir lastSentDirection = null;, NOT Dir lastSentDirection = Dir.NONE; as it is in C/C++
• My PR initialized lastSentDirection by changing the declaration to Dir lastSentDirection = Dir.NONE;
• Currently (prior to my PR), I believe that executing https://github.com/BuildCraft/BuildCraft/blob/8.0.x/common/buildcraft/transport/pipe/flow/PipeFlowFluids.java#L524 before https://github.com/BuildCraft/BuildCraft/blob/8.0.x/common/buildcraft/transport/pipe/flow/PipeFlowFluids.java#L741 would throw a NullPointerException
• In my PR, we initialize lastSentDirection to Dir.NONE, which should proactively fix any possible future problems with a possible NullPointerException at https://github.com/BuildCraft/BuildCraft/blob/8.0.x/common/buildcraft/transport/pipe/flow/PipeFlowFluids.java#L524

Note that I work mostly in C/C++ and dream in Lisp, so please be patient as I try to comprehend the intricacies of Java lol.

Keep up the great work 👍

[AlexIIL]

You're nearly correct - with one exception: you can only get a NullPointerException when dereferencing a value with null - which only happens when you use . on a variable/field/method result containing null. Using the identity comparison operators (== or !=) on null is perfectly safe. As such lastSentDirection being null is actually always safe, since the only place it was dereferenced is when it was saved to disk - which needs to be removed. It's also completely harmless (and probably beneficial) to initialise it though - since Dir.NONE is what the client starts out with.

[AlexIIL]

Basically this looks good now, thanks!

[anonyco]

You're nearly correct - with one exception: you can only get a NullPointerException when dereferencing a value with null - which only happens when you use . on a variable/field/method result containing null. Using the identity comparison operators (== or !=) on null is perfectly safe. As such lastSentDirection being null is actually always safe, since the only place it was dereferenced is when it was saved to disk - which needs to be removed. It's also completely harmless (and probably beneficial) to initialise it though - since Dir.NONE is what the client starts out with.

Thank you for taking the time to explain this to me. I learn something new about Java every day lol.