Certain ConfigContexts cause NullPointerExceptions despite having documented defaults

[tjcelaya]

TL;DR: we aren't handling some configuration parameters in the execution path which have documented default values

Despite the defaults listed in README.md, certain implementations of ConfigContext result in NullPointerExceptions. Adding getters for disableNativeSignatures and noAuth to these classes would save some trouble for developers that choose to use these classes. One option is to implement getters in these classes if the behavior of null is best decided by the class itself. Most of these classes use MantaUtils.parseBooleanOrNull

Problematic Classes Classes that do what they say they do:

• MapConfigContext: uses MantaUtils.parseBooleanOrNull which returns null when passed null
• EnvVarConfigContext: uses MantaUtils.parseBooleanOrNull as well
• StandardConfigContext: doesn't implement anything, which is probably as intended and implies the null-handling for default behavior belongs outside these classes
• ChainedConfigContext: abstracts over arrays of multiple ConfigContexts and probably shouldn't be concerned with this kind of thing

Alternatively, we could modify the lines that cause the NullPointerExceptions codepath to check handle nulls explicitly, specifically the following two lines

• https://github.com/joyent/java-manta/blob/master/java-manta-client/src/main/java/com/joyent/manta/client/MantaClient.java#L207
• https://github.com/joyent/java-manta/blob/master/java-manta-client/src/main/java/com/joyent/manta/http/MantaConnectionFactory.java#L171 Interestingly enough line 165 in that same file does explicit null checking for useNativeCodeToSign

Code used to verify which classes throw and which don't:

https://gist.github.com/tjcelaya/df63bfe932afff07de4f501d6daff241

[dekobon]

The fundamental problem is that we don't have a stable value for UNSET. I could see us refactor to a wrapping object that would allow for that information to be stored, but that would be a fair bit of work.