[SQLite3] Add length function

[avishmehta68710]

Add support for length built in aggregate function in sqlite3

[SuriZhang]

Please update the PR with a bit more details in your description to let reviewers know what are the changes in this PR also follow the contributing doc on requirements of commit history https://github.com/sqlancer/sqlancer/blob/master/CONTRIBUTING.md#commit-history

[mrigger]

Thanks for the PR! The apply implementation is used only in PQS. I'm not sure if the Java string length maps directly to the SQLite one (also considering the different character encodings). Perhaps you could just return null (see

sqlancer/src/sqlancer/sqlite3/ast/SQLite3Function.java

Line 69 in b19a41e

return null;

), so that a LENGTH function is used by NoREC and TLP, but not by PQS?

[mrigger]

I just noticed that we actually have a separate enum with functions for non-PQS implementations, which already includes LENGTH (see

sqlancer/src/sqlancer/sqlite3/gen/SQLite3ExpressionGenerator.java

Line 420 in 5533c4e

LENGTH("LENGTH", 1), //

). Thus, I'm closing this PR. Thanks for spending time on this, and perhaps you can still identify some other functions not yet generated functions in that enum.