readFile it's a bit of an interesting beast that probably tries to do too many things all together.
As far as I know, due to the name, what it does and how it's used, its purpose it's to read an entire file.
The special file (normally a device) is a special case, and to be fair although it permits partial reads, I think it shouldn't.

Currently as far as I can see is used 90% or more to read an entire file and then there are a few cases where it chooses if it's synchronous or not.

I would much prefer for it to be splitted in multiple functions with less arguments.

I'm probably not the right person to split out the function. I can at least update the function comment in the header file so it matches the behavior though instead of change the behavior though if that's preferable.

100% yes, readFile could use love and a refactor. I think we should merge this fix as the intention of this overload is to only read the requested size.

Prescient comment on the need to refactor readFile. I've tracked down a bug this change tickled in the pidfile handling code in osquery/core/system.cpp createPidFile(). I'll have a PR up shortly. If you're seeing unit test failures on Windows shutting down the osquery service, this is why.

I've opened the follow-up fix as PR6578.