Export connection error information to client

[sjpotter]

when one is using hiredis (at least in async mode), it is very difficult (if not impossible) to understand why a connection failed, especially if one is using TLS.

i.e. the onConnect handler is just called with a NULL reply object. as it is NULL, you know it failed, but have no idea why. For plain connections, it might not matter, but when TLS is involved, you will want to know if it was a TLS issue.

while openssl allows you to query it for its last error, in an async environment, you are no longer guaranteed to get the correct result associated to the right connection.

[michael-grunder]

I need to dig into this to see what could be done about it.

It's been a while since I've used the async API but can you query the error state through the attached redisContext structure (inside of the redisAsyncContext), or has that been clobbered by the time you are in your onConnect handler?

One solution could be to allow users to create a new onConnect callback that hiredis could pass a context structure with TCP/TLS error information.

[sjpotter]

need to think about this, not so much in my current memory context. @yossigo might have ideas as well.