[2018.3] Fixes to verify_login in mysql module

[garethgreenaway]

What does this PR do?

Ensure that verify_login is using the host from the connection_args and not the host associated with the user. Adding a test to ensure user_exists when the passed host is the MySQL wildcard %.

What issues does this PR fix or reference?

#50542

Previous Behavior

When user_exists was called with a wildcard hostname, %, then verify login would fail because it was trying to connect to that as a real hostname.

New Behavior

We should ensure we're using the hostname from connection_args when verifying the login.

Tests written?

Yes

Commits signed with GPG?

Yes

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[ghost]

Thanks for taking a look at and fixing my issue..

However, I think there is a problem with the approach here for checking if a user exists. The hostname passed into the user_exists call is supposed to be the client host not the server host. Mysql users are specified by client host so "user1@host1" is different from "user1@host2". Using verify_login to check if "user1@host1" exists won't work because the verify method may be running for a host other than where "user1" intends to connect from. I believe this is why the original code was using a query against the INFORMATION_SCHEMA to determine existence.

[garethgreenaway]

@doug-stratoscale the user_exists function is still checking the user table that both the user and host are valid, the host parameter is not used to determine which MySQL host to connect to, that is found in connection_args. The verify_login function is checking that the password is valid for MySQL versions higher than 8.0.11 when the password hashing mechanism changed and can no longer use the PASSWORD function from MySQL.

[ghost]

@garethgreenaway Thanks for the clarification - I looked at the code again ( :) ) and it makes sense.

[garethgreenaway]

@doug-stratoscale No worries! Appreciate the second pair of eyes 😄