Some SQL clients may default to a different authentication plugin than
"mysql_native_password". Since this is the only one supported by MySQL-
authenticator, the client is instructed to swap its plugin.
Due to MDEV-15556 and MDEV-15840 recursive CTEs can't be reliably used
with older 10.2 versions. To prevent problems, only use the query that
extracts composite roles with newer versions.
Allocating the session before a DCB guarantees that at no point will a DCB
have a null session. This further clarifies the concept of the session and
also allows the listener reference to be moved there.
Ideally, the session itself would allocate and assign the client DCB but
since the Listener is the only one who does it, it's acceptable for now.
More of the DCB initialization is now done in the DCB constructor. This
makes the creation of new DCBs simpler but it can be even simpler. By
passing the file descriptor that the DCB should use into the constructor
almost all of the initialization would be done inside it.
Also removed the unused path member variable.
By storing the reference in the DCB, the two-way dependency between the
listeners and services is severed. Now the services have no direct link to
listeners and after the destruction of a listener it will be freed once
all connections through it have closed.
Due to the fact that a listener itself has a DCB that must point to a
valid listener, a self-reference is stored in the listener DCB. This is
extremely confusing and is only here to keep the code functional until the
DCB part of the listener can be factored out.
The class is still mostly the same as the old C version but it now uses
std::string instead of char pointers. Changed configuration default values
so that the parameters passed to the listener allocation are always valid.
The authentication code did not initialize one of the buffers used to
calculate the password hashes. This resulted in the use of uninitialized
memory when the user provided no password.
