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What version of Go are you using (go version)?

go 1.8.3

What did you do?

database/sql/driver defines ErrBadConn that a driver should return if a connection is rendered unusable. The mssql driver recently merged a pr to use this in more instances. It corrected a few intermittent issues I was having with bad connection reuse, it now gives error messages that are almost completely unhelpful.

Where I used to have things like "IO timeout" returned to the user code, I now only get "driver: bad connection". This seems unavoidable at present because ErrBadConn is defined simply as:

var ErrBadConn = errors.New("driver: bad connection")

Ideally, the driver would be able to return any error message it likes, while still being able to signal to the database/sql package to not reuse this connections.

I have a couple of ideas for this perhaps:

1. Allow drivers to return an error that implements some other interface. Perhaps interface{ IsBadConn() bool} or something. Could be exported or unexported.
2. Have a wrapper type available in the driver package. Maybe something like type BadConnError struct{ error }. Or perhaps an unexported type with a func WrapAsBadConn(err error) error to create them.

In either case, the usages of err == driver.ErrBadConn would likely be replaced with a call to driver.IsBadConn(err) or similar, which would perform the additional type checks.

Does this make any sense? Would you be likely to consider a change to this effect for go 1.10?

I don't envision it being a largely impactful change, except perhaps if someone outside the std lib is ever checking errors for equality with driver.ErrBadConn. It is a possibility I suppose.