database/sql: *Rows.Err() does not return errors from *Rows.Scan()

[zombiezen]

I'm not really advocating for the behavior to change, but the usage of the word "iteration" in the current documentation of Rows.Err() is ambiguous with respect to the behavior of *Rows.Scan(). More examples and/or more precise documentation would help.

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

go version go1.10.2 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

https://play.golang.org/p/1xbP8gEMNtb

What did you expect to see?

(Given a certain reading of the Rows.Err() docs:)

2018/06/07 10:24:45 Error from Scan(...): sql: Scan error on column index 0: sql/driver: couldn't convert 42 into type bool
2018/06/07 10:24:45 Error from Err(): sql: Scan error on column index 0: sql/driver: couldn't convert 42 into type bool

What did you see instead?

2018/06/07 10:24:45 Error from Scan(...): sql: Scan error on column index 0: sql/driver: couldn't convert 42 into type bool
2018/06/07 10:24:45 No error from Err()

[bcmills]

CC @bradfitz @kardianos for SQL, @robpike for prose

[kardianos]

There is a change on master that may address this. Scan should return errs now if rows.lasterr == nil.
Does this help?

[kardianos]

Oh wait, you want the opposite. You would like Scan to store the scan error in lasterr if lasterr is nil so it can get reported in rows.Err().

[bcmills]

Yep. It's not obvious that the phrase “during iteration” is intended to exclude Scan, since the Scan call occurs after the start of iteration and before the end.