Its possible this is a doc change to sql.Scanner.Scan to reflect the expectations of sql.Scanner.Scan; it would be slightly nicer to copy src byte slices before passing them to sql.Scanner.Scan to avoid such surprises.

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

go version go1.10 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

package proto_test

import (
	"database/sql"
	"os"
	"reflect"
	"testing"

	_ "github.com/lib/pq"
)

func (w wrapper) Scan(src interface{}) error {
	srcBytes := src.([]byte)
	*w.dst = srcBytes
	return nil
}

type wrapper struct {
	dst *[]byte
}

func TestWrapper(t *testing.T) {
	dsn, ok := os.LookupEnv("DATABASE_URL")
	if !ok {
		t.Skip("no DATABASE_URL")
	}

	db, err := sql.Open("postgres", dsn)
	if err != nil {
		t.Fatal("database error", err)
	}

	var want = []byte{0x41, 0x42, 0x43}
	var got []byte

	var wrapped = wrapper{dst: &got}
	if err := db.QueryRow(`select $1::bytea`, &want).Scan(wrapped); err != nil {
		t.Error("select", err)
	}
	if !reflect.DeepEqual(got, want) {
		t.Errorf("got = %#v, want = %#v", got, want)
	}
}

What did you expect to see?

I expected the test to pass; from the docs on sql.Scanner, I assumed I could just assign the source to my destination. Digging a bit deeper, it looks like convertAssign makes a deep copy of dest via copyBytes when src is a primitive; but it doesn't do so when using the Scanner interface:

	if scanner, ok := dest.(Scanner); ok {
		return scanner.Scan(src)
	}

https://golang.org/src/database/sql/convert.go#L340

My guess is this violates the expectations of Rows.Scan`:

If a dest argument has type *[]byte, Scan saves in that argument a copy of the corresponding data. The copy is owned by the caller and can be modified and held indefinitely. The copy can be avoided by using an argument of type *RawBytes instead; see the documentation for RawBytes for restrictions on its use.

What did you see instead?

=== RUN   TestWrapper
--- FAIL: TestWrapper (0.01s)
	proto_test.go:41: got = []byte{0x20, 0x31, 0x0}, want = []byte{0x41, 0x42, 0x43}
FAIL