Make ExchangeWithConnContext public

If we want to use a custom mechanism of obtaining a Conn that doesn't
match the net.Dialer type, but retain the timeout behaviour from
ExchangeContext, there was previously no way to accomplish this.

This PR makes the underlying ExchangeWithConnContext function public,
which allows this behaviour.

Now that miekg#1454 is merged, there is no longer any interaction between the
context provided and the singleflight behaviour, so I removed the
comment from ExchangeWithConn.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

client.go

@@ -183,14 +183,13 @@ func (c *Client) Exchange(m *Msg, address string) (r *Msg, rtt time.Duration, er
 // This allows users of the library to implement their own connection management,
 // as opposed to Exchange, which will always use new connections and incur the added overhead
 // that entails when using "tcp" and especially "tcp-tls" clients.
-//
-// When the singleflight is set for this client the context is _not_ forwarded to the (shared) exchange, to
-// prevent one cancellation from canceling all outstanding requests.
 func (c *Client) ExchangeWithConn(m *Msg, conn *Conn) (r *Msg, rtt time.Duration, err error) {
-	return c.exchangeWithConnContext(context.Background(), m, conn)
+	return c.ExchangeWithConnContext(context.Background(), m, conn)
 }
 
-func (c *Client) exchangeWithConnContext(ctx context.Context, m *Msg, co *Conn) (r *Msg, rtt time.Duration, err error) {
+// ExchangeWithConnContext has the same behaviour as ExchangeWithConn and
+// additionally obeys deadlines from the passed Context.
+func (c *Client) ExchangeWithConnContext(ctx context.Context, m *Msg, co *Conn) (r *Msg, rtt time.Duration, err error) {
 	opt := m.IsEdns0()
 	// If EDNS0 is used use that for size.
 	if opt != nil && opt.UDPSize() >= MinMsgSize {
@@ -460,5 +459,5 @@ func (c *Client) ExchangeContext(ctx context.Context, m *Msg, a string) (r *Msg,
 	}
 	defer conn.Close()
 
-	return c.exchangeWithConnContext(ctx, m, conn)
+	return c.ExchangeWithConnContext(ctx, m, conn)
 }