INTERRUPTING A MEAL WHEN SHABBOS OR YOM TOV ARRIVES [Erev Shabbos:eating:interrupting]
R. Yehudah: If one was eating and Shabbos came, he must interrupt (remove the table and say Birkas ha'Mazon);
R. Yosi: He need not interrupt.
Shmuel: The Halachah does not follow either of them. Rather, one spreads a cloth and says Kidush.
Rif and Rosh (10:2-3): The Halachah follows Shmuel, for a Beraisa supports him.
After Kidush, one must say ha'Motzi again (Rosh - Kidush is an interruption, for the coming of Shabbos forbade eating until making Kidush);
One Birkas ha'Mazon exempts what was eaten before and after Kidush.
Rambam (Hilchos Shabbos 29:12): The Halachah follows Shmuel.
Magid Mishnah: He need not repeat ha'Motzi. We learn this from the Yerushalmi in Berachos.
Rosh (10:3): The Yerushalmi says that we cover the bread at the time of Kidush lest it be 'shamed' that we make Kidush on wine, which is less important.
Shulchan Aruch (OC 271:4): Once Shabbos begins, one may not taste anything, even water, before making Kidush.
Source - Beis Yosef (DH v'Asur): Amora'im (106b) argue about whether or not one who tasted makes Kidush, but no one permits l'Chatchilah to taste before Kidush.
Magen Avraham (5): One may rinse his mouth, for he does not intend to taste the water.
Shulchan Aruch (ibid.): If one started eating on Erev Shabbos, he spreads a cloth and says Kidush.
Magen Avraham (7): The cloth is spread over the bread and after Kidush it is removed, to make it appear that the meal is for the sake of Shabbos.
Shulchan Aruch (ibid.): If one was drinking wine, he says the Berachah of Kidush (Mekadesh ha'Shabbos) but he does not repeat Borei Peri ha'Gafen.
Beis Yosef (DH u'Mihu): Perhaps the Rambam requires Borei Peri ha'Gafen, like he said in Hilchos Berachos. Nevertheless, the Yerushalmi, the Rif and the Rosh do not require it, and we are lenient regarding Safek Berachos.
Taz (4): Kidush is not a Hefsek to require another Borei Peri ha'Gafen, for at the time Borei Peri ha'Gafen is said in Kidush, there has not yet been a Hefsek (of a Berachah; "Va'Ychulu..." is not Me'akev Kidush).
Shulchan Aruch (ibid.): After Kidush he says ha'Motzi again, unless he made Kidush on bread. Some exempt from ha'Motzi in any case.
Mishnah Berurah (17): The first opinion requires repeating ha'Motzi because Kidush was an interruption. It is better not to repeat it, for it is a Safek Berachah.
Kaf ha'Chayim (21,22): One may not eat from sundown, or from when one accepts Shabbos. One may accept Shabbos and make Kidush before dark, if it is at least half an hour before dark (but after this, one must wait to say Shma after dark before starting a meal). The Ari Zal says that one should pray Ma'ariv before Kidush in order to properly receive Kedushas Shabbos.