Gemara

Rav Avya was weak. He missed Rav Yosef's class (on Shabbos morning).

Abaye (to Rav Avya): Why didn't you come to Rav Yosef's class?

Rav Avya: I was weak.

Abaye: You should have eaten something beforehand!

Rav Avya: Rav Huna forbids eating before praying Musaf!

Abaye: You should have prayed Musaf alone, and then eaten something!

The Halachah does not follow Rav Huna, nor R. Yehoshua ben Levi;

(R. Yehoshua ben Levi): Once the time for Minchah comes, one may not eat before praying Minchah.

Ta'anis (26b): We have Birkas Kohanim at Shacharis and Musaf because it is not common to be intoxicated then.

Rishonim

Rosh (4:11): The Halachah does not follow Rav Huna or R. Yehoshua ben Levi. One may eat even after the time for Minchah came. R. Tam says that the Mishnah (Shabbos 9b) forbids only a big meal before Minchah Gedolah, but one may begin a small meal before Minchah Gedolah, and even before Minchah Ketanah.

Rosh (Ta'anis 4:31): R. Chananel learns from our Gemara here that (it is not common to be intoxicated at Musaf because) one may not eat before Musaf.

Rashba (28b DH v'Leis): The Halachah does not follow Rav Huna, i.e. one may taste before Musaf. The Ra'avad says that only tasting is permitted, but not eating. One may taste even enough to sustain the heart, like the case of Rav Avya.

R. Yonah (19a): Only tasting is permitted before Minchah, e.g. eating Peros, or a small amount of bread, even after the time for Minchah Ketanah, One may not eat a meal. The Rif (Shabbos 4a) rules like the stringent version that forbids even a small meal from the time of Minchah Gedolah, until he prays Minchah.

Poskim

Shulchan Aruch (OC 286:3): One may taste before praying Minchah, i.e. eating Peros, or a small amount of bread enough to sustain the heart. One may not eat a meal.

Tur: Even though the Halachah does not follow Rav Huna, who forbids tasting before Musaf, one must pray before eating. We permit Birkas Kohanim at Shacharis and Musaf because it is not common to be intoxicated then. This shows that it is not normal to eat beforehand. R. Chananel says 'we learn from here that one may not eat before Musaf.'

Beis Yosef (DH Af): The Rashba says that the Ra'avad permits only tasting before praying Minchah, but not a meal. R. Yonah says so about eating before Minchah. It seems that the same applies to eating before praying Musaf, like the Ra'avad. The Tur connotes like this. Do not explain that tasting and eating are permitted, and it is merely not common to eat before Musaf. He brings from R. Chananel that one may not eat before Musaf!

Rebuttal (Bach 2): The Tur holds that l'Chatchilah one may eat before Musaf, and all the more so if he began, he need not stop. However, l'Chatchilah one must pray before eating, even if he is weak, like the case of Rav Avya. Letter of the law, it is permitted. He brings a support from R. Chananel. R. Chananel did not say 'it is forbidden to eat before Musaf', rather, 'one is not Reshai (allowed).' I.e. since it is not normal to do so, one may not do so, due to "Al Titosh Toras Imecha." It is not common to be intoxicated then. Even though some people eat then, since letter of the law it is permitted, most do not transgress the custom of all Yisrael, therefore it is not common.

Bach (DH v'Af): R. Yehoshua ben Levi forbids tasting before Minchah, and the Gemara ruled unlike Rav Huna and R. Yehoshua ben Levi. The Tur forbids eating before Minchah. This is like Tosfos and the Rosh in Shabbos, that the Halachah follows R. Yehoshua ben Levi in one of two matters (eating, but not regarding tasting), and unlike Tosfos and the Rosh in Shabbos, who rule totally unlike R. Yehoshua ben Levi, and permit even eating. However, surely the Halachah does not follow Rav Huna at all. There is no source to rule like him even regarding eating. R. Yonah, Tosfos and the Rosh agree. Letter of the law, the Halachah permits eating before Musaf. (Even though the custom is not to eat), this affects one who is weak, and it is very hard to pray Musaf before eating. He may eat first. In such a case, one may be lenient against the custom. This is unlike the Beis Yosef. Tosfos and the Rosh in Berachos hold that the Gemara in Shabbos is not conclusive about eating before Minchah, so they rule totally unlike R. Yehoshua ben Levi and permit even eating. Likewise, the Halachah does not follow Rav Huna at all.

Taz (2): The Beis Yosef is correct. There is a proper proof from Ta'anis. Really, no proof is needed. Surely the argument is only about tasting, like Rav Huna's words connote. Heaven forbid to be lenient to eat a meal before Musaf!

Gra (DH Achal): We must say that one cannot be drunk at Musaf at all (for it is forbidden to eat beforehand, like the Beis Yosef), since also before Minchah it is not common (yet we forbid Birkas Kohanim at Minchah and permit before Musaf.)

Kaf ha'Chayim (31): It seems that the Magen Avraham agrees with the Bach. The Taz and many disagree. Sha'agas Aryeh says that even if he was eating a small meal, he must stop, lest he be negligent (and not pray in time). It seems that if he is weak, and he cannot properly with Kavanah unless he eats more than k'Beitzah, he may be lenient.

Beis Yosef (ibid.): R. Yerucham says 'one may not taste, i.e. eat, before Musaf, like it says in Berachos.' This is astounding! Perhaps he means that one may not taste a lot, i.e. eating, like the Ra'avad says.

Eliyahu Rabah (9): R. Yerucham says that there is no Isur to taste, i.e. eat, before Musaf. The Beis Yosef missed the word 'no'. We discuss one who made Kidush beforehand. He drank a Revi'is of wine, or ate a k'Zayis of bread. We permit even k'Beitzah. In pressed circumstances if one has only Peros, but not wine or bread (i.e. at night, when one may say Kidush on bread - PF), and he is weak, he may taste without Kidush. One may rely on the Ra'avad (Hilchos Shabbos 29:10) who permits tasting before Kidush. Also, (here) he may rely on Ateres Tzvi (289:3), who says that the Chiyuv Kidush is only after Musaf, even though he permits only water before Shacharis.

Bach (DH u'Mah she'Yesh): Some ask how one may eat before Musaf on Shabbos and Yom Tov. One may not even taste before Kidush! I ask, how do they (who ask) understand the case of Rav Avya? Rather, one says Kidush, eats, blesses Birkas ha'Mazon and then prays Musaf. There is no question at all.

Magen Avraham (1): Even though one may not eat before Kidush, and Kidush must be where one has a meal, it suffices to drink a cup of wine after (Kidush. This is considered a meal.) Alternatively, one tastes the Peros, prays there and then eats bread. Do not say that there is no Chiyuv to make Kidush yet (so the Kidush is invalid). Since he may eat a little bread, Kidush applies. The Bach connotes like this. He says that it is merely a custom not to eat a meal before Musaf. The Rosh connotes that it is no less than Minchah.

Machatzis ha'Shekel (DH uv'Rosh): I.e. just like one may not eat before Minchah, one may not eat before Musaf.

Question (R. Akiva Eiger): The Magen Avraham (232:17) forbids drinking more than k'Beitzah before Minchah. If so, the same applies before Musaf!

Mishnah Berurah (7): The Chiyuv Kidush is already after Shacharis. One may not eat or drink afterwards before Kidush. If one does not have enough wine, he may rely on the Poskim who say that it suffices to drink the entire cup of Kidush wine, which is at least a Revi'is, and this is considered a meal.

Magen Avraham (1): We can say that since one opinion in the Gemara exempts an individual from Musaf, one may eat beforehand.

Magen Avraham (2): One may even eat k'Beitzah of bread, like it says in 232:3 (regarding eating before Minchah).

Kaf ha'Chayim (26): Olas Tamid allows only less than a k'Zayis. This is unlike the Acharonim.

Mishnah Berurah (8): One may eat a k'Beitzah of bread, and all the more so (this amount of) other foods made from the five grains.

Mishnah Berurah (9): One may eat even much Peros. If one is weak, he may eat bread until his mind is settled, even if it is more than k'Beitzah. If one is weak and has no wine or anything else on which to say Kidush, he may eat Mezonos without Kidush. If not, he should not be lenient.

Kaf ha'Chayim (27): He says Kidush and eats before Musaf, and says Kidush again after Musaf to fulfill all opinions.

Kaf ha'Chayim (29): If they do a Bris Milah before Musaf, they may have a small meal, i.e. dairy, even with Birkas ha'Mazon, if they do not drink intoxicating drinks, and afterwards they will remind each other to pray Musaf. This is like an Arai (haphazard) meal.

Kaf ha'Chayim (30): The Magen Avraham (106:2) says that (on weekdays,) once women say some requests, they may eat. If so, after some requests they may not drink even water before Shacharis, for they are obligated in Kidush. I say that this is only if they intended to fulfill the Chiyuv of Tefilah through the request. If they normally pray, and requests they say beforehand are like praise, just like men who say Tehilim before Shacharis, they may drink before Shacharis, like men. Eshel Avraham says so.

Bi'ur Halachah (DH Achilas): Even a Kohen, who must give Birkas Kohanim at Musaf, may drink a Revi'is of our wine, for surely it has water in it.

Chasam Sofer (YD 7 DH Ach): On Rosh Hashanah, if one must eat before Musaf, he should eat before dawn. If this is not enough, he makes Kidush after Shacharis, before the Teki'os.

Ashrei ha'Ish (citing ha'Gaon R. Y.S. Elyashiv Ztz"l, 3:16:18,19): The custom in Yeshivos is to eat more than k'Beitzah, but one should do so on the first day only for a great need. One may eat bread only if he is sick, but not if he is just weak. On the second day, one who is weak may eat up to a k'Beitzah of bread, if needed. If Rosh Hashanah is on Shabbos he may eat more than a k'Beitzah, until his mind is settled. This is better than drinking [after Shacharis and] before Musaf without Kidush.

Teshuvos v'Hanhagos (5:179): We are just as stringent about eating when Rosh Hashanah is on Shabbos, for in Musaf one fulfills Malchuyos, Zichronos and Shofaros.

Doleh u'Mashke (p. 215): One opinion holds that one should eat (at most a Shi'ur) beforehand if the Tefilah will finish after midday.