Gemara

27a - R. Yochanan: We learn from a Gezerah Shavah "Chamishah Asar- Chamishah Asar" from Pesach. Just like one must eat Matzah the first night, and afterwards it is optional, the same applies to eating in the Sukah.

28b - Mishnah: If rain came, one may leave the Sukah once enough descends to ruin the porridge. This is like a servant who came to mix a cup for his master, and the master poured the flask on his face.

29a - Beraisa: The Mishnah refers to a porridge of grits.

Beraisa: If one was eating in the Sukah and descended (to the house) when rain came, he need not return until finishing his meal.

Rishonim

The Rif and Rosh (2:18,19) bring the Mishnah and Beraisa.

Ran (DH Masnisin): 'Porridge' is a food that is neither thin nor thick. It seems that one must wait for this Shi'ur only if he already started to eat, for when he leaves it looks like he rejects the Sukah. If he did not yet begin to eat, if there is enough rain to pain him he is exempt, just like one is exempt due to flies or odors. The Beraisa teaches that even if one began the Mitzvah and left due to rain he may finish eating outside. All the more so, one who began to eat outside need not enter the Sukah if the rain ceases. This is my reasoning; l'Ma'aseh, I would not rely on it without a solid proof.

Hagahos Ashri: One may leave when enough rain descends to ruin porridge, even if there is no porridge in the Sukah. The Yerushalmi says that one may similarly leave due to great heat or flies.

Rambam (Hilchos Sukah 6:10): If rain came, one may leave the Sukah if enough drops descend to ruin a Tavshil, even of beans. If one was eating in the Sukah and entered his house when rain came, he need not return until finishing his meal.

Rosh (Berachos 7:23): One must eat in the Sukah on the first night even if he would normally be exempt from Sukah due to rain.

Poskim

Shulchan Aruch (OC 639:5): If rain came, one may leave the Sukah if enough drops descends to ruin a Tavshil, even of beans.

Mishnah Berurah (33): If the rain is not falling in part of the Sukah, he should eat there. Perhaps he can even bless. If a finicky person is bothered by this he may leave. If he stays he should not bless Leshev ba'Sukah.

Kaf ha'Chayim (72): The Shulchan Aruch holds like Rashi, that porridge spoils easily. Some say that it does not spoil easily. If one is stringent like the latter opinion to eat in the Sukah when the rain ruins only foods that are easily ruined, he should not bless.

Rema: This is even if there is no Tavshil in the Sukah.

Tur: The Mishnah says 'from when the porridge is ruined', not 'enough to ruin porridge.' This shows that it discusses when there is porridge in the Sukah.

Rejection #1 (Beis Yosef DH u'Mah she'Chosav Rabeinu d'Ein): Surely, the Mishnah would not neglect to teach when one may leave if there is no porridge inside! Sefer ha'Mitzvos, the Rambam and Hagahos Ashri say that the Shi'ur applies even if there is no porridge in the Sukah.

Rejection #2 (Taz 15): A Beraisa teaches that the Mishnah refers to porridge of grits. According to the Tur, we gauge according to whatever food is in the Sukah at the time! The Mishnah says 'from when the porridge is ruined' to teach that it discusses when one is eating. A smaller Shi'ur of rain permits leaving at the time of sleeping.

Rema: One who is unsure of how much rain this is should estimate if he would leave his house if so much rain came in. If so, he may leave the Sukah. This refers to after the first night. On the first night one must eat in the Sukah even if it is raining.

Rashba (Teshuvah 4:78, brought in Beis Yosef DH veha'Ran) One is exempt from Sukah even on the first night if it is raining or he is in pain. If one were obligated in any case, the Gemara would have said so! I never saw or heard of anyone who is stringent about this.

Terumas ha'Deshen (2:160, brought in Beis Yosef DH uv'Kisvei): The Rosh and SMaK obligate eating in the Sukah on the first night even if it is raining. Tosfos permits to eat in the house on the first night due to rain. If the rain stops he must eat a k'Zayis in the Sukah. The Or Zaru'a and SMaG exempt if it is raining. One may rely on them regarding Yom Tov Sheni, for Rabeinu Yehudah says that now that the calendar is fixed it is not a Safek mid'Oraisa. This is unlike the Rosh (3:3), who says that all stringencies of Yom Tov Rishon apply to Yom Tov Sheni, because it is a Safek.

Kaf ha'Chayim (47): Some obligate eating a k'Zayis in the rain also on the second night, because Chachamim enacted like mid'Oraisa.

Rema (ibid.): He makes Kidush in the Sukah in order to say Shehecheyanu on the Sukah.

Question (Taz 17): On the first night, the Rema obligates eating in the Sukah even if it is raining. On the second night he relies on the Or Zaru'a and SMaG who exempt if it is raining. If so, he should be concerned for their opinion on the first night not to bless Leshev ba'Sukah in the rain! ('He makes Kidush...to say Shehecheyanu on the Sukah' implies that he says Leshev ba'Sukah.)

Answer (Taz 17): Rather, the Rema holds like the Rosh and SMaK, who obligate on the first night even if it is raining. He also obligates eating a k'Zayis in the Sukah on the second night, even if it is raining. However, on the second night one does not bless Leshev ba'Sukah in the rain. The Terumas ha'Deshen (1:95) says that on the second night one eats his fill in the house, and a k'Zayis at the end in the Sukah.

Mishnah Berurah (35): The Rema holds that the Gezeirah Shavah obligates Sukah on the first night even if one is in pain. Many exempt one who is in pain. One should be stringent to eat in the Sukah in such a case, but not to bless. Some say that one should wait until midnight; perhaps the rain will clear and he can eat in the Sukah and fulfill the Mitzvah according to everyone. Others disagree, for this is the ultimate Bitul of Simchas Yom Tov for the entire family. Some say that one should wait an hour or two, and if it is still raining to make Kidush in the Sukah like the Rema says.

Kaf ha'Chayim (73,74): One should wait until it becomes painful for him. If he invited poor guests he should not wait, for they need not fulfill all the opinions, and perhaps he transgresses Bal Te'acher (delaying to give Tzedakah).

Gra (DH va'Ykadesh): Even though one could begin outside the Sukah and eat a k'Zayis in the Sukah at the end, this would cause saying Shehecheyanu an extra time, for the Sukah. It is forbidden to cause making extra Berachos.

Mishnah Berurah (36): All agree that one is Yotzei Shehecheyanu on the Sukah if he says it there. Even if he is exempt at the time because he is in pain, this is no worse than blessing when making the Sukah before Yom Tov, which is proper (just our custom is to wait until Kidush on Yom Tov - OC 641:1).

Kaf ha'Chayim (79,80): When making Kidush and ha'Motzi he should have in mind to finish the meal in his house.

Mishnah Berurah (ibid.): If the rain stopped after one said Birkas ha'Mazon, he must now eat more than a k'Beitzah in the Sukah to fulfill the opinion that he was not Yotzei by eating in the rain. However, if he already lied down to sleep he is exempt, for it is a pain to get up, and if this night one is obligated even in pain, he was already Yotzei. On the second night he need not wait, for he was already Yotzei Shehecheyanu on the Sukah the first night. Nevertheless, it is good to wait a little, in case the rain will stop.

Shulchan Aruch (6): If one was eating in the Sukah and entered his house when rain came and then it stopped, he need not return until finishing his meal.

Mishnah Berurah (38): This is only if the rain stopped after he sat to eat in the house. If it stopped beforehand, he must return to the Sukah.