Pesachim page 110A speaks of giving mourners wine, a practice we saw also on page 65A of Eruvin. Nowadays it is assur for a mourner to drink wine. When did the modern practice develop?
In Kesuvos 8b The Gemara describes how the custom of 10 cups was increased with the addition of 4 extra cups and this eventually lead to drunkeness. The Chachamim then restored the original practice. Rashi explains that the original practice means the 10 cups, but the Ramban says that they did away with the 10 cups as well and they left no obligation of drinking at all in a Beis Avel.
The Aruch haShulchan (378 #10) writes that we no longer have the custom of the 10 cups described in our Gemora and there is no obligation on an Avel to drink wine. He ends with the words "u'Lema'et Adif," that is, it is preferable for an Avel to minimize wine drinking. The Shulchan Aruch in Y.D. 378 #8 rules that an Avel may drink a little wine in order to accompany the food he is eating. and I have not been able to find any authority who forbids an Avel to drink wine.
In Gesher Hachayim (vol 2 chap 16) Rav Tukechinsky asks why an Avel should be permitted to drink wine which is normally associated with Simcha. He explains that wine does not create Simcha, rather it alleviates anxiety and allows a person to overcome his worries. After drinking wine a person is able to achieve Simchah. Therefore Chazal permitted an Avel to drink wine as drinking itself is not an act of Simchah.