Remove and discard any string from the pork roast, and trim excess fat.

Combine chili powder, oil, garlic, black pepper, cumin, coriander, paprika, allspice, and salt in a bowl; rub the mixture all over the roast and work it into the pork. Marinate in the refrigerator for at least 30 minutes, or up to overnight.

Transfer the roast to a slow cooker. Combine ketchup, vinegar, molasses, and mustard; pour the mixture over the roast. Cover the slow cooker.

Cook on Low heat until the pork is very tender, about 8 to 10 hours.

Transfer the roast to a large bowl, and discard any visible fat. Use two forks to separate the pork into long strands, then return the shredded pork to the slow cooker and stir well to combine with the juices.

Strain 1 ½ cups of the cooking liquid into a saucepan. Whisk in cornstarch and bring to a boil. Cook, stirring, until thickened and bubbling.