As
Iride01 said, most stop with a lout "NO!" or "BACK!" Even the 80-100 pit bull with clipped ears and tail... yeah, I was shocked. He came out of the woods like a silverback - all shoulders and muscly... I yelled "BACK!" and he literally skidded and turned back. I actually felt bad for scaring the little fella... okay, not really.
For the one dog that didn't react like that and the did want to latch on to my foot, I managed a solid and direct shot in the eyes with the water bottle. It worked like you'd hope it would.
I've found the biggest danger isn't being "attacked" it's being (accidentally) tackled. Often, the dog focuses on one rider... then turns away - into another rider. Most dogs that chase us aren't mean... just being dogs.