Sabre gel. I have a couple, one keychain size, another larger. Yes, it works.
A couple of years ago I had to hose a large dog that appeared to be rabid and was definitely aggressive. It was in the middle of the street eating roadkill, ignoring traffic whizzing by. From about 30 yards away it saw me walking and charged toward me, loping sideways and off balance, snarling and definitely not fooling around. I had worried a bit that the gel might be hard to aim but the stream hit it right in the schnozz. The dog immediately turned around and ran off. Poor thing was clipped by a passing car, but got up and continued running.
I encounter chasing dogs often on my rides and runs, but I know most of those dogs and am accustomed to their behaviors. They're just "playing," doing that territorial thing many yard dogs do -- they stop as soon as I pass their personal boundaries. While I wouldn't dismiss the potential danger, I haven't felt it necessary to spray those dogs.
I have considered spraying those dogs too, in order to discourage them from chasing. Someone could be seriously injured by a dog that didn't intend any harm. The problem is that even "good" dogs can become vicious in an instant when they sense fear. I've seen it many times, including sibling dogs attacking and mutilating a sibling dog that was injured. Any dog is potentially dangerous if they sense fear, or if the person is knocked down and bleeds. But I'd rather try to at least talk with the owners first.
The only tricky bit is choosing a canister that you'll actually carry and can reach. For bike rides that pretty much rules out large cans of "bear spray." There are smaller canisters that we can clip onto a wrist, waistband, jersey pocket, wear around our neck, etc. If you get a neck band choose one that can snap under reasonable pressure, and be very careful wearing it for mountain biking, gravel, or any locations with overhanging tree limbs, etc., that might snag the neck leash. I use one with a large plastic clip for my jersey, and another smaller version with a carabiner that's secure but easy to operate with one hand.