Go to a pet store and get citronella spray it is harmless and will stop the dog in his tracks. I have it when I walk. my dog.
many years ago when I was a young man I was chased down by a Newfoundland and gave up put the bike between me and him we then proceed to do a keystone cops routine running around the bike.
The dog got bored with this reared up on his hind legs and put his fore paws on to my shoulders... I thought I was done for he proceeded to lick my face until his owner call him back then he trotted away.