Fix a data race in EnsureImageExistsCallbackIncludesSourceURL
Fix a data race in EnsureImageExistsCallbackIncludesSourceURL

gabriel-samfira/juju,dooferlad/juju,AdamIsrael/juju,axw/juju,macgreagoir/juju,AdamIsrael/juju,bac/juju,mikemccracken/juju,ericsnowcurrently/juju,makyo/juju,bac/juju,macgreagoir/juju,perrito666/juju,reedobrien/juju,macgreagoir/juju,axw/juju,howbazaar/juju,anastasiamac/juju,howbazaar/juju,marcmolla/juju,alesstimec/juju,bac/juju,frankban/juju,makyo/juju,dimitern/juju,perrito666/juju,perrito666/juju,frankban/juju,makyo/juju,mjs/juju,gabriel-samfira/juju,mikemccracken/juju,anastasiamac/juju,perrito666/juju,kat-co/juju,marcmolla/juju,waigani/juju,bz2/juju,bz2/juju,howbazaar/juju,dooferlad/juju,mjs/juju,reedobrien/juju,gabriel-samfira/juju,reedobrien/juju,dimitern/juju,waigani/juju,waigani/juju,frankban/juju,AdamIsrael/juju,kat-co/juju,bz2/juju,macgreagoir/juju,anastasiamac/juju,bogdanteleaga/juju,ericsnowcurrently/juju,marcmolla/juju,mjs/juju,marcmolla/juju,kat-co/juju,waigani/juju,mjs/juju,bogdanteleaga/juju,macgreagoir/juju,mikemccracken/juju,axw/juju,frankban/juju,dooferlad/juju,gabriel-samfira/juju,mikemccracken/juju,howbazaar/juju,marcmolla/juju,howbazaar/juju,bogdanteleaga/juju,alesstimec/juju,axw/juju,mjs/juju,ericsnowcurrently/juju,gabriel-samfira/juju,anastasiamac/juju,alesstimec/juju,kat-co/juju,makyo/juju,bac/juju,AdamIsrael/juju,bogdanteleaga/juju,ericsnowcurrently/juju,reedobrien/juju,dimitern/juju,dimitern/juju,alesstimec/juju,frankban/juju,reedobrien/juju,perrito666/juju,axw/juju,ericsnowcurrently/juju,bz2/juju,bac/juju,kat-co/juju,anastasiamac/juju,bz2/juju,mikemccracken/juju,alesstimec/juju,mjs/juju,dooferlad/juju,dooferlad/juju,dimitern/juju,mjs/juju