fix(gatsby-plugin-offline): delay adding resources for paths until we have urls

[vtenfys]

This adds a new API called onPostPrefetchPathname, which runs after resources have been fetched for a pathname, as opposed to onPrefetchPathname which runs directly upon fetching resources.

In plugin-offline, the existing API onPrefetchPathname was being used - this was causing problems, since the resources weren't always fetched in time for getResourceURLsForPathname to return the data (see https://github.com/gatsbyjs/gatsby/pull/8613/files#diff-450f89b4ad11867864b3d8d21f83e442R35 - getResourceURLsForPathname sometimes returned null).

Now that the API onPostPrefetchPathname is used instead, getResourceURLsForPathname will always work properly, since the API runs after the resources are fetched and only if they are fetched successfully.

In addition to this, I've passed onPostPrefetchPathname into the API runner for onPrefetchPathname so that plugins which use custom prefetching can run this API at the right time.

[vtenfys]

AppVeyor why...

[KyleAMathews]

This is called when the "prefetching" has started not when the resources has actually been downloaded.

Can you explain more about the error plugin-offline is causing?

[vtenfys]

@KyleAMathews We need to guarantee that getResourceURLsForPathname will actually work e.g. in gatsby-plugin-offline (see link in updated description) - currently it's sometimes returning null, since the URLs have not been populated in time, or it returns null if the pathname is a 404 (e.g. integration-tests/production-runtime for "/page-3").

With the new API, it's called at a time when getResourceURLsForPathname is guaranteed to work, so there's no longer an error like "Cannot call forEach of null` in plugin-offline (again at the link in the updated description).

[pieh]

Could we refactor this a bit so loader.js exposes prefetchResourcesForPath function and guess plugin and default gatsby prefetching use same implementation (and onPostPrefetchPathname api is called just from loader.js) ?

[pieh]

Thanks @davidbailey00!