Fix some link previews being incorrectly generated from other prior links (#16885)

* Add tests

* Fix some link previews being incorrectly generated from different prior links

PR #12403 added a cache to avoid redundant queries when the OEmbed endpoint can
be guessed from the URL. This caching mechanism is not perfectly correct as
there is no guarantee that all pages from a given domain share the same
OEmbed provider endpoint.

This PR prevents the FetchOEmbedService from caching OEmbed endpoint that
cannot be generalized by replacing a fully-qualified URL from the endpoint's
parameters, greatly reducing the number of incorrect cached generalizations.

1 files changed, 4 insertions, 1 deletions

diff --git a/app/services/fetch_oembed_service.rb b/app/services/fetch_oembed_service.rb
index 60be9b9dc..4cbaa04c6 100644
--- a/app/services/fetch_oembed_service.rb
+++ b/app/services/fetch_oembed_service.rb
@@ -2,6 +2,7 @@
 
 class FetchOEmbedService
   ENDPOINT_CACHE_EXPIRES_IN = 24.hours.freeze
+  URL_REGEX                 = /(=(http[s]?(%3A|:)(\/\/|%2F%2F)))([^&]*)/i.freeze
 
   attr_reader :url, :options, :format, :endpoint_url
 
@@ -65,10 +66,12 @@ class FetchOEmbedService
   end
 
   def cache_endpoint!
+    return unless URL_REGEX.match?(@endpoint_url)
+
     url_domain = Addressable::URI.parse(@url).normalized_host
 
     endpoint_hash = {
-      endpoint: @endpoint_url.gsub(/(=(http[s]?(%3A|:)(\/\/|%2F%2F)))([^&]*)/i, '={url}'),
+      endpoint: @endpoint_url.gsub(URL_REGEX, '={url}'),
       format: @format,
     }