Add URL checks to Doctor (#5760)

Merge pull request 5760

lib/jekyll/commands/doctor.rb

@@ -1,3 +1,5 @@
+require "addressable/uri"
+
 module Jekyll
   module Commands
     class Doctor < Command
@@ -36,6 +38,7 @@ def healthy?(site)
             !deprecated_relative_permalinks(site),
             !conflicting_urls(site),
             !urls_only_differ_by_case(site),
+            proper_site_url?(site),
           ].all?
         end
 
@@ -91,6 +94,15 @@ def urls_only_differ_by_case(site)
           urls_only_differ_by_case
         end
 
+        def proper_site_url?(site)
+          url = site.config["url"]
+          [
+            url_exists?(url),
+            url_valid?(url),
+            url_absolute(url),
+          ].all?
+        end
+
         private
         def collect_urls(urls, things, destination)
           things.each do |thing|
@@ -110,6 +122,29 @@ def case_insensitive_urls(things, destination)
             (memo[dest.downcase] ||= []) << dest
           end
         end
+
+        def url_exists?(url)
+          return true unless url.nil? || url.empty?
+          Jekyll.logger.warn "Warning:", "You didn't set an URL in the config file, "\
+              "you may encounter problems with some plugins."
+          false
+        end
+
+        def url_valid?(url)
+          Addressable::URI.parse(url)
+          true
+        rescue
+          Jekyll.logger.warn "Warning:", "The site URL does not seem to be valid, "\
+              "check the value of `url` in your config file."
+          false
+        end
+
+        def url_absolute(url)
+          return true if Addressable::URI.parse(url).absolute?
+          Jekyll.logger.warn "Warning:", "Your site URL does not seem to be absolute, "\
+              "check the value of `url` in your config file."
+          false
+        end
       end
     end
   end