Yum is replaced by DNF in newer Fedora releases

Check if yum is really present on a 'redhat' machine.  Only if it is,
enable the Yum bucket. The same for DNF since older versions of CentOS
and Fedora do not ship DNF.

lib/vagrant-cachier/bucket/dnf.rb

@@ -8,7 +8,7 @@ module VagrantPlugins
 
         def install
           if guest.capability?(:dnf_cache_dir)
-            guest_path = guest.capability(:dnf_cache_dir)
+            if guest_path = guest.capability(:dnf_cache_dir)
               return if @env[:cache_dirs].include?(guest_path)
 
               # Ensure caching is enabled
@@ -16,6 +16,7 @@ module VagrantPlugins
               comm.sudo("sed -i '/^[main]/a keepcache=1' /etc/dnf/dnf.conf")
 
               symlink(guest_path)
+            end
           else
             @env[:ui].info I18n.t('vagrant_cachier.skipping_bucket', bucket: 'DNF')
           end

lib/vagrant-cachier/bucket/yum.rb

@@ -8,13 +8,14 @@ module VagrantPlugins
 
         def install
           if guest.capability?(:yum_cache_dir)
-            guest_path = guest.capability(:yum_cache_dir)
+            if guest_path = guest.capability(:yum_cache_dir)
               return if @env[:cache_dirs].include?(guest_path)
 
               # Ensure caching is enabled
               comm.sudo("sed -i 's/keepcache=0/keepcache=1/g' /etc/yum.conf")
 
               symlink(guest_path)
+            end
           else
             @env[:ui].info I18n.t('vagrant_cachier.skipping_bucket', bucket: 'Yum')
           end

lib/vagrant-cachier/cap/redhat/dnf_cache_dir.rb

@@ -4,7 +4,12 @@ module VagrantPlugins
       module RedHat
         module DnfCacheDir
           def self.dnf_cache_dir(machine)
-            '/var/cache/dnf'
+            dnf_cache_dir = nil
+            machine.communicate.tap do |comm|
+              return unless comm.test('which dnf')
+              dnf_cache_dir = '/var/cache/dnf'
+            end
+            return dnf_cache_dir
           end
         end
       end

lib/vagrant-cachier/cap/redhat/yum_cache_dir.rb

@@ -4,7 +4,13 @@ module VagrantPlugins
       module RedHat
         module YumCacheDir
           def self.yum_cache_dir(machine)
-            '/var/cache/yum'
+            yum_cache_dir = nil
+            machine.communicate.tap do |comm|
+              # In case yum is only forwarding to dnf do not cache
+              return unless not comm.test('yum --version 2>&1 | grep /usr/bin/dnf')
+              yum_cache_dir = '/var/cache/yum'
+            end
+            return yum_cache_dir
           end
         end
       end