Raise an error when either the filtered hosts or roles are empty.

Keeps us confusingly running things on the primary_host when nothing
matches.

lib/kamal/commander.rb

@@ -28,11 +28,27 @@ def specific_primary!
   end
 
   def specific_roles=(role_names)
-    @specific_roles = Kamal::Utils.filter_specific_items(role_names, config.roles) if role_names.present?
+    if role_names.present?
+      @specific_roles = Kamal::Utils.filter_specific_items(role_names, config.roles)
+
+      if @specific_roles.empty?
+        raise ArgumentError, "No --roles match for #{role_names.join(',')}"
+      end
+
+      @specific_roles
+    end
   end
 
   def specific_hosts=(hosts)
-    @specific_hosts = Kamal::Utils.filter_specific_items(hosts, config.all_hosts) if hosts.present?
+    if hosts.present?
+      @specific_hosts = Kamal::Utils.filter_specific_items(hosts, config.all_hosts)
+
+      if @specific_hosts.empty?
+        raise ArgumentError, "No --hosts match for #{hosts.join(',')}"
+      end
+
+      @specific_hosts
+    end
   end
 
   def primary_host

test/commander_test.rb

@@ -24,15 +24,22 @@ class CommanderTest < ActiveSupport::TestCase
     @kamal.specific_hosts = [ "*" ]
     assert_equal [ "1.1.1.1", "1.1.1.2", "1.1.1.3", "1.1.1.4" ], @kamal.hosts
 
-    @kamal.specific_hosts = [ "*miss" ]
-    assert_equal [], @kamal.hosts
+    exception = assert_raises(ArgumentError) do
+      @kamal.specific_hosts = [ "*miss" ]
+    end
+    assert_match /hosts match for \*miss/, exception.message
   end
 
   test "filtering hosts by filtering roles" do
     assert_equal [ "1.1.1.1", "1.1.1.2", "1.1.1.3", "1.1.1.4" ], @kamal.hosts
 
     @kamal.specific_roles = [ "web" ]
     assert_equal [ "1.1.1.1", "1.1.1.2" ], @kamal.hosts
+
+    exception = assert_raises(ArgumentError) do
+      @kamal.specific_roles = [ "*miss" ]
+    end
+    assert_match /roles match for \*miss/, exception.message
   end
 
   test "filtering roles" do
@@ -50,8 +57,10 @@ class CommanderTest < ActiveSupport::TestCase
     @kamal.specific_roles = [ "*" ]
     assert_equal [ "web", "workers" ], @kamal.roles.map(&:name)
 
-    @kamal.specific_roles = [ "*miss" ]
-    assert_equal [], @kamal.roles.map(&:name)
+    exception = assert_raises(ArgumentError) do
+      @kamal.specific_roles = [ "*miss" ]
+    end
+    assert_match /roles match for \*miss/, exception.message
   end
 
   test "filtering roles by filtering hosts" do