linking: handle defined target properly with smart filters

This patch fixes the policy to not link the client to the default filter if the
client's defined target is found, is not a filter, does not have any other
filters linked with it. In this case, the client is therefore linked to the
actual defined target. On the other hand, if the client's defined target is a
filter, the client is linked to the first filter in the chain that has the same
target as the defined filter's target.

src/scripts/linking/find-defined-target.lua

@@ -102,6 +102,7 @@ SimpleEventHook {
       end
     end
 
+    si_flags.has_defined_target = false
     if target_picked and target then
       log:info (si,
         string.format ("... defined target picked: %s (%s), can_passthrough:%s",
@@ -110,6 +111,7 @@ SimpleEventHook {
           tostring (can_passthrough)))
       si_flags.has_node_defined_target = node_defined
       si_flags.can_passthrough = can_passthrough
+      si_flags.has_defined_target = true
       event:set_data ("target", target)
     elseif target_value and dont_fallback then
       -- send error to client and destroy node if linger is not set

src/scripts/linking/get-filter-from-target.lua

@@ -35,11 +35,17 @@ SimpleEventHook {
       return
     end
 
-    -- Get the filter from the given target, if any
+    -- Get the filter from the given target if it exists, otherwise get the
+    -- default filter, but only if target was not defined
     local target_direction = cutils.getTargetDirection (si.properties)
     local filter_target = futils.get_filter_from_target (target_direction, target)
     if filter_target ~= nil then
       target = filter_target
+    elseif filter_target == nil and not si_flags.has_defined_target then
+      filter_target = futils.get_filter_from_target (target_direction, nil)
+      if filter_target ~= nil then
+        target = filter_target
+      end
     end
 
     local can_passthrough, passthrough_compatible