link-target.lua: change into a async hook

link setup is a asynchronous action, async hook serves the purpose well
here.

The code dependent on links will be less error prone with this design.

src/scripts/linking/link-target.lua

@@ -10,7 +10,7 @@
 local putils = require ("policy-utils")
 local cutils = require ("common-utils")
 
-SimpleEventHook {
+AsyncEventHook {
   name = "linking/link-target",
   after = "linking/prepare-link",
   interests = {
@@ -18,12 +18,16 @@ SimpleEventHook {
       Constraint { "event.type", "=", "select-target" },
     },
   },
-  execute = function (event)
+  steps = {
+    start = {
+      next = "link_activated",
+      execute = function (event, transition)
         local source, om, si, si_props, si_flags, target =
             putils:unwrap_find_target_event (event)
 
-    -- bypass the hook, nothing to link to
         if not target then
+          -- bypass the hook, nothing to link to.
+          transition:advance ()
           return
         end
 
@@ -45,8 +49,8 @@ SimpleEventHook {
             si_flags.failed_peer_id == target.id and
             si_flags.failed_count ~= nil and
             si_flags.failed_count > 5 then
-      Log.warning (si, "tried to link on last rescan, not retrying")
+          transition:return_error ("tried to link on last rescan, not retrying "
-      goto done
+              .. tostring (si_link))
         end
 
         if si_props ["item.node.direction"] == "output" then
@@ -77,8 +81,9 @@ SimpleEventHook {
           ["in.item.port.context"] = "input",
           ["is.policy.item.link"] = true,
         } then
-      Log.warning (si_link, "failed to configure si-standard-link")
+          transition:return_error ("failed to configure si-standard-link "
-      goto done
+            .. tostring (si_link))
+          return
         end
 
         si_link:connect("link-error", function (_, error_msg)
@@ -117,25 +122,42 @@ SimpleEventHook {
         -- activate
         si_link:activate (Feature.SessionItem.ACTIVE, function (l, e)
           if e then
-        Log.info (l, "failed to activate si-standard-link: " .. tostring (si) .. " error:" .. tostring (e))
+            transition:return_error ("failed to activate si-standard-link: "
+                .. tostring (si) .. " error:" .. tostring (e))
             if si_flags ~= nil then
               si_flags.peer_id = nil
             end
             l:remove ()
           else
+            si_flags.si_link = si_link
+            transition:advance ()
+          end
+        end)
+      end,
+    },
+    link_activated = {
+      next = "none",
+      execute = function (event, transition)
+        local source, om, si, si_props, si_flags, target =
+            putils:unwrap_find_target_event (event)
+        if not target then
+          -- bypass the hook, nothing to link to.
+          transition:advance ()
+          return
+        end
+
         if si_flags ~= nil then
           si_flags.failed_peer_id = nil
           if si_flags.peer_id == nil then
-            si_flags.peer_id = target.id
+            si_flags.peer_id = si_target.id
           end
           si_flags.failed_count = 0
         end
-        Log.info (l, "activated si-standard-link " .. tostring (si))
+        Log.info (si_flags.si_link, "activated si-standard-link between "
-      end
+            .. tostring (si).." and "..tostring(si_target))
-    end)
 
-    ::done::
+        transition:advance ()
-    si_flags.was_handled = true
+      end,
-    putils.checkFollowDefault (si, target, si_flags.has_node_defined_target)
+    },
-  end
+  },
 }:register ()