swim: fix a leak when a trigger is installed

SWIM wraps user triggers to prepare arguments. The wrapper
function kept a reference to SWIM object, and prevented its
automatic deletion at GC.

The patch makes this reference weak.

Follow up #4250

src/lua/swim.lua

@@ -786,10 +786,17 @@ local swim_member_event_mt = {
 -- @return A function to set as a trigger.
 --
 local function swim_on_member_event_new(s, callback, ctx)
+    -- Do not keep a hard reference to a SWIM instance. Otherwise
+    -- it is a cyclic reference, and both the instance and the
+    -- trigger will never be GC-ed.
+    s = setmetatable({s}, {__mode = 'v'})
     return function(member_ptr, event_mask)
-        local m = swim_wrap_member(s, member_ptr)
-        local event = setmetatable({event_mask}, swim_member_event_mt)
-        return callback(m, event, ctx)
+        local s = s[1]
+        if s then
+            local m = swim_wrap_member(s, member_ptr)
+            local event = setmetatable({event_mask}, swim_member_event_mt)
+            return callback(m, event, ctx)
+        end
     end
 end
 

test/swim/swim.result

@@ -1574,6 +1574,27 @@ s1:delete()
 s2:delete()
 ---
 ...
+--
+-- Check that Lua triggers don't keep a reference of SWIM instance
+-- preventing its GC.
+--
+s = swim.new({uri = 0, uuid = uuid(1)})
+---
+...
+_ = s:on_member_event(function() end)
+---
+...
+s = setmetatable({s}, {__mode = 'v'})
+---
+...
+collectgarbage('collect')
+---
+- 0
+...
+s
+---
+- []
+...
 test_run:cmd("clear filter")
 ---
 - true

test/swim/swim.test.lua

@@ -535,4 +535,14 @@ s1:cfg({generation = 5})
 s1:delete()
 s2:delete()
 
+--
+-- Check that Lua triggers don't keep a reference of SWIM instance
+-- preventing its GC.
+--
+s = swim.new({uri = 0, uuid = uuid(1)})
+_ = s:on_member_event(function() end)
+s = setmetatable({s}, {__mode = 'v'})
+collectgarbage('collect')
+s
+
 test_run:cmd("clear filter")