fix(timers) reduce timers used

spec/balancer/generic_spec.lua

@@ -1662,8 +1662,8 @@ for algorithm, balancer_module in helpers.balancer_types() do
 
 
       it("recovers when dns entries are replaced by healthy ones", function()
-        local record = dnsA({
-          { name = "getkong.org", address = "1.2.3.4" },
+        dnsA({
+          { name = "getkong.org", address = "1.2.3.4", ttl = 2 },
         })
         b:addHost("getkong.org", 8000, 50)
         assert.not_nil(b:getPeer())
@@ -1677,11 +1677,10 @@ for algorithm, balancer_module in helpers.balancer_types() do
           }
         )
 
-        -- expire DNS and add a new backend IP
+        -- update DNS with a new backend IP
         -- balancer should now recover since a new healthy backend is available
-        record.expire = 0
         dnsA({
-          { name = "getkong.org", address = "5.6.7.8" },
+          { name = "getkong.org", address = "5.6.7.8", ttl = 60 },
         })
 
         local timeout = ngx.now() + 5   -- we'll try for 5 seconds
@@ -1873,8 +1872,8 @@ for algorithm, balancer_module in helpers.balancer_types() do
         })
         b:addHost("127.0.0.1", 8000, 100)
 
-        local test_table = setmetatable({}, { __mode = "k" })
-        test_table[b] = true
+        local test_table = setmetatable({}, { __mode = "v" })
+        test_table.key = b
         assert.not_nil(next(test_table))
 
         -- destroy it
@@ -1884,7 +1883,8 @@ for algorithm, balancer_module in helpers.balancer_types() do
         collectgarbage()
         collectgarbage()
         --assert.is_nil(next(test_table))  -- doesn't work, hangs if failed, luassert bug
-        assert.equal("nil", tostring(next(test_table)))
+        assert.is_nil(test_table.key)
+        assert.equal("nil", tostring(test_table.key))
       end)
 
     end)

src/resty/dns/balancer/base.lua

@@ -176,6 +176,13 @@ local errors = setmetatable({
 })
 
 
+-- global binary heap for all balancers to share as a single update timer for
+-- renewing DNS records
+local renewal_heap = require("binaryheap").minUnique()
+local renewal_weak_cache = setmetatable({}, { __mode = "v" })
+local renewal_timer  --luacheck: ignore
+
+
 local _M = {}
 
 
@@ -427,6 +434,55 @@ local function assert_atomicity(f, self, ...)
 end
 
 
+-- Timer invoked to update DNS records
+local function resolve_timer_callback()
+  local now = time()
+  --print("running timer:",tostring(renewal_heap:peekValue()), " ", now)
+
+  while (renewal_heap:peekValue() or math.huge) < now do
+    local key = renewal_heap:pop()
+    local host = renewal_weak_cache[key] --- can return nil if GC'ed
+
+    --print("timer on: ",key, " the value is: ", tostring((host or EMPTY).hostname))
+    if host then
+      ngx_log(ngx_DEBUG, host.balancer.log_prefix, "executing requery for: ", host.hostname)
+      host:queryDns(false) -- timer-context; cacheOnly always false
+    end
+  end
+end
+
+
+
+-- schedules a DNS update for a host in the global timer queue. This uses only
+-- a single timer for all balancers.
+-- IMPORTANT: this construct should not prevent GC of the Host object
+local function schedule_dns_renewal(host)
+  local record_expiry = (host.lastQuery or EMPTY).expire or 0
+  local key = host.balancer.id .. ":" .. host.hostname .. ":" .. host.port
+
+  local new_renew_at = record_expiry + 0.05 -- ensure expired, but within stale_ttl
+  local old_renew_at = renewal_heap:valueByPayload(key)
+
+  -- always store the host in the registry, because the same key might be reused
+  -- by a new host-object for the same hostname in case of quick delete/add sequence
+  renewal_weak_cache[key] = host
+
+  if old_renew_at then
+    renewal_heap:update(key, new_renew_at)
+  else
+    renewal_heap:insert(new_renew_at, key)
+  end
+end
+
+
+-- remove a Host from the DNS renewal timer
+local function cancel_dns_renewal(host)
+  local key = host.balancer.id .. ":" .. host.hostname .. ":" .. host.port
+  renewal_weak_cache[key] = nil
+  renewal_heap:remove(key)
+end
+
+
 local function update_dns_result(self, newQuery, dns)
   local oldQuery = self.lastQuery or {}
   local oldSorted = self.lastSorted or {}
@@ -607,6 +663,8 @@ function objHost:queryDns(cacheOnly)
 
   assert_atomicity(update_dns_result, self, newQuery, dns)
 
+  schedule_dns_renewal(self)
+
   return true
 end
 
@@ -727,9 +785,12 @@ function objHost:delete()
 
   for i = #self.addresses, 1, -1 do  -- reverse traversal as we're deleting
     self.addresses[i]:delete()
+    self.addresses[i] = nil
   end
 
   self.balancer = nil
+  self.lastQuery = nil
+  self.lastSorted = nil
 end
 
 
@@ -1023,6 +1084,8 @@ function objBalancer:removeHost(hostname, port)
 
       ngx_log(ngx_DEBUG, self.log_prefix, "removing host ", hostname, ":", port)
 
+      cancel_dns_renewal(host)
+
       -- set weights to 0
       host:disable()
 
@@ -1254,20 +1317,6 @@ function objBalancer:setHostStatus(available, hostname, port)
 end
 
 
--- Timer invoked to update DNS records
-function objBalancer:resolveTimerCallback()
-  --check all hosts for expired records, including those with errors
-  --we update, so changes on the list while traversing can happen, keep track of that
-
-  for _, host in ipairs(self.hosts) do
-    if ((host.lastQuery or EMPTY).expire or 0) < time() then
-      ngx_log(ngx_DEBUG, self.log_prefix, "executing requery for: ", host.hostname)
-      host:queryDns(false) -- timer-context; cacheOnly always false
-    end
-  end
-end
-
-
 --- Sets an event callback for user code. The callback is invoked for
 -- every address added to/removed from the balancer, and on health changes.
 --
@@ -1417,6 +1466,7 @@ _M.new = function(opts)
   balancer_id_counter = balancer_id_counter + 1
   local self = {
     -- properties
+    id = balancer_id_counter,
     log_prefix = "[" .. (opts.log_prefix or "balancer") .. " " .. tostring(balancer_id_counter) .. "] ",
     hosts = {},    -- a list a host objects
     addresses = {}, -- a list of addresses, including reverse lookup
@@ -1435,24 +1485,22 @@ _M.new = function(opts)
 
   self:setCallback(opts.callback or function() end) -- callback for address mutations
 
-  do
-    local err
-    self.resolveTimer, err = resty_timer({
-        recurring = true,
-        interval = 1,  -- check for expired records every 1 second
-        detached = false,
-        expire = self.resolveTimerCallback,
-      }, self)
-
-    if not self.resolveTimer then
-      return nil, "failed to create timer for background DNS resolution: " .. err
-    end
-  end
-
   ngx_log(ngx_DEBUG, self.log_prefix, "balancer_base created")
   return self
 end
 
+-- start global renewal timer
+renewal_timer = assert(resty_timer({
+  recurring = true,
+  interval = 1,
+  detached = false, -- not anchored, so reloading GC's timer for test purposes
+  expire = resolve_timer_callback,
+}))
+-- LuaJIT is smart enough to collect module level vars that are not referenced
+-- anymore, hence we add it to the module table as a workaround
+_M._renewal_timer = renewal_timer
+
+
 -- export the error constants
 _M.errors = errors
 objBalancer.errors = errors