perf(tracing): do not create spans in timer phase

Before this change timers would generate spans, which means DB and DNS
spans in recurring timers would be continuously generated and
garbage-collected.

This commit checks the exact ngx phase and runs it against a whitelist
to ensure `timer` phase does not generate spans.

changelog/unreleased/kong/perf-tracing-from-timers.yml

@@ -0,0 +1,3 @@
+message: "Performance optimization to avoid unnecessary creations and garbage-collections of spans"
+type: "performance"
+scope: "PDK"

kong/pdk/tracing.lua

@@ -9,7 +9,6 @@ local require = require
 local ffi = require "ffi"
 local tablepool = require "tablepool"
 local new_tab = require "table.new"
-local base = require "resty.core.base"
 local utils = require "kong.tools.utils"
 local phase_checker = require "kong.pdk.private.phases"
 
@@ -421,6 +420,15 @@ noop_tracer.set_active_span = NOOP
 noop_tracer.process_span = NOOP
 noop_tracer.set_should_sample = NOOP
 
+local VALID_TRACING_PHASES = {
+  rewrite = true,
+  access = true,
+  header_filter = true,
+  body_filter = true,
+  log = true,
+  content = true,
+}
+
 --- New Tracer
 local function new_tracer(name, options)
   name = name or "default"
@@ -450,7 +458,7 @@ local function new_tracer(name, options)
   -- @phases rewrite, access, header_filter, response, body_filter, log, admin_api
   -- @treturn table span
   function self.active_span()
-    if not base.get_request() then
+    if not VALID_TRACING_PHASES[ngx.get_phase()] then
       return
     end
 
@@ -463,7 +471,7 @@ local function new_tracer(name, options)
   -- @phases rewrite, access, header_filter, response, body_filter, log, admin_api
   -- @tparam table span
   function self.set_active_span(span)
-    if not base.get_request() then
+    if not VALID_TRACING_PHASES[ngx.get_phase()] then
       return
     end
 
@@ -482,7 +490,7 @@ local function new_tracer(name, options)
   -- @tparam table options TODO(mayo)
   -- @treturn table span
   function self.start_span(...)
-    if not base.get_request() then
+    if not VALID_TRACING_PHASES[ngx.get_phase()] then
       return noop_span
     end
 

spec/01-unit/26-tracing/01-tracer_pdk_spec.lua

@@ -49,17 +49,23 @@ end
 local unhook_log_spy = debug.sethook
 
 describe("Tracer PDK", function()
-  local ok, err, _
+  local ok, err, old_ngx_get_phase, _
   local log_spy
 
   lazy_setup(function()
     local kong_global = require "kong.global"
     _G.kong = kong_global.new()
     kong_global.init_pdk(kong)
     log_spy = hook_log_spy()
+    old_ngx_get_phase = ngx.get_phase
+    -- trick the pdk into thinking we are not in the timer context
+    _G.ngx.get_phase = function() return "access" end  -- luacheck: ignore
   end)
 
-  lazy_teardown(unhook_log_spy)
+  lazy_teardown(function()
+    unhook_log_spy()
+    _G.ngx.get_phase = old_ngx_get_phase  -- luacheck: ignore
+  end)
 
   describe("initialize tracer", function()
 

spec/03-plugins/37-opentelemetry/01-otlp_spec.lua

@@ -44,16 +44,22 @@ local pb_decode_span = function(data)
 end
 
 describe("Plugin: opentelemetry (otlp)", function()
+  local old_ngx_get_phase
+
   lazy_setup(function ()
     -- overwrite for testing
     pb.option("enum_as_value")
     pb.option("auto_default_values")
+    old_ngx_get_phase = ngx.get_phase
+    -- trick the pdk into thinking we are not in the timer context
+    _G.ngx.get_phase = function() return "access" end  -- luacheck: ignore
   end)
 
   lazy_teardown(function()
     -- revert it back
     pb.option("enum_as_name")
     pb.option("no_default_values")
+    _G.ngx.get_phase = old_ngx_get_phase  -- luacheck: ignore
   end)
 
   after_each(function ()