perf: cache policy manifests

Previously, every time the policy chain was rebuilt, the gateway would look
for the manifests file on the disk. This caused a delay for incoming requests.

Since the manifests are static and do not change at runtime, it is more
efficient to cache them at the module level. This caching will speed up the
lookup process.

Author: tkan145

gateway/src/apicast/policy_loader.lua

@@ -16,6 +16,11 @@ local concat = table.concat
 local setmetatable = setmetatable
 local pcall = pcall
 
+local tab_new = require('table.new')
+local isempty = require('table.isempty')
+
+local manifests_cache = tab_new(32, 0)
+
 local _M = {}
 
 local resty_env = require('resty.env')
@@ -70,8 +75,22 @@ local function lua_load_path(load_path)
   return format('%s/?.lua', load_path)
 end
 
+local function get_manifest(name, version)
+  local manifests = manifests_cache[name]
+  if manifests then
+    for _, manifest in ipairs(manifests) do
+      if version == manifest.version then
+        return manifest
+      end
+    end
+  end
+end
+
 local function load_manifest(name, version, path)
-  local manifest = read_manifest(path)
+  local manifest = get_manifest(name, version)
+  if not manifest then
+    manifest = read_manifest(path)
+  end
 
   if manifest then
       if manifest.version ~= version then
@@ -110,8 +129,8 @@ end
 function _M:load_path(name, version, paths)
   local failures = {}
 
-  for _, path in ipairs(paths or self.policy_load_paths()) do
-    local manifest, load_path = load_manifest(name, version, format('%s/%s/%s', path, name, version) )
+  if version == 'builtin' then
+    local manifest, load_path = load_manifest(name, version, format('%s/%s', self.builtin_policy_load_path(), name) )
 
     if manifest then
       return load_path, manifest.configuration
@@ -120,8 +139,8 @@ function _M:load_path(name, version, paths)
     end
   end
 
-  if version == 'builtin' then
-    local manifest, load_path = load_manifest(name, version, format('%s/%s', self.builtin_policy_load_path(), name) )
+  for _, path in ipairs(paths or self.policy_load_paths()) do
+    local manifest, load_path = load_manifest(name, version, format('%s/%s/%s', path, name, version) )
 
     if manifest then
       return load_path, manifest.configuration
@@ -130,6 +149,7 @@ function _M:load_path(name, version, paths)
     end
   end
 
+
   return nil, nil, failures
 end
 
@@ -173,9 +193,15 @@ end
 -- Returns all the policy modules
 function _M:get_all()
   local policy_modules = {}
+  local manifests
 
-  local policy_manifests_loader = require('apicast.policy_manifests_loader')
-  local manifests = policy_manifests_loader.get_all()
+  if isempty(manifests_cache) then
+    local policy_manifests_loader = require('apicast.policy_manifests_loader')
+    manifests = policy_manifests_loader.get_all()
+    manifests_cache = manifests
+  else
+    manifests = manifests_cache
+  end
 
   for policy_name, policy_manifests in pairs(manifests) do
     for _, manifest in ipairs(policy_manifests) do