perf(pdk) faster request.get_header (#8716)

* perf(pdk) faster reqeust.get_header
* typo
* cache ngx.var
* more effecient implementation of string process
* no need to lower
* bug fix
* try to archive better performance
* style
* bug fix

kong/pdk/request.lua

@@ -12,6 +12,7 @@ local phase_checker = require "kong.pdk.private.phases"
 
 
 local ngx = ngx
+local var = ngx.var
 local sub = string.sub
 local find = string.find
 local lower = string.lower
@@ -568,10 +569,8 @@ local function new(self)
       error("header name must be a string", 2)
     end
 
-    local header_value = _REQUEST.get_headers()[name]
-    if type(header_value) == "table" then
-      return header_value[1]
-    end
+    -- Do not localize ngx.re.gsub! It will crash because ngx.re is monkey patched.
+    local header_value = var["http_" .. ngx.re.gsub(name, "-", "_", "jo")]
 
     return header_value
   end

t/01-pdk/04-request/13-get_header.t

@@ -104,7 +104,7 @@ X-Foo-Header: ''
 
 
 
-=== TEST 5: request.get_header() returns nil when requested header does not fit in default max_headers
+=== TEST 5: request.get_header() have no limit on header numbers
 --- http_config eval: $t::Util::HttpConfig
 --- config
     location = /t {
@@ -133,7 +133,7 @@ X-Foo-Header: ''
 --- request
 GET /t
 --- response_body
-accept header value: nil
+accept header value: text/html
 --- no_error_log
 [error]