feat(core) add ability to inject X-Kong-Upstream-Status header

X-Kong-Upstream-Status header returns the status
code returned by the upstream API.
This header is disabled by default and can be enabled
by adding it to `headers` config property in kong.conf.

kong/conf_loader.lua

@@ -25,6 +25,7 @@ local header_key_to_name = {
   [string.lower(headers.SERVER)] = headers.SERVER,
   [string.lower(headers.PROXY_LATENCY)] = headers.PROXY_LATENCY,
   [string.lower(headers.UPSTREAM_LATENCY)] = headers.UPSTREAM_LATENCY,
+  [string.lower(headers.UPSTREAM_STATUS)] = headers.UPSTREAM_STATUS,
 }
 
 local PREFIX_PATHS = {

kong/constants.lua

@@ -50,6 +50,7 @@ return {
     HOST_OVERRIDE = "X-Host-Override",
     PROXY_LATENCY = "X-Kong-Proxy-Latency",
     UPSTREAM_LATENCY = "X-Kong-Upstream-Latency",
+    UPSTREAM_STATUS = "X-Kong-Upstream-Status",
     CONSUMER_ID = "X-Consumer-ID",
     CONSUMER_CUSTOM_ID = "X-Consumer-Custom-ID",
     CONSUMER_USERNAME = "X-Consumer-Username",

kong/runloop/handler.lua

@@ -27,6 +27,7 @@ local ngx         = ngx
 local log         = ngx.log
 local null        = ngx.null
 local ngx_now     = ngx.now
+local re_match    = ngx.re.match
 local unpack      = unpack
 
 
@@ -687,12 +688,26 @@ return {
   },
   header_filter = {
     before = function(ctx)
+      local var = ngx.var
+      local header = ngx.header
+
       if ctx.KONG_PROXIED then
         local now = get_now()
         -- time spent waiting for a response from upstream
         ctx.KONG_WAITING_TIME             = now - ctx.KONG_ACCESS_ENDED_AT
         ctx.KONG_HEADER_FILTER_STARTED_AT = now
       end
+
+      local upstream_status_header = constants.HEADERS.UPSTREAM_STATUS
+      if singletons.configuration.headers[upstream_status_header] then
+        local matches, err = re_match(var.upstream_status,
+                                          "[0-9]+$", "oj")
+        if err then
+          log(ERR, "failed to set ", upstream_status_header, " header: ", err)
+        else
+          header[upstream_status_header] = matches[0]
+        end
+      end
     end,
     after = function(ctx)
       local header = ngx.header

spec/02-integration/05-proxy/14-upstream-status-header_spec.lua

@@ -0,0 +1,141 @@
+local helpers = require "spec.helpers"
+local constants = require "kong.constants"
+
+local function setup_db()
+  local bp = helpers.get_db_utils()
+  assert(helpers.dao:run_migrations())
+
+  local service = assert(bp.services:insert {
+    host = helpers.mock_upstream_host,
+    port = helpers.mock_upstream_port,
+    protocol = helpers.mock_upstream_protocol,
+  })
+
+  assert(bp.routes:insert {
+    protocols = { "http" },
+    service = service,
+    paths = { "/status/200" },
+  })
+
+  local route2 = assert(bp.routes:insert {
+    protocols = { "http" },
+    service = service,
+    paths = { "/status/plugin-changes-200-to-500" },
+  })
+
+  assert(bp.plugins:insert({
+    name = "dummy",
+    route_id = route2.id,
+    config = {
+      resp_code = 500,
+    }
+  }))
+end
+
+describe(constants.HEADERS.UPSTREAM_STATUS .. " header", function()
+  local client
+
+  describe("should be same as upstream status code", function()
+    setup(function()
+      setup_db()
+      assert(helpers.start_kong {
+        headers = "server_tokens,latency_tokens,x-kong-upstream-status",
+        custom_plugins = "dummy",
+        nginx_conf = "spec/fixtures/custom_nginx.template",
+      })
+      client = helpers.proxy_client()
+    end)
+
+    teardown(function()
+      if client then
+        client:close()
+      end
+      helpers.stop_kong()
+    end)
+
+    it("when no plugin changes status code", function()
+      local res = assert(client:send {
+        method  = "GET",
+        path    = "/status/200",
+        headers = {
+          host = helpers.mock_upstream_host,
+        }
+      })
+      assert.res_status(200, res)
+      assert.equal("200", res.headers[constants.HEADERS.UPSTREAM_STATUS])
+    end)
+
+    it("when a plugin changes status code", function()
+      local res = assert(client:send {
+        method  = "GET",
+        host = helpers.mock_upstream_host,
+        path    = "/status/plugin-changes-200-to-500",
+        headers = {
+          ["Host"]  = helpers.mock_upstream_host,
+        }
+      })
+      assert.res_status(500, res)
+      assert.equal("200", res.headers[constants.HEADERS.UPSTREAM_STATUS])
+    end)
+  end)
+
+  describe("is not injected with default configuration", function()
+    setup(function()
+      setup_db()
+      assert(helpers.start_kong{
+        nginx_conf = "spec/fixtures/custom_nginx.template",
+      })
+    end)
+
+    teardown(function()
+      if client then
+        client:close()
+      end
+      helpers.stop_kong()
+    end)
+
+    it("", function()
+      local client = helpers.proxy_client()
+      local res = assert(client:send {
+        method  = "GET",
+        path    = "/status/200",
+        headers = {
+          host = helpers.mock_upstream_host,
+        }
+      })
+      assert.res_status(200, res)
+      assert.is_nil(res.headers[constants.HEADERS.UPSTREAM_STATUS])
+    end)
+  end)
+
+  describe("is injected with configuration [headers=X-Kong-Upstream-Status]", function()
+
+    setup(function()
+      setup_db()
+      assert(helpers.start_kong{
+        nginx_conf = "spec/fixtures/custom_nginx.template",
+        headers="X-Kong-Upstream-Status",
+      })
+    end)
+
+    teardown(function()
+      if client then
+        client:close()
+      end
+      helpers.stop_kong()
+    end)
+
+    it("", function()
+      local client = helpers.proxy_client()
+      local res = assert(client:send {
+        method  = "GET",
+        path    = "/status/200",
+        headers = {
+          host = helpers.mock_upstream_host,
+        }
+      })
+      assert.res_status(200, res)
+      assert("200", res.headers[constants.HEADERS.UPSTREAM_STATUS])
+    end)
+  end)
+end)

spec/fixtures/custom_plugins/kong/plugins/dummy/handler.lua

@@ -29,6 +29,10 @@ function DummyHandler:header_filter(conf)
 
   ngx.header["Dummy-Plugin"] = conf.resp_header_value
 
+  if conf.resp_code then
+    ngx.status = conf.resp_code
+  end
+
   if conf.append_body then
     ngx.header["Content-Length"] = nil
   end

spec/fixtures/custom_plugins/kong/plugins/dummy/schema.lua

@@ -2,5 +2,6 @@ return {
   fields = {
     resp_header_value = { type = "string", default = "1" },
     append_body = { type = "string" },
+    resp_code = { type = "number" },
   }
 }