[resty.ctx] library for sharing ctx to subrequests

this is needed to share ngx.ctx to subrequests or internal redirects

quite similar to https://github.com/tokers/lua-resty-ctxdump
implementation based on suggestions from openresty/lua-nginx-module#1057

gateway/src/resty/ctx.lua

@@ -0,0 +1,84 @@
+--- resty.ctx
+-- Module for sharing ngx.ctx to subrequests.
+-- @module resty.ctx
+
+local ffi = require 'ffi'
+local debug = require 'debug'
+local base = require "resty.core.base"
+
+-- to get FFI definitions
+require 'resty.core.ctx'
+
+local registry = debug.getregistry()
+local getfenv = getfenv
+local C = ffi.C
+local FFI_NO_REQ_CTX = base.FFI_NO_REQ_CTX
+local error = error
+local tonumber = tonumber
+
+local _M = {
+}
+
+--- Return ctx reference number
+-- @raise no request found, no request ctx found
+-- @treturn int
+function _M.ref()
+  local r = getfenv(0).__ngx_req
+
+  if not r then
+    return error("no request found")
+  end
+
+  local _ = ngx.ctx -- load context
+
+  local ctx_ref = C.ngx_http_lua_ffi_get_ctx_ref(r)
+
+  if ctx_ref == FFI_NO_REQ_CTX then
+    return error("no request ctx found")
+  end
+
+  -- The context should not be garbage collected until all the subrequests are completed.
+  -- That includes internal redirects and post action.
+
+  return ctx_ref
+end
+
+_M.var = 'ctx_ref'
+
+--- Store ctx reference in ngx.var
+-- @tparam ?string var variable name, defaults to ctx_ref
+function _M.stash(var)
+  ngx.var[var or _M.var] = _M.ref()
+end
+
+local function get_ctx(ref)
+  local r = getfenv(0).__ngx_req
+
+  if not r then
+    return error("no request found")
+  end
+
+  local ctx_ref = tonumber(ref)
+  if not ctx_ref then
+    return
+  end
+
+  return registry.ngx_lua_ctx_tables[ctx_ref] or error("no request ctx found")
+end
+
+--- Apply stored ctx to the current request
+-- @tparam ?string var variable name, defaults to ctx_ref
+-- @raise no request found
+-- @treturn table
+function _M.apply(var)
+  local ctx = get_ctx(ngx.var[var or _M.var])
+
+  -- this will actually store the reference again
+  -- so each request that gets the context applied will hold own reference
+  -- this is a very safe way to ensure it is not GC'd or released by another requests
+  ngx.ctx = ctx
+
+  return ctx
+end
+
+return _M

t/resty-ctx.t

@@ -0,0 +1,132 @@
+use lib 't';
+use Test::APIcast 'no_plan';
+
+run_tests();
+
+__DATA__
+
+=== TEST 1: get context reference
+get context reference number
+--- http_config
+  lua_package_path "$TEST_NGINX_LUA_PATH";
+--- config
+  location = /t {
+    content_by_lua_block {
+      ngx.say(require('resty.ctx').ref())
+    }
+  }
+--- request
+GET /t
+--- response_body
+1
+--- no_error_log
+[error]
+
+
+
+=== TEST 2: stash context reference
+--- http_config
+  lua_package_path "$TEST_NGINX_LUA_PATH";
+--- config
+  set $ctx_ref -1;
+
+  location = /t {
+    rewrite_by_lua_block {
+      require('resty.ctx').stash()
+    }
+
+    content_by_lua_block {
+      ngx.say(ngx.var.ctx_ref)
+    }
+  }
+--- request
+GET /t
+--- response_body
+1
+--- no_error_log
+[error]
+
+
+
+=== TEST 3: apply context reference
+--- http_config
+  lua_package_path "$TEST_NGINX_LUA_PATH";
+--- config
+  set $ctx_ref -1;
+
+  location = /t {
+    rewrite_by_lua_block {
+      require('resty.ctx').stash()
+      ngx.ctx.foo = 'bar'
+    }
+
+    content_by_lua_block {
+      ngx.exec('@redirect')
+    }
+  }
+
+  location @redirect {
+    internal;
+
+    rewrite_by_lua_block {
+      require('resty.ctx').apply()
+      ngx.say(ngx.ctx.foo)
+    }
+  }
+--- request
+GET /t
+--- response_body
+bar
+--- no_error_log
+[error]
+
+
+
+=== TEST 4: context is not garbage collected
+--- http_config
+  lua_package_path "$TEST_NGINX_LUA_PATH";
+--- config
+  set $ctx_ref -1;
+
+  location = /t {
+    rewrite_by_lua_block {
+      require('resty.ctx').stash()
+      ngx.ctx.foo = 'bar'
+    }
+
+    content_by_lua_block {
+      ngx.exec('@redirect')
+    }
+  }
+
+  location @redirect {
+    internal;
+
+    rewrite_by_lua_block {
+      collectgarbage()
+      require('resty.ctx').apply()
+    }
+
+    post_action @out_of_band;
+
+    content_by_lua_block {
+      collectgarbage()
+      ngx.say(ngx.ctx.foo)
+    }
+  }
+
+  location @out_of_band {
+    rewrite_by_lua_block {
+      collectgarbage()
+      require('resty.ctx').apply()
+    }
+
+    content_by_lua_block {
+      collectgarbage()
+      ngx.say(ngx.ctx.foo)
+    }
+  }
+--- request
+GET /t
+--- response_body
+bar