Headers policy: allow templating using liquid

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - OpenTracing support [PR #669](https://github.com/3scale/apicast/pull/669)
 - Generate new policy scaffold from the CLI [PR #682](https://github.com/3scale/apicast/pull/682)
 - 3scale batcher policy [PR #685](https://github.com/3scale/apicast/pull/685), [PR #710](https://github.com/3scale/apicast/pull/710)
+- Liquid templating support in the headers policy configuration [PR #716](https://github.com/3scale/apicast/pull/716)
 
 ### Changed
 

gateway/src/apicast/policy/headers/apicast-policy.json

@@ -42,6 +42,21 @@
             "value": {
               "description": "Value that will be added, set or pushed in the header",
               "type": "string"
+            },
+            "value_type": {
+              "description": "How to evaluate 'value'",
+              "type": "string",
+              "oneOf": [
+                {
+                  "enum": ["plain"],
+                  "title": "Evaluate 'value' as plain text."
+                },
+                {
+                  "enum": ["liquid"],
+                  "title": "Evaluate 'value' as liquid."
+                }
+              ],
+              "default": "plain"
             }
           },
           "required": ["op", "header", "value"]

gateway/src/apicast/policy/headers/headers.lua

@@ -9,6 +9,10 @@ local ipairs = ipairs
 local type = type
 local insert = table.insert
 
+local TemplateString = require 'apicast.template_string'
+
+local default_value_type = 'plain'
+
 local policy = require('apicast.policy')
 local _M = policy.new('Headers policy')
 
@@ -69,10 +73,12 @@ local command_functions = {
 }
 
 -- header_type can be 'request' or 'response'.
-local function run_commands(commands, header_type, ...)
+local function run_commands(context, commands, header_type, ...)
   for _, command in ipairs(commands) do
     local command_func = command_functions[header_type][command.op]
-    command_func(command.header, command.value, ...)
+    local value = command.template_string:render(context)
+
+    command_func(command.header, value, ...)
   end
 end
 
@@ -85,6 +91,13 @@ local function init_config(config)
   return res
 end
 
+local function build_templates(commands)
+  for _, command in ipairs(commands) do
+    command.template_string = TemplateString.new(
+      command.value, command.value_type or default_value_type)
+  end
+end
+
 --- Initialize a Headers policy
 -- @tparam[opt] table config
 -- @field[opt] request Table with the operations to apply to the request headers
@@ -93,6 +106,7 @@ end
 --   1) op: can be 'add', 'set' or 'push'.
 --   2) header
 --   3) value
+--   4) value_type (can be 'liquid' or 'plain'). Defaults to 'plain'.
 -- The push operation:
 --   1) When the header is not set, creates it with the given value.
 --   2) When the header is set, it creates a new header with the same name and
@@ -108,18 +122,23 @@ end
 function _M.new(config)
   local self = new(config)
   self.config = init_config(config)
+
+  for _, commands in ipairs({ self.config.request, self.config.response } ) do
+    build_templates(commands)
+  end
+
   return self
 end
 
-function _M:rewrite()
+function _M:rewrite(context)
   -- This is here to avoid calling ngx.req.get_headers() in every command
   -- applied to the request headers.
   local req_headers = ngx.req.get_headers() or {}
-  run_commands(self.config.request, 'request', req_headers)
+  run_commands(context, self.config.request, 'request', req_headers)
 end
 
-function _M:header_filter()
-  run_commands(self.config.response, 'response')
+function _M:header_filter(context)
+  run_commands(context, self.config.response, 'response')
 end
 
 return _M

gateway/src/apicast/template_string.lua

@@ -0,0 +1,116 @@
+local Liquid = require 'liquid'
+local LiquidTemplate = Liquid.Template
+local LiquidInterpreterContext = Liquid.InterpreterContext
+local LiquidFilterSet = Liquid.FilterSet
+local LiquidResourceLimit = Liquid.ResourceLimit
+local ngx = ngx
+
+local setmetatable = setmetatable
+local pairs = pairs
+local format = string.format
+
+local LiquidTemplateString = {}
+local liquid_template_string_mt = { __index = LiquidTemplateString }
+
+-- Expose only ngx.* functions that we think could be useful and that do not
+-- have any side-effects.
+
+-- Table of ngx.* functions that we think can be useful for templates and do
+-- not have any side-effects.
+-- @field escape_uri https://github.com/openresty/lua-nginx-module#ngxescape_uri
+-- @field unescape_uri https://github.com/openresty/lua-nginx-module#ngxunescape_uri
+-- @field encode_base64 https://github.com/openresty/lua-nginx-module#ngxencode_base64
+-- @field decode_base64 https://github.com/openresty/lua-nginx-module#ngxdecode_base64
+-- @field crc32_short https://github.com/openresty/lua-nginx-module#ngxcrc32_short
+-- @field crc32_long https://github.com/openresty/lua-nginx-module#ngxcrc32_long
+-- @field hmac_sha1 https://github.com/openresty/lua-nginx-module#ngxhmac_sha1
+-- @field md5 https://github.com/openresty/lua-nginx-module#ngxmd5
+-- @field md5_bin https://github.com/openresty/lua-nginx-module#ngxmd5_bin
+-- @field sha1_bin https://github.com/openresty/lua-nginx-module#ngxsha1_bin
+-- @field quote_sql_str https://github.com/openresty/lua-nginx-module#ngxquote_sql_str
+-- @field today https://github.com/openresty/lua-nginx-module#ngxtoday
+-- @field time https://github.com/openresty/lua-nginx-module#ngxtime
+-- @field now https://github.com/openresty/lua-nginx-module#ngxnow
+-- @field localtime https://github.com/openresty/lua-nginx-module#ngxlocaltime
+-- @field utctime https://github.com/openresty/lua-nginx-module#ngxutctime
+-- @field cookie_time https://github.com/openresty/lua-nginx-module#ngxcookie_time
+-- @field http_time https://github.com/openresty/lua-nginx-module#ngxhttp_time
+-- @field parse_http_time https://github.com/openresty/lua-nginx-module#ngxparse_http_time
+local liquid_filters = {
+  escape_uri = ngx.escape_uri,
+  unescape_uri = ngx.unescape_uri,
+  encode_base64 = ngx.encode_base64,
+  decode_base64 = ngx.decode_base64,
+  crc32_short = ngx.crc32_short,
+  crc32_long = ngx.crc32_long,
+  hmac_sha1 = ngx.hmac_sha1,
+  md5 = ngx.md5,
+  md5_bin = ngx.md5_bin,
+  sha1_bin = ngx.sha1_bin,
+  quote_sql_str = ngx.quote_sql_str,
+  today = ngx.today,
+  time = ngx.time,
+  now = ngx.now,
+  localtime = ngx.localtime,
+  utctime = ngx.utctime,
+  cookie_time = ngx.cookie_time,
+  http_time = ngx.http_time,
+  parse_http_time = ngx.parse_http_time
+}
+
+local liquid_filter_set = LiquidFilterSet:new()
+for name, func in pairs(liquid_filters) do
+  liquid_filter_set:add_filter(name, func)
+end
+
+-- Set resource limits to avoid loops
+local liquid_resource_limit = LiquidResourceLimit:new(nil, nil, 0)
+
+function LiquidTemplateString.new(string)
+  return setmetatable({ template = LiquidTemplate:parse(string) },
+                      liquid_template_string_mt)
+end
+
+function LiquidTemplateString:render(context)
+  return self.template:render(
+    LiquidInterpreterContext:new(context),
+    liquid_filter_set,
+    liquid_resource_limit
+  )
+end
+
+local PlainTemplateString = {}
+local plain_template_string_mt = { __index = PlainTemplateString }
+
+function PlainTemplateString.new(string)
+  return setmetatable({ string = string }, plain_template_string_mt)
+end
+
+function PlainTemplateString:render()
+  return self.string
+end
+
+local template = {
+  plain = PlainTemplateString,
+  liquid = LiquidTemplateString
+}
+
+local _M = {}
+
+--- Initialize a template
+-- Initialize a liquid or a plain text template according to the given type.
+-- @tparam string value String to construct the template from
+-- @tparam string type Render the template as this type.
+--   Can be 'liquid' or 'plain'
+-- @treturn a template string and nil, err when an invalid type is given
+function _M.new(value, type)
+  local template_mod = template[type]
+
+  if template_mod then
+    return template_mod.new(value)
+  else
+    return nil, format('Invalid type specified: %s', type)
+  end
+end
+
+return _M

spec/policy/headers/headers_spec.lua

@@ -105,6 +105,55 @@ describe('Headers policy', function()
         assert.same({ '2', '3' }, ngx.req.get_headers()[header])
       end)
     end)
+
+    describe('when the type of the value is specified', function()
+      describe("and it is 'liquid'", function()
+        it('evaluates the value as liquid', function()
+          local context = { var_in_context = 'some_value' }
+
+          local config = {
+            [request_headers] = {
+              {
+                op = 'push',
+                header = header,
+                value = '{{ var_in_context }}',
+                value_type = 'liquid'
+              }
+            }
+          }
+
+          local headers_policy = HeadersPolicy.new(config)
+
+          headers_policy:rewrite(context)
+
+          assert.same({ context.var_in_context }, ngx.req.get_headers()[header])
+        end)
+      end)
+
+      describe("and it is 'plain'", function()
+        it('evaluates the value as plain text', function()
+          local context = { var_in_context = 'some_value' }
+          local value = '{{ var_in_context }}'
+
+          local config = {
+            [request_headers] = {
+              {
+                op = 'push',
+                header = header,
+                value = value,
+                value_type = 'plain'
+              }
+            }
+          }
+
+          local headers_policy = HeadersPolicy.new(config)
+
+          headers_policy:rewrite(context)
+
+          assert.same({ value }, ngx.req.get_headers()[header])
+        end)
+      end)
+    end)
   end)
 
   describe('.header_filter', function()
@@ -201,5 +250,54 @@ describe('Headers policy', function()
         assert.same({ '2', '3' }, ngx.header[header])
       end)
     end)
+
+    describe('when the type of the value is specified', function()
+      describe("and it is 'liquid'", function()
+        it('evaluates the value as liquid', function()
+          local context = { var_in_context = 'some_value' }
+
+          local config = {
+            [response_headers] = {
+              {
+                op = 'push',
+                header = header,
+                value = '{{ var_in_context }}',
+                value_type = 'liquid'
+              }
+            }
+          }
+
+          local headers_policy = HeadersPolicy.new(config)
+
+          headers_policy:header_filter(context)
+
+          assert.same({ context.var_in_context }, ngx.header[header])
+        end)
+      end)
+
+      describe("and it is 'plain'", function()
+        it('evaluates the value as plain text', function()
+          local context = { var_in_context = 'some_value' }
+          local value = '{{ var_in_context }}'
+
+          local config = {
+            [response_headers] = {
+              {
+                op = 'push',
+                header = header,
+                value = value,
+                value_type = 'plain'
+              }
+            }
+          }
+
+          local headers_policy = HeadersPolicy.new(config)
+
+          headers_policy:header_filter(context)
+
+          assert.same({ value }, ngx.header[header])
+        end)
+      end)
+    end)
   end)
 end)

spec/template_string_spec.lua

@@ -0,0 +1,26 @@
+local TemplateString = require 'apicast.template_string'
+
+describe('template string', function()
+  it('renders plain text', function()
+    local plain_text_template = TemplateString.new('{{ a_key }}', 'plain')
+    assert.equals('{{ a_key }}', plain_text_template:render())
+  end)
+
+  it('renders liquid', function()
+    local liquid_template = TemplateString.new('{{ a_key }}', 'liquid')
+    assert.equals('a_value', liquid_template:render({ a_key = 'a_value' }))
+  end)
+
+  it('can apply liquid filters', function()
+    local liquid_template = TemplateString.new('{{ "something" | md5 }}', 'liquid')
+    assert.equals(ngx.md5('something'), liquid_template:render({}))
+  end)
+
+  describe('.new', function()
+    it('returns nil and an error with invalid type', function()
+      local template, err = TemplateString.new('some_string', 'invalid_type')
+      assert.is_nil(template)
+      assert.is_not_nil(err)
+    end)
+  end)
+end)