feat(expressions): add contentLength to bot expressions (#1120)

Signed-off-by: Xe Iaso <me@xeiaso.net>

docs/docs/CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 <!-- This changes the project to: -->
 
+- Add `contentLength` variable to bot expressions.
 - Add `COOKIE_SAME_SITE_MODE` to force anubis cookies SameSite value, and downgrade automatically from `None` to `Lax` if cookie is insecure.
 - Fix lock convoy problem in decaymap ([#1103](https://github.com/TecharoHQ/anubis/issues/1103)).
 - Fix lock convoy problem in bbolt by implementing the actor pattern ([#1103](https://github.com/TecharoHQ/anubis/issues/1103)).

docs/docs/admin/configuration/expressions.mdx

@@ -103,6 +103,7 @@ Anubis exposes the following variables to expressions:
 | :-------------- | :-------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------- | :----------------------------------------------------------- |
 | `headers`       | `map[string, string]` | The [headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers) of the request being processed.                            | `{"User-Agent": "Mozilla/5.0 Gecko/20100101 Firefox/137.0"}` |
 | `host`          | `string`              | The [HTTP hostname](https://web.dev/articles/url-parts#host) the request is targeted to.                                                      | `anubis.techaro.lol`                                         |
+| `contentLength` | `int64`               | The numerical value of the `Content-Length` header.                                                                                           |
 | `load_1m`       | `double`              | The current system load average over the last one minute. This is useful for making [load-based checks](#using-the-system-load-average).      |
 | `load_5m`       | `double`              | The current system load average over the last five minutes. This is useful for making [load-based checks](#using-the-system-load-average).    |
 | `load_15m`      | `double`              | The current system load average over the last fifteen minutes. This is useful for making [load-based checks](#using-the-system-load-average). |

lib/policy/celchecker.go

@@ -61,6 +61,8 @@ func (cr *CELRequest) ResolveName(name string) (any, bool) {
 	switch name {
 	case "remoteAddress":
 		return cr.Header.Get("X-Real-Ip"), true
+	case "contentLength":
+		return cr.ContentLength, true
 	case "host":
 		return cr.Host, true
 	case "method":

lib/policy/expressions/environment.go

@@ -19,6 +19,7 @@ func BotEnvironment() (*cel.Env, error) {
 	return New(
 		// Variables exposed to CEL programs:
 		cel.Variable("remoteAddress", cel.StringType),
+		cel.Variable("contentLength", cel.IntType),
 		cel.Variable("host", cel.StringType),
 		cel.Variable("method", cel.StringType),
 		cel.Variable("userAgent", cel.StringType),