Set a Path on the CSRF cookie (#944)

Without setting a Path, the UA will infer the Path from the URL path,
meaning that it will take the "directory name" of a URL path (so for a
response for a resource at `/index`, the path would be inferred as `/`,
and for a response for `/foo/bar` it would be inferred to be `/foo/`
etc.).

This is problematic with CSRF for multiple reasons:

- It causes a proliferation of CSRF cookies which pollute client-side
  storage.
- It breaks CSRF when requests are sent across paths. For example, if a
  resource at `/foo/bar` contains a form which submits to `/index`, they
  would be theoretically using different CSRF states.
- Poem only processes a single cookie, which means that we have to rely
  on the ordering specified in RFC 6265 Section 5.4. This is bad for two
  reasons:

  1. The ordering is only a SHOULD, not a MUST.
  2. Poem does, in fact, not process cookies in the correct ordering
     (this is subject of another commit to fix).

Note that this commit may break applications if they share a CSRF cookie
name with other applications hosted at different paths on the same
domain.

poem/src/middleware/csrf.rs

@@ -1,4 +1,4 @@
-use std::{sync::Arc, time::Duration};
+use std::{borrow::Cow, sync::Arc, time::Duration};
 
 use base64::{engine::general_purpose::STANDARD, Engine};
 use libcsrf::{
@@ -83,6 +83,10 @@ pub struct Csrf {
     http_only: bool,
     same_site: Option<SameSite>,
     ttl: Duration,
+    // Using Arc<Cow<'static, _>> here because the path is most likely going
+    // to be a static str, and if it's not, we don't want to have to copy it
+    // into every endpoint.
+    path: Arc<Cow<'static, str>>,
 }
 
 impl Default for Csrf {
@@ -94,6 +98,7 @@ impl Default for Csrf {
             http_only: true,
             same_site: Some(SameSite::Strict),
             ttl: Duration::from_secs(24 * 60 * 60),
+            path: Arc::new(Cow::Borrowed("/")),
         }
     }
 }
@@ -147,6 +152,17 @@ impl Csrf {
         }
     }
 
+    /// Set the path for which the CSRF cookie should be set.
+    ///
+    /// By default, this is `"/"`.
+    #[must_use]
+    pub fn path(self, value: impl Into<Cow<'static, str>>) -> Self {
+        Self {
+            path: Arc::new(value.into()),
+            ..self
+        }
+    }
+
     /// Sets the protection ttl. This will be used for both the cookie
     /// expiry and the time window over which CSRF tokens are considered
     /// valid.
@@ -170,6 +186,7 @@ impl<E: Endpoint> Middleware<E> for Csrf {
             http_only: self.http_only,
             same_site: self.same_site,
             ttl: self.ttl,
+            path: Arc::clone(&self.path),
         })
     }
 }
@@ -184,6 +201,7 @@ pub struct CsrfEndpoint<E> {
     http_only: bool,
     same_site: Option<SameSite>,
     ttl: Duration,
+    path: Arc<Cow<'static, str>>,
 }
 
 impl<E> CsrfEndpoint<E> {
@@ -226,6 +244,7 @@ impl<E: Endpoint> Endpoint for CsrfEndpoint<E> {
             cookie.set_http_only(self.http_only);
             cookie.set_same_site(self.same_site);
             cookie.set_max_age(self.ttl);
+            cookie.set_path(&**self.path);
             cookie
         };