Fix the upstream OAuth 2.0 callback form deserialisation (#4010)

Fixes #3957

This was broken since #3893

crates/handlers/src/upstream_oauth2/callback.rs

@@ -41,32 +41,33 @@ use crate::{impl_from_error_for_route, upstream_oauth2::cache::MetadataCache, Pr
 
 #[derive(Serialize, Deserialize)]
 pub struct Params {
-    state: String,
+    state: Option<String>,
 
     /// An extra parameter to track whether the POST request was re-made by us
     /// to the same URL to escape Same-Site cookies restrictions
     #[serde(default)]
     did_mas_repost_to_itself: bool,
 
+    code: Option<String>,
+
+    error: Option<ClientErrorCode>,
+    error_description: Option<String>,
+    #[allow(dead_code)]
+    error_uri: Option<String>,
+
     #[serde(flatten)]
-    code_or_error: CodeOrError,
+    extra_callback_parameters: Option<serde_json::Value>,
 }
 
-#[derive(Serialize, Deserialize)]
-#[serde(untagged)]
-enum CodeOrError {
-    Code {
-        code: String,
-
-        #[serde(flatten)]
-        extra_callback_parameters: Option<serde_json::Value>,
-    },
-    Error {
-        error: ClientErrorCode,
-        error_description: Option<String>,
-        #[allow(dead_code)]
-        error_uri: Option<String>,
-    },
+impl Params {
+    /// Returns true if none of the fields are set
+    pub fn is_empty(&self) -> bool {
+        self.state.is_none()
+            && self.code.is_none()
+            && self.error.is_none()
+            && self.error_description.is_none()
+            && self.error_uri.is_none()
+    }
 }
 
 #[derive(Debug, Error)]
@@ -86,6 +87,12 @@ pub(crate) enum RouteError {
     #[error("State parameter mismatch")]
     StateMismatch,
 
+    #[error("Missing state parameter")]
+    MissingState,
+
+    #[error("Missing code parameter")]
+    MissingCode,
+
     #[error("Could not extract subject from ID token")]
     ExtractSubject(#[source] minijinja::Error),
 
@@ -161,7 +168,7 @@ pub(crate) async fn handler(
     PreferredLanguage(locale): PreferredLanguage,
     cookie_jar: CookieJar,
     Path(provider_id): Path<Ulid>,
-    Form(params): Form<Option<Params>>,
+    Form(params): Form<Params>,
 ) -> Result<Response, RouteError> {
     let provider = repo
         .upstream_oauth_provider()
@@ -172,7 +179,7 @@ pub(crate) async fn handler(
 
     let sessions_cookie = UpstreamSessionsCookie::load(&cookie_jar);
 
-    let Some(params) = params else {
+    if params.is_empty() {
         if let Method::GET = method {
             return Err(RouteError::MissingQueryParams);
         }
@@ -204,8 +211,19 @@ pub(crate) async fn handler(
         (Some(expected), _) => return Err(RouteError::InvalidResponseMode { expected }),
     }
 
+    if let Some(error) = params.error {
+        return Err(RouteError::ClientError {
+            error,
+            error_description: params.error_description.clone(),
+        });
+    }
+
+    let Some(state) = params.state else {
+        return Err(RouteError::MissingState);
+    };
+
     let (session_id, _post_auth_action) = sessions_cookie
-        .find_session(provider_id, &params.state)
+        .find_session(provider_id, &state)
         .map_err(|_| RouteError::MissingCookie)?;
 
     let session = repo
@@ -219,7 +237,7 @@ pub(crate) async fn handler(
         return Err(RouteError::ProviderMismatch);
     }
 
-    if params.state != session.state_str {
+    if state != session.state_str {
         // The state in the session cookie should match the one from the params
         return Err(RouteError::StateMismatch);
     }
@@ -230,21 +248,8 @@ pub(crate) async fn handler(
     }
 
     // Let's extract the code from the params, and return if there was an error
-    let (code, extra_callback_parameters) = match params.code_or_error {
-        CodeOrError::Error {
-            error,
-            error_description,
-            ..
-        } => {
-            return Err(RouteError::ClientError {
-                error,
-                error_description,
-            })
-        }
-        CodeOrError::Code {
-            code,
-            extra_callback_parameters,
-        } => (code, extra_callback_parameters),
+    let Some(code) = params.code else {
+        return Err(RouteError::MissingCode);
     };
 
     let mut lazy_metadata = LazyProviderInfos::new(&metadata_cache, &provider, &client);
@@ -326,7 +331,7 @@ pub(crate) async fn handler(
         context = context.with_id_token_claims(claims);
     }
 
-    if let Some(extra_callback_parameters) = extra_callback_parameters.clone() {
+    if let Some(extra_callback_parameters) = params.extra_callback_parameters.clone() {
         context = context.with_extra_callback_parameters(extra_callback_parameters);
     }
 
@@ -432,7 +437,7 @@ pub(crate) async fn handler(
             session,
             &link,
             token_response.id_token,
-            extra_callback_parameters,
+            params.extra_callback_parameters,
             userinfo,
         )
         .await?;