Have a better error on registration if the username is already taken

crates/handlers/src/views/register.rs

@@ -26,7 +26,7 @@ use mas_axum_utils::{
 };
 use mas_config::Encrypter;
 use mas_router::Route;
-use mas_storage::user::{register_user, start_session};
+use mas_storage::user::{register_user, start_session, username_exists};
 use mas_templates::{
     FieldError, FormError, RegisterContext, RegisterFormField, TemplateContext, Templates,
     ToFormState,
@@ -96,6 +96,8 @@ pub(crate) async fn post(
 
         if form.username.is_empty() {
             state.add_error_on_field(RegisterFormField::Username, FieldError::Required);
+        } else if username_exists(&mut txn, &form.username).await? {
+            state.add_error_on_field(RegisterFormField::Username, FieldError::Exists);
         }
 
         if form.password.is_empty() {

crates/storage/sqlx-data.json

@@ -1974,6 +1974,26 @@
     },
     "query": "\n            UPDATE compat_sessions\n            SET deleted_at = NOW()\n            FROM compat_access_tokens\n            WHERE compat_access_tokens.token = $1\n              AND compat_sessions.id = compat_access_tokens.id \n              AND compat_sessions.deleted_at IS NULL\n        "
   },
+  "af77bad7259175464c5ad57f9662571c17b29552ebb70e4b6022584b41bdff0d": {
+    "describe": {
+      "columns": [
+        {
+          "name": "exists!",
+          "ordinal": 0,
+          "type_info": "Bool"
+        }
+      ],
+      "nullable": [
+        null
+      ],
+      "parameters": {
+        "Left": [
+          "Text"
+        ]
+      }
+    },
+    "query": "\n            SELECT EXISTS(\n                SELECT 1 FROM users WHERE username = $1\n            ) AS \"exists!\"\n        "
+  },
   "b0fec01072df856ba9cd8be0ecf7a58dd4709a0efca4035a2c6f99c43d5a12be": {
     "describe": {
       "columns": [

crates/storage/src/user.rs

@@ -476,6 +476,22 @@ pub async fn lookup_user_by_username(
     })
 }
 
+pub async fn username_exists(
+    executor: impl PgExecutor<'_>,
+    username: &str,
+) -> Result<bool, sqlx::Error> {
+    sqlx::query_scalar!(
+        r#"
+            SELECT EXISTS(
+                SELECT 1 FROM users WHERE username = $1
+            ) AS "exists!"
+        "#,
+        username
+    )
+    .fetch_one(executor)
+    .await
+}
+
 #[derive(Debug, Clone)]
 struct UserEmailLookup {
     user_email_id: i64,

crates/templates/src/forms.rs

@@ -27,11 +27,14 @@ pub trait FormField: Copy + Hash + PartialEq + Eq + Serialize + for<'de> Deseria
 #[derive(Debug, Serialize)]
 #[serde(rename_all = "snake_case", tag = "kind")]
 pub enum FieldError {
-    /// A reuired field is missing
+    /// A required field is missing
     Required,
 
     /// An unspecified error on the field
     Unspecified,
+
+    /// That value already exists
+    Exists,
 }
 
 /// An error on the whole form

crates/templates/src/res/components/field.html

@@ -44,6 +44,8 @@ limitations under the License.
           <div class="mx-4 text-sm text-alert">
             {% if error.kind == "required" %}
               This field is required
+            {% elif error.kind == "exists" and name == "username" %}
+              This username is already taken
             {% else %}
               {{ error.kind }}
             {% endif %}