refactor: simplify recent changes — extract helper, named struct, atomic entry, pre-alloc

- Extract auth::create_and_send_magic_link() to deduplicate token rollback logic
- Replace (i32, i32, RateLimiter) tuple with named UserRateLimitEntry struct
- Use DashMap entry API for atomic rate limiter lookup (fixes TOCTOU race)
- Pre-allocate scraper body Vec from Content-Length when available

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/app_state.rs

@@ -23,13 +23,20 @@ pub struct AppState {
     pub provider_rate_limiter: ProviderRateLimiter,
     /// Per-user rate limiters for generation jobs.
     /// Keyed by user_id. Created on first generation, replaced if settings change.
-    /// Tuple: (max_requests, window_seconds, limiter).
-    pub user_rate_limiters: DashMap<Uuid, (i32, i32, RateLimiter)>,
+    pub user_rate_limiters: DashMap<Uuid, UserRateLimitEntry>,
     /// In-memory store for active generation jobs.
     /// Maps job_id -> progress watch channel.
     pub job_store: JobStore,
 }
 
+/// Cached per-user rate limiter with the settings it was created from.
+#[derive(Clone)]
+pub struct UserRateLimitEntry {
+    pub max_requests: i32,
+    pub window_seconds: i32,
+    pub limiter: RateLimiter,
+}
+
 impl AppState {
     /// Create a new `AppState` instance.
     pub fn new(config: AppConfig, pool: PgPool, http_client: reqwest::Client) -> Self {

backend/src/handlers/auth.rs

@@ -21,7 +21,7 @@ use crate::db;
 use crate::errors::AppError;
 use crate::middleware::auth::AuthUser;
 use crate::models::user::{MeResponse, UserRole};
-use crate::services::{auth, email, turnstile};
+use crate::services::{auth, turnstile};
 use crate::util::token;
 
 /// Generic auth response used for register and login.
@@ -96,25 +96,16 @@ pub async fn register(
         tracing::info!(email = %email_lower, "New user registered");
     }
 
-    // Generate and send magic link — roll back token if email send fails
-    if let Some(raw_token) = auth::create_magic_link(&state.pool, &email_lower).await? {
-        if let Err(e) = email::send_magic_link(
-            &state.http_client,
-            &state.config.resend_api_key,
-            &state.config.email_from,
-            &email_lower,
-            &state.config.app_url,
-            &raw_token,
-        )
-        .await
-        {
-            let token_hash = crate::util::token::hash_token(&raw_token);
-            db::magic_links::delete_by_hash(&state.pool, &token_hash)
-                .await
-                .ok();
-            return Err(e);
-        }
-    }
+    // Generate and send magic link (rolls back token if email delivery fails)
+    auth::create_and_send_magic_link(
+        &state.pool,
+        &state.http_client,
+        &state.config.resend_api_key,
+        &state.config.email_from,
+        &state.config.app_url,
+        &email_lower,
+    )
+    .await?;
 
     Ok((
         StatusCode::OK,
@@ -157,24 +148,15 @@ pub async fn login(
     let existing = db::users::find_by_email(&state.pool, &email_lower).await?;
 
     if existing.is_some() {
-        if let Some(raw_token) = auth::create_magic_link(&state.pool, &email_lower).await? {
-            if let Err(e) = email::send_magic_link(
-                &state.http_client,
-                &state.config.resend_api_key,
-                &state.config.email_from,
-                &email_lower,
-                &state.config.app_url,
-                &raw_token,
-            )
-            .await
-            {
-                let token_hash = crate::util::token::hash_token(&raw_token);
-                db::magic_links::delete_by_hash(&state.pool, &token_hash)
-                    .await
-                    .ok();
-                return Err(e);
-            }
-        }
+        auth::create_and_send_magic_link(
+            &state.pool,
+            &state.http_client,
+            &state.config.resend_api_key,
+            &state.config.email_from,
+            &state.config.app_url,
+            &email_lower,
+        )
+        .await?;
     } else {
         // Add a small random delay to prevent timing attacks
         let delay_ms = rand::random::<u64>() % 150 + 50; // 50-200ms

backend/src/services/auth.rs

@@ -10,6 +10,7 @@ use uuid::Uuid;
 use crate::db;
 use crate::errors::AppError;
 use crate::models::user::{User, UserRole};
+use crate::services::email;
 use crate::util::token;
 
 /// Magic link token expiry: 15 minutes.
@@ -51,6 +52,32 @@ pub async fn create_magic_link(pool: &PgPool, email: &str) -> Result<Option<Stri
     Ok(Some(raw_token))
 }
 
+/// Create a magic link token, send it by email, and roll back if delivery fails.
+///
+/// Combines token creation and email sending with automatic cleanup: if the
+/// email fails to send, the token is deleted so it doesn't consume a quota slot.
+pub async fn create_and_send_magic_link(
+    pool: &PgPool,
+    http_client: &reqwest::Client,
+    resend_api_key: &str,
+    email_from: &str,
+    app_url: &str,
+    to_email: &str,
+) -> Result<(), AppError> {
+    let Some(raw_token) = create_magic_link(pool, to_email).await? else {
+        return Ok(());
+    };
+
+    if let Err(e) =
+        email::send_magic_link(http_client, resend_api_key, email_from, to_email, app_url, &raw_token).await
+    {
+        let token_hash = token::hash_token(&raw_token);
+        db::magic_links::delete_by_hash(pool, &token_hash).await.ok();
+        return Err(e);
+    }
+    Ok(())
+}
+
 /// Verify a magic link token and return the associated email.
 ///
 /// The token is consumed atomically (marked as used) to enforce single-use.

backend/src/services/scraper.rs

@@ -146,16 +146,20 @@ pub async fn scrape_url(
     let final_url = response.url().clone();
 
     // Read body with streaming size limit — bail early to avoid OOM on huge responses.
-    // Check Content-Length first (fast path), then enforce while streaming chunks.
-    if let Some(content_length) = response.content_length() {
-        if content_length as usize > MAX_BODY_SIZE {
+    // Check Content-Length first (fast reject + pre-allocation), then enforce per chunk.
+    let content_length = response.content_length();
+    if let Some(len) = content_length {
+        if len as usize > MAX_BODY_SIZE {
             return Err(AppError::BadRequest(
                 "Response body exceeds 5 MB limit".into(),
             ));
         }
     }
 
-    let mut bytes = Vec::new();
+    let mut bytes = match content_length {
+        Some(len) => Vec::with_capacity(len as usize),
+        None => Vec::new(),
+    };
     while let Some(chunk) = response
         .chunk()
         .await

backend/src/services/synthesis.rs

@@ -383,38 +383,44 @@ fn emit_progress(tx: &watch::Sender<ProgressEvent>, step: &str, message: &str, p
 /// Returns `None` if the user has no rate limit overrides, in which case the
 /// global provider rate limiter should be used instead.
 ///
-/// The limiter is stored in `state.user_rate_limiters` keyed by `user_id` so that
-/// rate limit history persists across generation jobs. If the user's settings have
-/// changed since the limiter was created, a fresh limiter replaces the old one.
+/// Uses DashMap's entry API for atomic check-and-insert, preventing concurrent
+/// generation jobs from creating independent limiters for the same user.
 fn get_user_rate_limiter(
     state: &AppState,
     settings: &UserSettings,
     user_id: Uuid,
 ) -> Option<crate::services::rate_limiter::RateLimiter> {
+    use crate::app_state::UserRateLimitEntry;
+
     match (
         settings.rate_limit_max_requests,
         settings.rate_limit_time_window_seconds,
     ) {
         (Some(max_req), Some(window_sec)) => {
-            // Reuse existing limiter if settings haven't changed
-            if let Some(entry) = state.user_rate_limiters.get(&user_id) {
-                let (stored_max, stored_window, ref limiter) = *entry;
-                if stored_max == max_req && stored_window == window_sec {
-                    return Some(limiter.clone());
+            let mut entry = state.user_rate_limiters.entry(user_id).or_insert_with(|| {
+                UserRateLimitEntry {
+                    max_requests: max_req,
+                    window_seconds: window_sec,
+                    limiter: crate::services::rate_limiter::RateLimiter::new(
+                        max_req as usize,
+                        Duration::from_secs(window_sec as u64),
+                    ),
                 }
+            });
+            // Replace if user's settings changed since the limiter was created
+            if entry.max_requests != max_req || entry.window_seconds != window_sec {
+                *entry = UserRateLimitEntry {
+                    max_requests: max_req,
+                    window_seconds: window_sec,
+                    limiter: crate::services::rate_limiter::RateLimiter::new(
+                        max_req as usize,
+                        Duration::from_secs(window_sec as u64),
+                    ),
+                };
             }
-            // Settings changed or first generation — create and store
-            let limiter = crate::services::rate_limiter::RateLimiter::new(
-                max_req as usize,
-                Duration::from_secs(window_sec as u64),
-            );
-            state
-                .user_rate_limiters
-                .insert(user_id, (max_req, window_sec, limiter.clone()));
-            Some(limiter)
+            Some(entry.limiter.clone())
         }
         _ => {
-            // User cleared their overrides — remove stale limiter
             state.user_rate_limiters.remove(&user_id);
             None
         }