fix(analysis): hold the OCR permit for the full blocking inference

The OCR dispatcher acquired a borrowed semaphore permit, then ran the
CPU-bound inference in `spawn_blocking`. On cancellation (graceful
shutdown) the future dropped the permit while the detached blocking task
kept running, briefly oversubscribing the blocking pool past
ANALYSIS_WORKERS.

Extract `run_ocr_blocking`, which acquires an *owned* permit and moves it
into the blocking task so the concurrency slot's lifetime matches the
actual work. Covered by a cancellation-invariant test asserting the
permit stays held after the caller future is aborted.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.93.5"
+version = "0.93.6"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.93.5"
+version = "0.93.6"
 edition = "2021"
 default-run = "mangalord"
 

backend/src/analysis/ocr.rs

@@ -155,6 +155,37 @@ pub fn ocr_concurrency_limit(workers: usize, cores: usize) -> usize {
     workers.min(cores.max(1)).max(1)
 }
 
+/// Run a CPU-bound OCR closure on the blocking pool while holding an
+/// **owned** permit for the entire duration of the work.
+///
+/// The permit is acquired with `acquire_owned` and moved *into* the
+/// blocking task rather than being held by the caller's future. This
+/// matters on cancellation: if the dispatcher future is dropped (graceful
+/// shutdown), a borrowed permit would be released the instant the future
+/// unwinds — but `spawn_blocking` work is not cancellable and keeps
+/// running detached, so the concurrency bound (ANALYSIS_WORKERS) would be
+/// briefly exceeded. Moving the permit into the task ties the slot's
+/// lifetime to the actual CPU work.
+async fn run_ocr_blocking<T, F>(
+    permits: Arc<tokio::sync::Semaphore>,
+    f: F,
+) -> anyhow::Result<T>
+where
+    F: FnOnce() -> T + Send + 'static,
+    T: Send + 'static,
+{
+    let permit = permits
+        .acquire_owned()
+        .await
+        .map_err(|e| anyhow::anyhow!("OCR semaphore closed: {e}"))?;
+    tokio::task::spawn_blocking(move || {
+        let _permit = permit; // held until f() returns, even if the caller is cancelled
+        f()
+    })
+    .await
+    .map_err(|e| anyhow::anyhow!("OCR task join error: {e}"))
+}
+
 /// Production dispatcher for the OCR backend: load the page, read its image
 /// from storage, run OCR on the blocking pool, and persist the lines. Mirrors
 /// [`crate::analysis::daemon::RealAnalyzeDispatcher`] but with no network I/O.
@@ -192,15 +223,10 @@ impl AnalyzeDispatcher for OcrAnalyzeDispatcher {
         // OCR inference is CPU-bound and synchronous — keep it off the async
         // worker's runtime thread, and gate it behind the shared permit pool so
         // ANALYSIS_WORKERS > cores can't oversubscribe the blocking pool.
-        let _permit = self
-            .ocr_permits
-            .acquire()
-            .await
-            .map_err(|e| anyhow::anyhow!("OCR semaphore closed: {e}"))?;
         let engine = Arc::clone(&self.engine);
-        let lines = tokio::task::spawn_blocking(move || engine.recognize(&bytes))
-            .await
-            .map_err(|e| anyhow::anyhow!("OCR task join error: {e}"))??;
+        let lines =
+            run_ocr_blocking(Arc::clone(&self.ocr_permits), move || engine.recognize(&bytes))
+                .await??;
         let analysis = lines_to_analysis(lines);
         repo::page_analysis::persist_analysis(&self.db, page_id, &analysis, OCR_MODEL_LABEL).await?;
         Ok(())
@@ -235,6 +261,55 @@ pub mod test_support {
 mod tests {
     use super::*;
 
+    #[tokio::test]
+    async fn run_ocr_blocking_holds_permit_until_work_completes_despite_cancellation() {
+        use std::sync::mpsc;
+        use tokio::sync::{Notify, Semaphore};
+
+        let permits = Arc::new(Semaphore::new(1));
+        let (release_tx, release_rx) = mpsc::channel::<()>();
+        let started = Arc::new(Notify::new());
+
+        // Model the dispatch path: acquire an owned permit and run blocking
+        // work that we hold open via a channel.
+        let sem = Arc::clone(&permits);
+        let started2 = Arc::clone(&started);
+        let caller = tokio::spawn(async move {
+            run_ocr_blocking(sem, move || {
+                // Signal that the blocking task now holds the permit, then
+                // block until the test releases us.
+                started2.notify_one();
+                release_rx.recv().ok();
+            })
+            .await
+            .unwrap();
+        });
+
+        // Wait until the blocking task is running and owns the permit.
+        started.notified().await;
+        assert_eq!(permits.available_permits(), 0, "permit taken by blocking work");
+
+        // Cancel the caller future (simulates graceful shutdown). The
+        // blocking task is not cancellable and keeps running detached; with
+        // an *owned* permit the slot must stay occupied. A borrowed permit
+        // would have been released here, regressing the concurrency bound.
+        caller.abort();
+        let _ = caller.await;
+        assert_eq!(
+            permits.available_permits(),
+            0,
+            "owned permit must remain held by the still-running blocking task"
+        );
+
+        // Let the blocking work finish; the permit is then returned.
+        release_tx.send(()).unwrap();
+        let permit = tokio::time::timeout(std::time::Duration::from_secs(5), permits.acquire())
+            .await
+            .expect("permit should be released once blocking work completes")
+            .unwrap();
+        drop(permit);
+    }
+
     #[test]
     fn lines_to_analysis_maps_lines_in_order_and_leaves_rest_empty() {
         let v = lines_to_analysis(vec!["Hello".to_string(), "world!".to_string()]);

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.93.5",
+  "version": "0.93.6",
   "private": true,
   "type": "module",
   "scripts": {