TraceMachina
diff --git a/‎nativelink-store/src/fast_slow_store.rs‎
Lines changed: 118 additions & 5 deletions b/‎nativelink-store/src/fast_slow_store.rs‎
Lines changed: 118 additions & 5 deletions
@@ -65,6 +65,12 @@ pub struct FastSlowStore {
     // actually it's faster because we're not downloading the file multiple
     // times are doing loads of duplicate IO.
     populating_digests: Mutex<HashMap<StoreKey<'static>, Loader>>,
+    // Tracks keys whose slow-store write is currently in flight, along with
+    // the best-known size. Consulted by `has_with_results` so that a
+    // concurrent writer that has not yet finished pushing to the slow store
+    // is still visible to a concurrent existence check, preventing redundant
+    // duplicate uploads of the same blob.
+    in_flight_slow_writes: Mutex<HashMap<StoreKey<'static>, u64>>,
 }
 
 // This guard ensures that the populating_digests is cleared even if the future
@@ -91,6 +97,27 @@ impl LoaderGuard<'_> {
     }
 }
 
+/// Cancel-safe RAII guard that removes an entry from
+/// `FastSlowStore::in_flight_slow_writes` when dropped. This ensures the
+/// map does not leak entries if the surrounding `update()` future is
+/// cancelled before the slow-store write completes.
+struct InFlightSlowWriteGuard {
+    weak_store: Weak<FastSlowStore>,
+    key: Option<StoreKey<'static>>,
+}
+
+impl Drop for InFlightSlowWriteGuard {
+    fn drop(&mut self) {
+        let Some(store) = self.weak_store.upgrade() else {
+            return;
+        };
+        let Some(key) = self.key.take() else {
+            return;
+        };
+        store.in_flight_slow_writes.lock().remove(&key);
+    }
+}
+
 impl Drop for LoaderGuard<'_> {
     fn drop(&mut self) {
         let Some(store) = self.weak_store.upgrade() else {
@@ -126,9 +153,40 @@ impl FastSlowStore {
             weak_self: weak_self.clone(),
             metrics: FastSlowStoreMetrics::default(),
             populating_digests: Mutex::new(HashMap::new()),
+            in_flight_slow_writes: Mutex::new(HashMap::new()),
         })
     }
 
+    /// Best-effort size for tracking in-flight slow writes. Falls back to 0
+    /// when neither the upload size nor the key carries an exact size
+    /// (e.g. `MaxSize` for a string key). Callers of `has_with_results` only
+    /// rely on `Some(_)` vs `None`, so a size of 0 still correctly signals
+    /// "this blob exists".
+    const fn track_size(key: &StoreKey<'_>, size_info: UploadSizeInfo) -> u64 {
+        match size_info {
+            UploadSizeInfo::ExactSize(s) => s,
+            UploadSizeInfo::MaxSize(_) => match key {
+                StoreKey::Digest(d) => d.size_bytes(),
+                StoreKey::Str(_) => 0,
+            },
+        }
+    }
+
+    fn register_in_flight_slow_write(
+        &self,
+        key: StoreKey<'_>,
+        size: u64,
+    ) -> InFlightSlowWriteGuard {
+        let owned = key.into_owned();
+        self.in_flight_slow_writes
+            .lock()
+            .insert(owned.borrow().into_owned(), size);
+        InFlightSlowWriteGuard {
+            weak_store: self.weak_self.clone(),
+            key: Some(owned),
+        }
+    }
+
     pub const fn fast_store(&self) -> &Store {
         &self.fast_store
     }
@@ -365,11 +423,55 @@ impl StoreDriver for FastSlowStore {
         if slow_store.optimized_for(StoreOptimizations::NoopDownloads) {
             return self.fast_store.has_with_results(key, results).await;
         }
-        // Only check the slow store because if it's not there, then something
-        // down stream might be unable to get it.  This should not affect
-        // workers as they only use get() and a CAS can use an
-        // ExistenceCacheStore to avoid the bottleneck.
-        self.slow_store.has_with_results(key, results).await
+        // Primary lookup is the slow store because that's authoritative for
+        // downstream consumers that fetch from there. But the slow store
+        // alone can miss two important cases:
+        //   1. A concurrent writer's slow-store write is still in flight.
+        //   2. The blob is present in the fast (local) store — either
+        //      fast-only by configuration, or because the slow write has
+        //      not yet started/completed.
+        // Reporting NotFound in those cases causes redundant duplicate
+        // uploads or unnecessary slow-store fetches.
+        self.slow_store.has_with_results(key, results).await?;
+
+        // Fill in any blobs whose slow-store write is currently in flight.
+        // Cheap when the map is empty (the common case).
+        {
+            let in_flight = self.in_flight_slow_writes.lock();
+            if !in_flight.is_empty() {
+                for (k, result) in key.iter().zip(results.iter_mut()) {
+                    if result.is_none() {
+                        let owned = k.borrow().into_owned();
+                        if let Some(size) = in_flight.get(&owned) {
+                            *result = Some(*size);
+                        }
+                    }
+                }
+            }
+        }
+
+        // Fall back to the fast store for anything still missing. This
+        // covers fast-only writes and the brief window between fast-store
+        // insertion and slow-store write start.
+        let missing_indices: Vec<usize> = results
+            .iter()
+            .enumerate()
+            .filter_map(|(i, r)| if r.is_none() { Some(i) } else { None })
+            .collect();
+        if !missing_indices.is_empty() {
+            let missing_keys: Vec<StoreKey<'_>> =
+                missing_indices.iter().map(|&i| key[i].borrow()).collect();
+            let mut fast_results = vec![None; missing_keys.len()];
+            self.fast_store
+                .has_with_results(&missing_keys, &mut fast_results)
+                .await?;
+            for (j, &orig_idx) in missing_indices.iter().enumerate() {
+                if fast_results[j].is_some() {
+                    results[orig_idx] = fast_results[j];
+                }
+            }
+        }
+        Ok(())
     }
 
     async fn update(
@@ -405,9 +507,14 @@ impl StoreDriver for FastSlowStore {
             return self.fast_store.update(key, reader, size_info).await;
         }
         if ignore_fast {
+            let _guard =
+                self.register_in_flight_slow_write(key.borrow(), Self::track_size(&key, size_info));
             return self.slow_store.update(key, reader, size_info).await;
         }
 
+        let _slow_in_flight_guard =
+            self.register_in_flight_slow_write(key.borrow(), Self::track_size(&key, size_info));
+
         let (mut fast_tx, fast_rx) = make_buf_channel_pair();
         let (mut slow_tx, slow_rx) = make_buf_channel_pair();
 
@@ -549,6 +656,10 @@ impl StoreDriver for FastSlowStore {
             {
                 trace!("FastSlowStore::update_with_whole_file: uploading to slow_store");
                 let slow_start = std::time::Instant::now();
+                let _guard = self.register_in_flight_slow_write(
+                    key.borrow(),
+                    Self::track_size(&key, upload_size),
+                );
                 slow_update_store_with_file(
                     self.slow_store.as_store_driver_pin(),
                     key.borrow(),
@@ -598,6 +709,8 @@ impl StoreDriver for FastSlowStore {
             if ignore_slow {
                 return Ok(Some(file));
             }
+            let _guard = self
+                .register_in_flight_slow_write(key.borrow(), Self::track_size(&key, upload_size));
             return self
                 .slow_store
                 .update_with_whole_file(key, path, file, upload_size)