Preserve semantic reader after failed generation rebuild

crates/searchi-service/src/lib.rs

@@ -289,22 +289,36 @@ impl HybridSearch {
     /// storage errors leave the previous generation active. A crashed build is
     /// reclaimed on the next explicit rebuild under an OS advisory lock.
     pub fn index_embeddings(&mut self) -> Result<EmbeddingReport> {
-        let previous_fingerprint = self.generation_fingerprint.clone();
-        self.generation_fingerprint = format!(
+        let fingerprint = format!(
             "{}:text-only",
             generation_fingerprint(self.embedder.fingerprint())
         );
+        self.rebuild_generation(fingerprint, Self::build)
+    }
+
+    /// Keep the reader's policy aligned with the active generation on every
+    /// failure boundary, including lock admission, activation and cleanup.
+    fn rebuild_generation(
+        &mut self,
+        fingerprint: String,
+        build: impl FnOnce(&mut Self, i64) -> Result<EmbeddingReport>,
+    ) -> Result<EmbeddingReport> {
         let _lock = self.store.build_lock()?;
-        let generation = self.store.begin(&self.generation_fingerprint)?;
-        let built = self.build(generation);
+        let generation = self.store.begin(&fingerprint)?;
+        let previous_fingerprint = std::mem::replace(&mut self.generation_fingerprint, fingerprint);
+        let built = build(self, generation).and_then(|report| {
+            self.store.activate(generation)?;
+            Ok(report)
+        });
         match built {
-            Ok(report) => {
-                self.store.activate(generation)?;
-                Ok(report)
-            }
+            Ok(report) => Ok(report),
             Err(error) => {
-                self.store.abandon(generation)?;
                 self.generation_fingerprint = previous_fingerprint;
+                if let Err(cleanup) = self.store.abandon(generation) {
+                    return Err(error.context(format!(
+                        "Failed to discard staging generation {generation}: {cleanup:#}"
+                    )));
+                }
                 Err(error)
             }
         }