From f672d84951cf7dea4f47429475b109426f2be944 Mon Sep 17 00:00:00 2001
From: salmonik <salmonik@tuta.io>
Date: Wed, 4 Nov 2026 01:12:12 +0100
Subject: tts: run piper loading and synthesis on blocking threads

---
 src/tts.rs | 44 ++++++++++++++++++++++++++++----------------
 1 file changed, 28 insertions(+), 16 deletions(-)

(limited to 'src')

diff --git a/src/tts.rs b/src/tts.rs
index dbe0d88..4754054 100644
--- a/src/tts.rs
+++ b/src/tts.rs
@@ -1,12 +1,11 @@
 use std::io::Cursor;
 use std::path::PathBuf;
-use std::sync::OnceLock;
 use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::{Arc, Mutex, OnceLock};
 use std::time::{Duration, Instant};
 
 use dashmap::DashMap;
 use piper_rs::Piper;
-use tokio::sync::Mutex;
 use tracing::{debug, info, warn};
 
 use crate::error::{Error, Result};
@@ -44,7 +43,7 @@ const EVENT_TYPES: &[&str] = &[
 /// Held in poise `Data`. Piper is loaded lazily on first synthesis call.
 /// Cache maps full text string → resampled 48 kHz stereo PCM bytes.
 pub struct TtsState {
-    piper: OnceLock<Mutex<Piper>>,
+    piper: OnceLock<Arc<Mutex<Piper>>>,
     cache: DashMap<String, (Vec<u8>, Instant)>,
     cache_bytes: AtomicUsize,
     model_path: Option<PathBuf>,
@@ -62,9 +61,9 @@ impl TtsState {
 
     /// Get or initialize the Piper model. Returns error if model path is
     /// unset or model fails to load.
-    async fn get_piper(&self) -> Result<&Mutex<Piper>> {
+    async fn get_piper(&self) -> Result<Arc<Mutex<Piper>>> {
         if let Some(piper) = self.piper.get() {
-            return Ok(piper);
+            return Ok(piper.clone());
         }
 
         let path = self
@@ -88,14 +87,21 @@ impl TtsState {
             config = %config_path.display(),
             "loading piper model"
         );
-        let piper = Piper::new(&onnx_path, &config_path).map_err(|e| {
-            Error::Custom(format!("Failed to load piper model: {e}"))
-        })?;
+        // Model loading is CPU/IO heavy; keep it off the async runtime too.
+        let piper = tokio::task::spawn_blocking(move || {
+            Piper::new(&onnx_path, &config_path)
+                .map_err(|e| Error::Custom(format!("Failed to load piper model: {e}")))
+        })
+        .await??;
         info!("piper model loaded successfully");
 
         // set() may fail if another task initialized first — that's fine
-        let _ = self.piper.set(Mutex::new(piper));
-        Ok(self.piper.get().unwrap())
+        let _ = self.piper.set(Arc::new(Mutex::new(piper)));
+        Ok(self
+            .piper
+            .get()
+            .expect("piper was just initialized")
+            .clone())
     }
 
     /// Look up cached audio for a text string. Returns `None` if uncached
@@ -181,15 +187,21 @@ impl TtsState {
             return Ok(cached);
         }
 
-        // Cache miss — synthesize
+        // Cache miss — synthesize on a blocking thread. Piper inference is
+        // synchronous and can take >100 ms; it must not stall a Tokio worker.
         let piper = self.get_piper().await?;
+        let text_owned = text.to_string();
         let started = Instant::now();
 
-        let (samples, sample_rate) = {
-            let mut lock = piper.lock().await;
-            lock.create(text, false, None, None, None, None)
-                .map_err(|e| Error::Custom(format!("piper synthesis failed: {e}")))?
-        };
+        let (samples, sample_rate) = tokio::task::spawn_blocking(move || {
+            let mut piper = piper
+                .lock()
+                .map_err(|_| Error::Custom("piper mutex poisoned".into()))?;
+            piper
+                .create(&text_owned, false, None, None, None, None)
+                .map_err(|e| Error::Custom(format!("piper synthesis failed: {e}")))
+        })
+        .await??;
 
         let elapsed_ms = started.elapsed().as_millis() as u64;
         debug!(
-- 
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