Heartbeat progress during long subprocess runs (keeps SSE stream alive)

app.py

@@ -18,6 +18,8 @@ import os
 import shutil
 import subprocess
 import sys
+import threading
+import time
 import uuid
 from dataclasses import dataclass
 from pathlib import Path
@@ -108,6 +110,10 @@ def _write_input_image(run_dir: Path, image: Image.Image) -> Path:
 
 def _run_subprocess(cmd: list[str], *, cwd: Path, progress: gr.Progress,
                     progress_fraction: float, phase_label: str) -> None:
+    """Invoke a long-running Lyra-2 subprocess while emitting periodic progress
+    updates. Heartbeats matter: HF's SSE proxy drops connections when no events
+    flow for ~60s, and Lyra-2's model-load phase can be silent for 2+ min.
+    """
     progress(progress_fraction, desc=phase_label)
     log.info("[%s] %s (cwd=%s)", phase_label, " ".join(cmd), cwd)
     env = os.environ.copy()
@@ -122,10 +128,34 @@ def _run_subprocess(cmd: list[str], *, cwd: Path, progress: gr.Progress,
         bufsize=1,
     )
     assert proc.stdout is not None
-    for line in proc.stdout:
-        sys.stdout.write(line)
-        sys.stdout.flush()
-    rc = proc.wait()
+
+    # Background heartbeat to keep the Gradio SSE stream alive during long
+    # silent phases (model load + diffusion steps don't flush to our stdout).
+    stop_heartbeat = threading.Event()
+    t_start = time.monotonic()
+
+    def _heartbeat() -> None:
+        while not stop_heartbeat.wait(20.0):
+            elapsed = time.monotonic() - t_start
+            try:
+                progress(progress_fraction, desc=f"{phase_label} — {elapsed:.0f}s elapsed")
+            except Exception:
+                # Progress updates can raise if the client disconnects; don't
+                # kill the subprocess just because the browser went away.
+                pass
+
+    hb = threading.Thread(target=_heartbeat, daemon=True)
+    hb.start()
+
+    try:
+        for line in proc.stdout:
+            sys.stdout.write(line)
+            sys.stdout.flush()
+        rc = proc.wait()
+    finally:
+        stop_heartbeat.set()
+        hb.join(timeout=1.0)
+
     if rc != 0:
         raise gr.Error(f"{phase_label} failed with exit code {rc}. Check Space logs for details.")