- brut: _merged/index.json has 586 activities — the count when merge_all last ran. The SSE rebuild bug (already fixed) meant it never re-ran after the full Strava sync

added 3256 more.
  - danilo: _merged/ is 8 KB — basically empty. merge_all likely ran concurrently (multiple file uploads trigger multiple rebuilds without a lock in --no-build mode),
  causing a race where shutil.rmtree(merged_acts) from one run wiped what another run was writing.

  Two fixes: serialize --no-build rebuilds with the same lock, and add a "Rebuild" button to the admin page.

 Root causes fixed:
  1. merge_all race condition — --no-build rebuilds now hold _rebuild_lock, same as full builds
  2. The SSE rebuild-trigger bug (already fixed earlier) was brut's original cause

bincio/serve/server.py

@@ -243,16 +243,19 @@ def _trigger_rebuild(handle: str) -> None:
     def _run() -> None:
         try:
             if _webroot is None:
-                # Fast: only update data, skip Astro build
-                subprocess.run(
-                    [uv, "run", "bincio", "render",
-                     "--data-dir", _data_dir,
-                     "--site-dir", _site_dir,
-                     "--handle", _handle,
-                     "--no-build"],
-                    stdout=subprocess.DEVNULL,
-                    stderr=subprocess.DEVNULL,
-                )
+                # Fast: only update data, skip Astro build.
+                # Serialised with the same lock: merge_all wipes and recreates
+                # _merged/activities/ — concurrent runs would corrupt each other.
+                with _rebuild_lock:
+                    subprocess.run(
+                        [uv, "run", "bincio", "render",
+                         "--data-dir", _data_dir,
+                         "--site-dir", _site_dir,
+                         "--handle", _handle,
+                         "--no-build"],
+                        stdout=subprocess.DEVNULL,
+                        stderr=subprocess.DEVNULL,
+                    )
             else:
                 # Full build + rsync — serialised so concurrent uploads don't race
                 with _rebuild_lock:
@@ -500,6 +503,20 @@ async def admin_disk(bincio_session: Optional[str] = Cookie(default=None)) -> JS
     })
 
 
+@app.post("/api/admin/users/{handle}/rebuild")
+async def admin_rebuild(
+    handle: str,
+    bincio_session: Optional[str] = Cookie(default=None),
+) -> JSONResponse:
+    """Trigger a merge_all + site rebuild for a user. Admin only."""
+    _require_admin(bincio_session)
+    user_dir = _get_data_dir() / handle
+    if not user_dir.is_dir():
+        raise HTTPException(404, f"No data directory for user '{handle}'")
+    _trigger_rebuild(handle)
+    return JSONResponse({"ok": True})
+
+
 @app.delete("/api/admin/users/{handle}/activities")
 async def admin_delete_activities(
     handle: str,