transcode_for_browser: return new path in fresh tmp dir to beat Gradio probe race

app.py

@@ -159,8 +159,10 @@ def strip_audio_from_video(video_path: str, output_path: str) -> None:
 def _transcode_for_browser(video_path: str) -> str:
     """Re-encode uploaded video to H.264/AAC MP4 so the browser preview widget can play it.
 
-    Overwrites the original file in-place so Gradio serves the H.264 version
-    and does NOT trigger its own slow fallback conversion.
+    Returns a NEW path in a fresh /tmp/gradio/ subdirectory. Gradio probes the
+    returned path fresh, sees H.264, and serves it directly without its own
+    slow fallback converter. The in-place overwrite approach loses the race
+    because Gradio probes the original path at upload time before this callback runs.
     Only called on upload — not during generation.
     """
     if video_path is None:
@@ -173,9 +175,14 @@ def _transcode_for_browser(video_path: str) -> str:
         if video_streams and video_streams[0].get("codec_name") == "h264":
             print(f"[transcode_for_browser] already H.264, skipping")
             return video_path
-        # Write to a temp file first, then replace original in-place.
-        # This avoids Gradio's own fallback converter triggering on the H.265 source.
-        tmp_path  = video_path + ".tmp.mp4"
+        # Write to a fresh gradio-served temp dir so Gradio re-probes from scratch
+        # and never sees the H.265 source for this returned value.
+        import hashlib as _hashlib, os as _os
+        basename  = _os.path.basename(video_path)
+        tag       = _hashlib.sha256((video_path + "_h264").encode()).hexdigest()
+        out_dir   = f"/tmp/gradio/{tag}"
+        _os.makedirs(out_dir, exist_ok=True)
+        out_path  = _os.path.join(out_dir, basename)
         kwargs = dict(
             vcodec="libx264", preset="fast", crf=18,
             pix_fmt="yuv420p", movflags="+faststart",
@@ -186,13 +193,11 @@ def _transcode_for_browser(video_path: str) -> str:
         else:
             kwargs["an"] = None
         # map 0:v:0 explicitly to skip non-video streams (e.g. data/timecode tracks)
-        ffmpeg.input(video_path).output(tmp_path, map="0:v:0", **kwargs).run(
+        ffmpeg.input(video_path).output(out_path, map="0:v:0", **kwargs).run(
             overwrite_output=True, quiet=True
         )
-        import os as _os
-        _os.replace(tmp_path, video_path)
-        print(f"[transcode_for_browser] transcoded to H.264 in-place: {video_path}")
-        return video_path
+        print(f"[transcode_for_browser] transcoded to H.264: {out_path}")
+        return out_path
     except Exception as e:
         print(f"[transcode_for_browser] failed, using original: {e}")
         return video_path