Fix additive audio duration handling

app/operations/mix_audio.py

@@ -53,6 +53,13 @@ def _audio_bitrate(params: dict[str, Any]) -> str:
     return value
 
 
+def _video_duration(inputs: Sequence[InputMedia]) -> float:
+    duration = inputs[0].duration
+    if duration is None or duration <= 0:
+        raise InputError("The source video duration is required for additive audio mixing")
+    return duration
+
+
 async def add_audio(
     ffmpeg: FFmpegService,
     inputs: Sequence[InputMedia],
@@ -62,14 +69,20 @@ async def add_audio(
     require_inputs(inputs, 2)
     tracks = _track_params(params, len(inputs) - 1)
     bitrate = _audio_bitrate(params)
+    duration = _video_duration(inputs)
     output = output_path(output_dir, "audio-mix", "mp4")
 
-    filters: list[str] = []
-    mix_inputs = ["[0:a]"]
+    duration_value = f"{duration:.6f}"
+    filters: list[str] = [
+        f"[0:a]apad=whole_dur={duration_value},atrim=duration={duration_value}[base]"
+    ]
+    mix_inputs = ["[base]"]
+
     for offset, track in enumerate(tracks, start=1):
         delay_ms = round(track["start"] * 1000)
         filters.append(
-            f"[{offset}:a]volume={track['volume']:.6g},adelay={delay_ms}:all=1[a{offset}]"
+            f"[{offset}:a]volume={track['volume']:.6g},adelay={delay_ms}:all=1,"
+            f"apad=whole_dur={duration_value},atrim=duration={duration_value}[a{offset}]"
         )
         mix_inputs.append(f"[a{offset}]")
 
@@ -105,5 +118,10 @@ async def add_audio(
         args,
         output,
         "video.add_audio",
-        {"audio_tracks": tracks, "preserve_video": True, "mix_mode": "additive"},
+        {
+            "audio_tracks": tracks,
+            "preserve_video": True,
+            "mix_mode": "additive",
+            "duration_seconds": duration,
+        },
     )