Fix Scene Detection frames mismatch causes invalid frame access crashes (#927)

* Fix frame count mismatch between FFmpeg and VapourSynth
Fix Scene Detection uses incompatible decode method leading to accessing invalid frames

* Fix race condition in Scene Detection causes End of File Error

---------

Co-authored-by: Boats McGee <{ID}+{username}@users.noreply.github.com>

Author: BoatsMcGee

av1an-core/src/context.rs

@@ -120,7 +120,7 @@ impl Av1anContext {
 
       // frames need to be recalculated in this case
       if self.frames == 0 {
-        self.frames = self.args.input.frames()?;
+        self.frames = self.args.input.frames(self.vs_script.clone())?;
         done.frames.store(self.frames, atomic::Ordering::Relaxed);
       }
 
@@ -732,9 +732,19 @@ impl Av1anContext {
   fn calc_split_locations(&self) -> anyhow::Result<(Vec<Scene>, usize)> {
     let zones = self.parse_zones()?;
 
+    // Create a new input with the generated VapourSynth script for Scene Detection
+    let input = if let Some(ref vs_script) = self.vs_script {
+      Input::VapourSynth {
+        path: vs_script.clone(),
+        vspipe_args: Vec::new(),
+      }
+    } else {
+      self.args.input.clone()
+    };
+
     Ok(match self.args.split_method {
       SplitMethod::AvScenechange => av_scenechange_detect(
-        &self.args.input,
+        &input,
         self.args.encoder,
         self.frames,
         self.args.min_scene_len,
@@ -770,8 +780,7 @@ impl Av1anContext {
             zone_overrides: None,
           });
         }
-
-        (scenes, self.args.input.frames()?)
+        (scenes, self.args.input.frames(self.vs_script.clone())?)
       }
     })
   }
@@ -810,7 +819,7 @@ impl Av1anContext {
         crate::split::read_scenes_from_file(scene_file.as_ref())?
       } else {
         used_existing_cuts = false;
-        self.frames = self.args.input.frames()?;
+        self.frames = self.args.input.frames(self.vs_script.clone())?;
         self.calc_split_locations()?
       };
     self.frames = frames;

av1an-core/src/lib.rs

@@ -116,16 +116,20 @@ impl Input {
     matches!(&self, Input::VapourSynth { .. })
   }
 
-  pub fn frames(&self) -> anyhow::Result<usize> {
+  pub fn frames(&self, vs_script_path: Option<PathBuf>) -> anyhow::Result<usize> {
     const FAIL_MSG: &str = "Failed to get number of frames for input video";
     Ok(match &self {
-      Input::Video { path } => {
+      Input::Video { path } if vs_script_path.is_none() => {
         ffmpeg::num_frames(path.as_path()).map_err(|_| anyhow::anyhow!(FAIL_MSG))?
       }
-      Input::VapourSynth { path, .. } => {
-        vapoursynth::num_frames(path.as_path(), self.as_vspipe_args_map()?)
-          .map_err(|_| anyhow::anyhow!(FAIL_MSG))?
-      }
+      path => vapoursynth::num_frames(
+        vs_script_path
+          .as_ref()
+          .map(|p| p.as_path())
+          .unwrap_or(path.as_path()),
+        self.as_vspipe_args_map()?,
+      )
+      .map_err(|_| anyhow::anyhow!(FAIL_MSG))?,
     })
   }
 

av1an-core/src/scene_detect.rs

@@ -39,14 +39,15 @@ pub fn av_scenechange_detect(
 
   let input2 = input.clone();
   let frame_thread = thread::spawn(move || {
-    let frames = input2.frames().unwrap();
+    let frames = input2.frames(None).unwrap();
     if verbosity != Verbosity::Quiet {
       progress_bar::convert_to_progress(0);
       progress_bar::set_len(frames as u64);
     }
     frames
   });
 
+  let frames = frame_thread.join().unwrap();
   let scenes = scene_detect(
     input,
     encoder,
@@ -66,8 +67,6 @@ pub fn av_scenechange_detect(
     zones,
   )?;
 
-  let frames = frame_thread.join().unwrap();
-
   progress_bar::finish_progress_bar();
 
   Ok((scenes, frames))