move blocking directory reading to external thread

src/main.rs

@@ -1,5 +1,5 @@
 use anyhow::{Context, Result};
-use futures::prelude::*;
+use futures::{channel::mpsc, prelude::*};
 use glib::GString;
 use gstreamer::Element;
 use gstreamer_audio::{prelude::*, AudioEncoder};
@@ -56,16 +56,29 @@ fn main() -> Result<()> {
     gstreamer::init()?;
     let input = std::env::args().nth(1).context("missing input")?;
     let output = std::env::args().nth(2).context("missing output")?;
-    futures::executor::block_on(
-        futures::stream::iter(get_path_pairs(input.into(), output.into())).for_each_concurrent(
-            num_cpus::get(),
-            |(src, dest)| async move {
+
+    let (pair_tx, pair_rx) = mpsc::channel(16);
+
+    // move blocking directory reading to an external thread
+    let pair_producer = std::thread::spawn(|| {
+        let produce_pairs = futures::stream::iter(get_path_pairs(input.into(), output.into()))
+            .map(Ok)
+            .forward(pair_tx)
+            .map(|res| res.context("sending path pairs failed"));
+        futures::executor::block_on(produce_pairs)
+    });
+
+    let transcoder = pair_rx.for_each_concurrent(num_cpus::get(), |(src, dest)| async move {
         if let Err(err) = transcode(src.as_path(), dest.as_path()).await {
             println!("err {} => {}:\n{:?}", src.display(), dest.display(), err);
         }
-            },
-        ),
-    );
+    });
+    futures::executor::block_on(transcoder);
+
+    pair_producer
+        .join()
+        .expect("directory reading thread panicked")?;
+
     Ok(())
 }