refactor: move compression engine copy calls to separate threads

src/main.rs

@@ -538,14 +538,32 @@ fn mode_diff(cmd: &mut Command, args: Args, ids: &mut Vec<i64>, tags: &mut Vec<S
 
             let stdout_a_raw = unsafe { std::fs::File::from_raw_fd(fd_a_write) };
             let stdout_b_raw = unsafe { std::fs::File::from_raw_fd(fd_b_write) };
-            let mut stdout_a = BufWriter::new(stdout_a_raw);
+            let stdout_a = BufWriter::new(stdout_a_raw);
-            let mut stdout_b = BufWriter::new(stdout_b_raw);
+            let stdout_b = BufWriter::new(stdout_b_raw);
 
             debug!("MAIN: Sending item A to diff");
-            compression_engine_a.copy(item_path_a.clone(), &mut stdout_a)?;
+            let handle_a = {
+                let stdout_a = stdout_a.clone();
+                let item_path_a = item_path_a.clone();
+                let compression_engine_a = compression_engine_a.clone();
+                std::thread::spawn(move || {
+                    compression_engine_a.copy(item_path_a, &mut stdout_a).unwrap()
+                })
+            };
+
             debug!("MAIN: Sending item B to diff");
-            compression_engine_b.copy(item_path_b.clone(), &mut stdout_b)?;
+            let handle_b = {
+                let stdout_b = stdout_b.clone();
+                let item_path_b = item_path_b.clone();
+                let compression_engine_b = compression_engine_b.clone();
+                std::thread::spawn(move || {
+                    compression_engine_b.copy(item_path_b, &mut stdout_b).unwrap()
+                })
+            };
+
             debug!("MAIN: Done copying data to FDs");
+            handle_a.join().unwrap();
+            handle_b.join().unwrap();
 
             let output = child.wait_with_output().expect("Failed to wait on diff command");
             if output.status.success() {