Add comment in Command::spawn

Signed-off-by: Jiahao XU <[email protected]>

Author: NobodyXu

src/native_mux_impl/command.rs

@@ -81,6 +81,15 @@ impl Command {
             .into_iter()
             .zip(is_inherited)
             .filter_map(|(stdio, is_inherited)| if !is_inherited { Some(stdio) } else { None })
+            // Note that once we do this, these file descriptors
+            // (and the Fds we got them from above) should no longer be used in
+            // any async context, as they'd start blocking.
+            //
+            // We give away the descriptors in stdios when we pass them to
+            // open_new_session (which doesn't use them in an async context),
+            // and the Fds are dropped when this function returns, meaning no
+            // owned file descriptors we set to be blocking here can be used in
+            // an async context in the future.
             .try_for_each(|stdio| set_blocking(stdio).map_err(Error::ChildIo))?;
 
         let cmd = NonZeroByteSlice::new(&self.cmd).ok_or(Error::InvalidCommand)?;