tar: Hold open input stream as long as possible

I'm hoping this will help us debug coreos/rpm-ostree#4567
```
[2023-08-30T15:00:16.554Z] Aug 30 15:00:15 qemu0 kola-runext-container-image[1957]: error: Importing: Parsing layer blob sha256:00623c39da63781bdd3fb00fedb36f8b9ec95e42cdb4d389f692457f24c67144: Failed to invoke skopeo proxy method FinishPipe: remote error: write |1: broken pipe
```

I haven't been able to reproduce it outside of CI yet, but we had
a prior ugly hack for this in
ostreedev@a27dac8

As the comments say - the goal is to hold open the input stream
as long as feasibly possible.

lib/src/tar/write.rs

@@ -259,16 +259,28 @@ async fn filter_tar_async(
     mut dest: impl AsyncWrite + Send + Unpin,
 ) -> Result<BTreeMap<String, u32>> {
     let (tx_buf, mut rx_buf) = tokio::io::duplex(8192);
+    // The source must be moved to the heap so we know it is stable for passing to the worker thread
     let src = Box::pin(src);
-    let tar_transformer = tokio::task::spawn_blocking(move || -> Result<_> {
-        let src = tokio_util::io::SyncIoBridge::new(src);
+    let tar_transformer = tokio::task::spawn_blocking(move || {
+        let mut src = tokio_util::io::SyncIoBridge::new(src);
         let dest = tokio_util::io::SyncIoBridge::new(tx_buf);
-        filter_tar(src, dest)
+        let r = filter_tar(&mut src, dest);
+        // Pass ownership of the input stream back to the caller - see below.
+        (r, src)
     });
     let copier = tokio::io::copy(&mut rx_buf, &mut dest);
     let (r, v) = tokio::join!(tar_transformer, copier);
     let _v: u64 = v?;
-    r?
+    let (r, src) = r?;
+    // Note that the worker thread took temporary ownership of the input stream; we only close
+    // it at this point, after we're sure we've done all processing of the input.  The reason
+    // for this is that both the skopeo process *or* us could encounter an error (see join_fetch).
+    // By ensuring we hold the stream open as long as possible, it ensures that we're going to
+    // see a remote error first, instead of the remote skopeo process seeing us close the pipe
+    // because we found an error.
+    drop(src);
+    // And pass back the result
+    r
 }
 
 /// Write the contents of a tarball as an ostree commit.