system-tests: Prevent out of order prints to stderr

If we get a new tokio::io::stderr() every time the output can be
scrambled because multiple threads try to output it at the same time.

Therefore, only use one stderr instance to keep the output in order.

Author: sysheap

system-tests/src/infra/read_asserter.rs

@@ -7,13 +7,18 @@ const DEFAULT_BUFFER_SIZE: usize = 1024;
 pub struct ReadAsserter<Reader: AsyncRead + Unpin> {
     reader: Reader,
     buffer: SearchableBuffer,
+    // It is important to only keep one stderr instance
+    // Otherwise the output could be interlaved, especially with
+    // write_all
+    stderr: tokio::io::Stderr,
 }
 
 impl<Reader: AsyncRead + Unpin> ReadAsserter<Reader> {
     pub fn new(reader: Reader) -> Self {
         Self {
             reader,
             buffer: SearchableBuffer::new(Vec::with_capacity(DEFAULT_BUFFER_SIZE)),
+            stderr: tokio::io::stderr(),
         }
     }
 
@@ -34,9 +39,8 @@ impl<Reader: AsyncRead + Unpin> ReadAsserter<Reader> {
         }
     }
 
-    async fn print_to_stderr(&self, data: &[u8]) {
-        let mut stderr = tokio::io::stderr();
-        stderr
+    async fn print_to_stderr(&mut self, data: &[u8]) {
+        self.stderr
             .write_all(data)
             .await
             .expect("Write to stderr must succeed.");