bash-mcp: warn when capturing a task's stdout or stderr fails

copy_stream_to_file dropped the copy and flush errors, so an ENOSPC or
pipe error left a truncated .out/.err with no trace while the task still
reported its exit status. It now returns the error and the runner logs
it with the stream and file path.

hive-bash-mcp/src/runner.rs

@@ -786,17 +786,18 @@ async fn exec_cmd(
     let stdout = child.stdout.take().expect("stdout piped");
     let stderr = child.stderr.take().expect("stderr piped");
 
-    let out_path = out_path.to_path_buf();
-    let err_path = err_path.to_path_buf();
-
-    let copy_out = tokio::spawn(copy_stream_to_file(
-        tokio::io::BufReader::new(stdout),
-        out_path,
-    ));
-    let copy_err = tokio::spawn(copy_stream_to_file(
-        tokio::io::BufReader::new(stderr),
-        err_path,
-    ));
+    let copy_out = {
+        let path = out_path.to_path_buf();
+        tokio::spawn(
+            async move { copy_stream_to_file(tokio::io::BufReader::new(stdout), &path).await },
+        )
+    };
+    let copy_err = {
+        let path = err_path.to_path_buf();
+        tokio::spawn(
+            async move { copy_stream_to_file(tokio::io::BufReader::new(stderr), &path).await },
+        )
+    };
 
     // A future that fires when the timeout elapses, or never if `None`.
     let timeout_fut = async {
@@ -835,8 +836,8 @@ async fn exec_cmd(
         }
     };
 
-    let _ = copy_out.await;
-    let _ = copy_err.await;
+    warn_if_copy_failed("stdout", out_path, copy_out.await);
+    warn_if_copy_failed("stderr", err_path, copy_err.await);
 
     let status = wait_res?;
     let outcome = if let Some(forced) = killed {
@@ -854,19 +855,36 @@ fn u32_to_i32(v: u32) -> i32 {
     i32::try_from(v).unwrap_or(i32::MAX)
 }
 
-async fn copy_stream_to_file<R>(mut reader: R, path: PathBuf)
+/// Copies `reader` into a new file at `path`, flushing at the end. On error
+/// the bytes copied before it stay in the file.
+async fn copy_stream_to_file<R>(mut reader: R, path: &Path) -> std::io::Result<()>
 where
     R: tokio::io::AsyncRead + Unpin,
 {
-    match tokio::fs::File::create(&path).await {
-        Ok(mut f) => {
-            let _ = tokio::io::copy(&mut reader, &mut f).await;
-            let _ = f.flush().await;
-        }
-        Err(e) => {
-            tracing::warn!(path = %path.display(), error = ?e, "bash_runner: open output file failed");
-        }
-    }
+    let mut f = tokio::fs::File::create(path).await?;
+    let copied = tokio::io::copy(&mut reader, &mut f).await;
+    f.flush().await?;
+    copied.map(|_| ())
+}
+
+/// Warns when a stream copy failed or its task panicked: the log file is then
+/// incomplete while the task still reports its exit status.
+fn warn_if_copy_failed(
+    stream: &str,
+    path: &Path,
+    joined: Result<std::io::Result<()>, tokio::task::JoinError>,
+) {
+    let error = match joined {
+        Ok(Ok(())) => return,
+        Ok(Err(e)) => format!("{e:?}"),
+        Err(e) => format!("{e:?}"),
+    };
+    tracing::warn!(
+        stream,
+        path = %path.display(),
+        error,
+        "bash_runner: capturing task output failed, the log file is incomplete"
+    );
 }
 
 fn tail_file(path: &Path, max_bytes: usize) -> Option<String> {
@@ -1208,4 +1226,55 @@ mod tests {
             assert_eq!(read_task("t1").unwrap().status, TaskStatus::Pending);
         });
     }
+
+    /// Yields `data`, then fails with an I/O error.
+    struct FailAfter {
+        data: &'static [u8],
+        sent: bool,
+    }
+
+    impl tokio::io::AsyncRead for FailAfter {
+        fn poll_read(
+            mut self: std::pin::Pin<&mut Self>,
+            _cx: &mut std::task::Context<'_>,
+            buf: &mut tokio::io::ReadBuf<'_>,
+        ) -> std::task::Poll<std::io::Result<()>> {
+            if self.sent {
+                return std::task::Poll::Ready(Err(std::io::Error::other("pipe broke")));
+            }
+            self.sent = true;
+            buf.put_slice(self.data);
+            std::task::Poll::Ready(Ok(()))
+        }
+    }
+
+    fn block_on<F: std::future::Future>(f: F) -> F::Output {
+        tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap()
+            .block_on(f)
+    }
+
+    #[test]
+    fn copy_error_is_returned_and_earlier_bytes_are_kept() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("t1.out");
+        let reader = FailAfter {
+            data: b"before",
+            sent: false,
+        };
+        let res = block_on(super::copy_stream_to_file(reader, &path));
+        assert_eq!(res.unwrap_err().to_string(), "pipe broke");
+        assert_eq!(std::fs::read(&path).unwrap(), b"before");
+    }
+
+    #[test]
+    fn successful_copy_writes_everything() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("t1.out");
+        let res = block_on(super::copy_stream_to_file(&b"all of it"[..], &path));
+        assert!(res.is_ok());
+        assert_eq!(std::fs::read(&path).unwrap(), b"all of it");
+    }
 }