From a6fe0ae4fc36becc55eb97e41ed3849c93ab3da9 Mon Sep 17 00:00:00 2001
From: Lukasz Sojka <lukasz.sojka@scylladb.com>
Date: Thu, 31 Oct 2024 09:32:50 +0100
Subject: [PATCH] fix(libssh2-runner): fix slow stderr read when no stdout

In case there's only stderr stream and no data on stdout, stderr stream
processing waits for stdout processing timeout (0.5s by default). This
causes long processing of many stderr lines as this delay happens for
each line.

Fix by verifying first if given stream has data and proceed to next if
not.

refs: https://github.com/scylladb/java-driver/issues/258
(cherry picked from commit a09dd1ecf657584e96e6c7003e1560cbb3443645)
---
 sdcm/remote/libssh2_client/__init__.py | 27 +++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/sdcm/remote/libssh2_client/__init__.py b/sdcm/remote/libssh2_client/__init__.py
index 2778e9ea14..74de560386 100644
--- a/sdcm/remote/libssh2_client/__init__.py
+++ b/sdcm/remote/libssh2_client/__init__.py
@@ -440,25 +440,30 @@ def _process_output(  # pylint: disable=too-many-arguments, too-many-branches
             if perf_counter() > end_time:
                 reader.stop()
                 return False
-            if stdout_stream is not None:
+
+            data_processed = False
+            if stdout_stream is not None and reader.stdout.qsize():
                 try:
                     stdout = reader.stdout.get(timeout=timeout_read_data_chunk)
                     data = stdout.decode(encoding, errors='ignore') + '\n'
                     stdout_stream.write(data)
                     for watcher in watchers:
                         watcher.submit_line(data)
+                    data_processed = True
+                except Exception:  # pylint: disable=broad-except  # noqa: BLE001
+                    pass
+            if stderr_stream is not None and reader.stderr.qsize():
+                try:
+                    stderr = reader.stderr.get(timeout=timeout_read_data_chunk)
+                    data = stderr.decode(encoding) + '\n'
+                    stderr_stream.write(data)
+                    for watcher in watchers:
+                        watcher.submit_line(data)
+                    data_processed = True
                 except Exception:  # pylint: disable=broad-except  # noqa: BLE001
                     pass
-            if stderr_stream is not None:
-                if reader.stderr.qsize():
-                    try:
-                        stderr = reader.stderr.get(timeout=timeout_read_data_chunk)
-                        data = stderr.decode(encoding) + '\n'
-                        stderr_stream.write(data)
-                        for watcher in watchers:
-                            watcher.submit_line(data)
-                    except Exception:  # pylint: disable=broad-except  # noqa: BLE001
-                        pass
+            if not data_processed:  # prevent unbounded loop in case no data on the streams
+                sleep(timeout_read_data_chunk or 0.5)
         return True
 
     @staticmethod