Fix extra erase of 2 output lines when rolling print is still empty

The reset function erase (len(last_lines) + 2) number of lines, where the extra 2 lines is for the starting and ending line such as '========== Rolling subprocess output =========='. When last_lines is still empty, these lines haven't been printed to the screen, and thus should not be erased. This CL adds a check for the emptiness of the rolling print buffer to avoid extra erasing.

Test: atest unittests
Change-Id: I79f72bb4cdf580e052ea88d322d6b400a1c915b4

atest/atest_utils.py

diff --git a/atest/atest_utils.py b/atest/atest_utils.py
index d8266b8..d46b2db 100644
--- a/atest/atest_utils.py
+++ b/atest/atest_utils.py
@@ -311,9 +311,8 @@
   is_rolling = True
 
   def reset_output():
-    if not is_rolling:
-      return
-    io_output.write(_BASH_CLEAR_PREVIOUS_LINE_CODE * (len(last_lines) + 2))
+    if is_rolling and last_lines:
+      io_output.write(_BASH_CLEAR_PREVIOUS_LINE_CODE * (len(last_lines) + 2))
 
   def write_output(new_lines: list[str]):
     if not is_rolling:

atest/atest_utils_unittest.py

diff --git a/atest/atest_utils_unittest.py b/atest/atest_utils_unittest.py
index 243b6b5..6fae039 100755
--- a/atest/atest_utils_unittest.py
+++ b/atest/atest_utils_unittest.py
@@ -137,6 +137,21 @@
         io_output.getvalue(),
     )
 
+  @mock.patch.object(atest_utils, 'get_terminal_size', return_value=(5, -1))
+  def test_stream_io_output_no_lines_written_no_lines_cleared(self, _):
+    """Test when nothing is written, no lines are cleared."""
+    io_input = StringIO()
+    io_output = StringIO()
+
+    atest_utils.stream_io_output(
+        io_input, max_lines=2, io_output=io_output, is_io_output_atty=True
+    )
+
+    self.assertNotIn(
+        atest_utils._BASH_CLEAR_PREVIOUS_LINE_CODE,
+        io_output.getvalue(),
+    )
+
 
 class ConcatenatePathTest(unittest.TestCase):
   """Class that tests path concatenation."""