ellipsis_truncate: pad a truncated string to the full width (1.4-maint)

src/borg/helpers/parseformat.py

@@ -983,6 +983,8 @@ def ellipsis_truncate(msg, space):
     """
     shorten a long string by adding ellipsis between it and return it, example:
     this_is_a_very_long_string -------> this_is..._string
+
+    The result is padded with spaces to a width of *space* terminal cells (if *space* >= 3).
     """
     from ..platform import swidth
     ellipsis_width = swidth('...')
@@ -991,8 +993,10 @@ def ellipsis_truncate(msg, space):
         # if there is very little space, just show ...
         return '...' + ' ' * (space - ellipsis_width)
     if space < ellipsis_width + msg_width:
-        return '{}...{}'.format(swidth_slice(msg, space // 2 - ellipsis_width),
-                                swidth_slice(msg, -space // 2))
+        # swidth_slice does not split a wide character, so a slice can be one cell short: pad below.
+        msg = '{}...{}'.format(swidth_slice(msg, space // 2 - ellipsis_width),
+                               swidth_slice(msg, -space // 2))
+        msg_width = swidth(msg)
     return msg + ' ' * (space - msg_width)
 
 

src/borg/testsuite/helpers.py

@@ -26,6 +26,7 @@
 from ..helpers import parse_timestamp, ChunkIteratorFileWrapper, ChunkerParams
 from ..helpers import ProgressIndicatorPercent, ProgressIndicatorEndless
 from ..helpers import swidth_slice
+from ..helpers import ellipsis_truncate
 from ..helpers import chunkit
 from ..helpers import safe_ns, safe_s, SUPPORT_32BIT_PLATFORMS
 from ..helpers import popen_with_error_handling
@@ -1128,6 +1129,19 @@ def test_swidth_slice_mixed_characters():
     assert swidth_slice(string, 6) == '나윤a'
 
 
+@pytest.mark.skipif(not working_swidth(), reason='swidth() is not supported / active')
+@pytest.mark.parametrize('msg', ['나윤선나윤선나윤선나윤선나윤선', '나윤a선나윤선나윤선나윤선나윤선', 'a' * 40, 'short'])
+@pytest.mark.parametrize('space', [8, 9, 20, 21])
+def test_ellipsis_truncate_width(msg, space):
+    # the result always fills the space, so it overwrites a longer line printed before it.
+    result = ellipsis_truncate(msg, space)
+    assert platform.swidth(result) == space
+    if platform.swidth(msg) > space:
+        assert '...' in result
+    else:
+        assert result == msg + ' ' * (space - platform.swidth(msg))
+
+
 def utcfromtimestamp(timestamp):
     """Returns a naive datetime instance representing the timestamp in the UTC timezone"""
     return datetime.fromtimestamp(timestamp, timezone.utc).replace(tzinfo=None)