[Bug 1562308] Re: missing or duplicate lines caused by a wrapped line with wide characters

[Chris J Arges]

Hello Toshio, or anyone else affected,

Accepted less into xenial-proposed. The package will build now and be
available at https://launchpad.net/ubuntu/+source/less/481-2.1ubuntu0.2
in a few hours, and then in the -proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to
enable and use -proposed.  Your feedback will aid us getting this update
out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested, and change the tag
from verification-needed to verification-done. If it does not fix the
bug for you, please add a comment stating that, and change the tag to
verification-failed.  In either case, details of your testing will help
us make a better decision.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance!

** Changed in: less (Ubuntu Xenial)
       Status: New => Fix Committed

** Tags added: verification-needed

** Changed in: less (Ubuntu Yakkety)
       Status: New => Fix Committed

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https://bugs.launchpad.net/bugs/1562308

Title:
  missing or duplicate lines caused by a wrapped line with wide
  characters

Status in less package in Ubuntu:
  Fix Released
Status in less source package in Xenial:
  Fix Committed
Status in less source package in Yakkety:
  Fix Committed

Bug description:
  [Impact]

  Currently less doesn't work correctly with UTF-8 encoded Japanese
  characters,  wrapping the lines in an invalid manner, missing or
  duplicating them (see original description below for exact error
  cases). For locale like these it breaks the general workflow, making
  the tool unreliable.

  [Test Case]

  1. Open xterm.
  2. Set the geometory of xterm to 71x22.
  3. Open the attached lesstest.long_jap.txt with less (maybe you need
     environment variable LANG=ja_JP.UTF-8)
  4. Type "j", then you will see "003" at the top and only the first part of 
     the wrapped line should be shown.
  5. Type "k", then you will see "001", "002" and "003" at the top.

  [Regression Potential]

  Rather low as the fix is present in the beta version of less for over
  5 months already, which seems to be enough time for general audience
  testing. But since the width tables are modified potentially this
  could lead to other similar breakages related to wide-character
  handling.

  [Original Description]

  When you scroll down text with "j" key and encounter a wrapped line
  with wide characters (such as UTF8-encoded Japanese characters),
  "less" seems to show the whole wrapped line at a single "j" key,
  causing the view scroll down by 2 lines at once. Strangely, if you
  type "k" to scroll up, it does that by only 1 line. As a result, there
  is a missing line that should have been shown.

  Even stranger stuff (i.e., duplicate lines) happens when you type "j"
  and "k" alternately when a wrapped line with wide characters is at the
  bottom of the view.

  # Steps to reproduce

  1. Open xterm.
  2. Set the geometory of xterm to 71x22.
  3. Open the attached lesstest.long_jap.txt with less (maybe you need
     environment variable LANG=ja_JP.UTF-8)
  4. Type "j", then you will see "003" at the top, and a long wrapped
     line with Japanese characters at the bottom.
  5. Type "k", then you will see "001" and "003" at the top. "002" is
     missing.

  # Expected behavior

  In step 4, only the first part of the wrapped line should be shown.

  In step 5, all "001", "002" and "003" should be shown.

  # Test Environment

  - Xubuntu 16.04 (Xenial) beta (in VirtualBox on Xubuntu 14.04)
  - less: 481-2.1
  - xterm: 322-1ubuntu1
  - Japanese environment (LANG=ja_JP.UTF-8)

  # Note

  - The same problem happens on xfce4-terminal (0.6.3-2ubuntu1)
  - lv (4.51-2.3build1) doesn't have such problem.
  - In "less" in Xubuntu 14.04 (version 458-2), this problem didn't
    exist.

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