Truncating file names with Unicode characters

Truncating file names with Unicode characters

[Vitaly Potyarkin]

# Truncating file names with Unicode characters

When shortening file names that contain Unicode characters, git performs
truncation without awareness of two-byte characters. That often leads to
splitting a character in half and displaying a garbage byte that's left.

Unawareness of Unicode also means that filename length is calculated incorrectly
and some output gets misaligned.

I have tested this with git 2.14.1 on Windows and with git 2.11.0 on Linux. My
configuration includes setting `core.quotepath = off` to display Unicode paths.

# Example: `git log --stat`

## Bad output: half-characters and wrong text alignment

The last file name gets truncated in the middle of the character (`ˆ` is
what's left of it). Text alignment is off because string lengths are calculated
in bytes instead of characters.

    Extension/README.md                                |  28 +++++++++
    .../Catalog.Номенклатура.xml           |  32 ++++++++++
    .../Configuration.xml                              |   5 +-
    ...етПереработчика.ObjectModule.txt |  39 ++++++++++++
    ...cument.ОтчетПереработчика.xml |  68 +++++++++++++++++++++
    .../Enum.СтавкиНДС.xml                    |  24 ++++++++
    ...ˆирениеERPПотяркин_2018-06-05.cfe | Bin 0 -> 22018 bytes
    7 files changed, 195 insertions(+), 1 deletion(-)

## Good output with ASCII file names

Truncation and alignment are done right because each character is represented
by a single byte.

    .../index.html                                             | 14
++++++++++++++
    docs/posts/2017/loops-in-power-query-m-language/index.html | 14
++++++++++++++
    .../index.html                                             |  7 +++++++
    .../temporary-virtual-environment-for-python/index.html    | 14
++++++++++++++
    .../index.html                                             | 14
++++++++++++++
    docs/posts/2018/getting-started-with-libpq/index.html      | 14
++++++++++++++
    .../index.html                                             | 14
++++++++++++++
    .../2018/unit-testing-in-power-query-m-language/index.html |  7 +++++++
    8 files changed, 98 insertions(+)

Re: Truncating file names with Unicode characters

[Jeff King]

On Fri, Jun 08, 2018 at 09:25:29PM +0300, Vitaly Potyarkin wrote:

> # Truncating file names with Unicode characters
> 
> When shortening file names that contain Unicode characters, git performs
> truncation without awareness of two-byte characters. That often leads to
> splitting a character in half and displaying a garbage byte that's left.
> 
> Unawareness of Unicode also means that filename length is calculated incorrectly
> and some output gets misaligned.

Yeah, I agree it would be nice for this to be more aware of multi-byte
glyphs.

Unfortunately, we don't actually know what encoding the paths are in
(and they might not even all be in the same encoding). But hopefully we
could at least make the common case work a bit better. If we know that
core.quotepath is off, then the user is expecting paths in their
terminal encoding. So possibly we could just rely on
i18n.logOutputEncoding at that point (which of course defaults to utf8).

If anyone is interested in working on it, the relevant code is in
diff.c:show_stats(). Looks like we do a straight strlen() to get the
width, which should become utf8_strwidth(). That would solve the
problem where we use the wrong width.

To deal with the broken code points, I think you'd need to copy the name
into a strbuf and use strbuf_utf8_replace() to truncate (it doesn't seem
to be well documented, but you can grep for a few other callers).

-Peff