[PATCH] console: handle Unicode surrogate pairs

[PATCH] console: handle Unicode surrogate pairs

[Johannes Schindelin]

When running Cygwin's Bash in the Windows Terminal (see
https://docs.microsoft.com/en-us/windows/terminal/ for details), Cygwin
is receiving keyboard input in the form of UTF-16 characters.

UTF-16 has that awkward challenge that it cannot map the full Unicode
range, and to make up for it, there are the ranges U+D800-U+DBFF and
U+DC00-U+DFFF which are illegal except when they come in a pair encoding
for Unicode characters beyond U+FFFF.

Cygwin does not handle such surrogate pairs correctly at the moment, as
can be seen e.g. when running Cygwin's Bash in the Windows Terminal and
then inserting an emoji (e.g. via Windows + <dot>, which opens an emoji
picker on recent Windows versions): Instead of showing an emoji, this
shows the infamous question mark in a black triangle, i.e. the invalid
Unicode character.

Let's special-case surrogate pairs in this scenario.

This fixes https://github.com/git-for-windows/git/issues/3281

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---

	This applies without merge conflict all the way back to
	cygwin_2_7_0-release.

 winsup/cygwin/fhandler_console.cc | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/fhandler_console.cc b/winsup/cygwin/fhandler_console.cc
index 3e17fd9a41..d11f4a4770 100644
--- a/winsup/cygwin/fhandler_console.cc
+++ b/winsup/cygwin/fhandler_console.cc
@@ -453,7 +453,22 @@ fhandler_console::read (void *pv, size_t& buflen)
 	    }
 	  else
 	    {
-	      nread = con.con_to_str (tmp + 1, 59, unicode_char);
+	      WCHAR second = unicode_char >= 0xd800 && unicode_char <= 0xdbff
+		  && i + 1 < total_read ?
+		  input_rec[i + 1].Event.KeyEvent.uChar.UnicodeChar : 0;
+
+	      if (second < 0xdc00 || second > 0xdfff)
+		{
+		  nread = con.con_to_str (tmp + 1, 59, unicode_char);
+		}
+	      else
+		{
+		  /* handle surrogate pairs */
+		  WCHAR pair[2] = { unicode_char, second };
+		  nread = sys_wcstombs (tmp + 1, 59, pair, 2);
+		  i++;
+		}
+
 	      /* Determine if the keystroke is modified by META.  The tricky
 		 part is to distinguish whether the right Alt key should be
 		 recognized as Alt, or as AltGr. */
--
2.34.0.rc2.windows.1

Re: [PATCH] console: handle Unicode surrogate pairs

[Takashi Yano]

On Tue, 16 Nov 2021 11:26:10 +0100 (CET)
Johannes Schindelin wrote:
> When running Cygwin's Bash in the Windows Terminal (see
> https://docs.microsoft.com/en-us/windows/terminal/ for details), Cygwin
> is receiving keyboard input in the form of UTF-16 characters.
> 
> UTF-16 has that awkward challenge that it cannot map the full Unicode
> range, and to make up for it, there are the ranges U+D800-U+DBFF and
> U+DC00-U+DFFF which are illegal except when they come in a pair encoding
> for Unicode characters beyond U+FFFF.
> 
> Cygwin does not handle such surrogate pairs correctly at the moment, as
> can be seen e.g. when running Cygwin's Bash in the Windows Terminal and
> then inserting an emoji (e.g. via Windows + <dot>, which opens an emoji
> picker on recent Windows versions): Instead of showing an emoji, this
> shows the infamous question mark in a black triangle, i.e. the invalid
> Unicode character.
> 
> Let's special-case surrogate pairs in this scenario.
> 
> This fixes https://github.com/git-for-windows/git/issues/3281
> 
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
> 
> 	This applies without merge conflict all the way back to
> 	cygwin_2_7_0-release.
> 
>  winsup/cygwin/fhandler_console.cc | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/winsup/cygwin/fhandler_console.cc b/winsup/cygwin/fhandler_console.cc
> index 3e17fd9a41..d11f4a4770 100644
> --- a/winsup/cygwin/fhandler_console.cc
> +++ b/winsup/cygwin/fhandler_console.cc
> @@ -453,7 +453,22 @@ fhandler_console::read (void *pv, size_t& buflen)
>  	    }
>  	  else
>  	    {
> -	      nread = con.con_to_str (tmp + 1, 59, unicode_char);
> +	      WCHAR second = unicode_char >= 0xd800 && unicode_char <= 0xdbff
> +		  && i + 1 < total_read ?
> +		  input_rec[i + 1].Event.KeyEvent.uChar.UnicodeChar : 0;
> +
> +	      if (second < 0xdc00 || second > 0xdfff)
> +		{
> +		  nread = con.con_to_str (tmp + 1, 59, unicode_char);
> +		}
> +	      else
> +		{
> +		  /* handle surrogate pairs */
> +		  WCHAR pair[2] = { unicode_char, second };
> +		  nread = sys_wcstombs (tmp + 1, 59, pair, 2);
> +		  i++;
> +		}
> +
>  	      /* Determine if the keystroke is modified by META.  The tricky
>  		 part is to distinguish whether the right Alt key should be
>  		 recognized as Alt, or as AltGr. */
> --
> 2.34.0.rc2.windows.1
> 

Thanks for the patch. LGTM.
I will push the patch to the master branch.


Corinna,

Should we apply this patch also to cygwin-3_3-branch?
Or should only the bug fix be for cygwin-3_3-branch?

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

Re: [PATCH] console: handle Unicode surrogate pairs

[Corinna Vinschen]

On Nov 16 22:19, Takashi Yano wrote:
> On Tue, 16 Nov 2021 11:26:10 +0100 (CET)
> Johannes Schindelin wrote:
> > When running Cygwin's Bash in the Windows Terminal (see
> > https://docs.microsoft.com/en-us/windows/terminal/ for details), Cygwin
> > is receiving keyboard input in the form of UTF-16 characters.
> > [...]
> > This fixes https://github.com/git-for-windows/git/issues/3281

Please put this into the release file.

> Thanks for the patch. LGTM.
> I will push the patch to the master branch.
> 
> 
> Corinna,
> 
> Should we apply this patch also to cygwin-3_3-branch?
> Or should only the bug fix be for cygwin-3_3-branch?

The entire patch looks like a bugfix to me, but it depends on how
one looks at it.  Your decision.


Thanks,
Corinna