gpg: Improve the photo image viewer selection.

* g10/exec.c (w32_system): Add "!ShellExecute" special.
* g10/photoid.c (get_default_photo_command): Use the new ShellExecute
under Windows and fallbac to 'display' and 'xdg-open' in the Unix
case.
(show_photos): Flush stdout so that the output is shown before the
image pops up.
--

For Unix this basically syncs the code with what we have in gpg 1.4.
Note that xdg-open may not be used when running as root which we
support here.

For Windows we now use ShellExecute as this seems to be preferred over
"cmd /c start"; however this does not solve the actual problem we had
in the bug report.  To solve that problem we resort to a wait
parameter which defaults to 400ms.  This works on my Windows-10
virtualized test box.  If we can figure out which simple viewers are
commonly installed on Windows we should enhance this patch to test for
them.

GnuPG-bug-id: 4334
Signed-off-by: Werner Koch <[email protected]>

1 files changed, 14 insertions, 9 deletions

diff --git a/doc/gpg.texi b/doc/gpg.texi
index 7858baf82..8aaf8db0f 100644
--- a/doc/gpg.texi
+++ b/doc/gpg.texi
@@ -1426,19 +1426,24 @@ viewed (e.g. "f"), "%V" for the calculated validity as a string (e.g.
 and "%%" for an actual percent sign. If neither %i or %I are present,
 then the photo will be supplied to the viewer on standard input.
 
-The default viewer is "xloadimage -fork -quiet -title 'KeyID 0x%k'
-STDIN". Note that if your image viewer program is not secure, then
-executing it from GnuPG does not make it secure.
+On Unix the default viewer is
+@code{xloadimage -fork -quiet -title 'KeyID 0x%k' STDIN}
+with a fallback to
+@code{display -title 'KeyID 0x%k' %i}
+and finally to
+@code{xdg-open %i}.
+On Windows
+@code{!ShellExecute 400 %i} is used; here the command is a meta
+command to use that API call followed by a wait time in milliseconds
+which is used to give the viewer time to read the temporary image file
+before gpg deletes it again.  Note that if your image viewer program
+is not secure, then executing it from gpg does not make it secure.
 
 @item --exec-path @var{string}
 @opindex exec-path
 @efindex PATH
-Sets a list of directories to search for photo viewers and keyserver
-helpers. If not provided, keyserver helpers use the compiled-in
-default directory, and photo viewers use the @code{PATH} environment
-variable.
-Note, that on W32 system this value is ignored when searching for
-keyserver helpers.
+Sets a list of directories to search for photo viewers If not provided
+photo viewers use the @code{PATH} environment variable.
 
 @item --keyring @var{file}
 @opindex keyring