is_Cygwin: avoid `exec`ing anything

The `is_Cygwin` function is used, among other things, to determine
how executables are discovered in the `PATH` list by the `_which` function.

We are about to change the behavior of the `_which` function on Windows
(but not Cygwin): On Windows, we want it to ignore empty elements of the
`PATH` instead of treating them as referring to the current directory
(which is a "legacy feature" according to
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03,
but apparently not explicitly deprecated, the POSIX documentation is
quite unclear on that even if the Cygwin project itself considers it to
be deprecated: https://github.com/cygwin/cygwin/commit/fc74dbf22f5c).

This is important because on Windows, `exec` does something very unsafe
by default (unless we're running a Cygwin version of Tcl, which follows
Unix semantics).

However, we try to `exec` something _inside_ `is_Cygwin` to determine
whether we're running within Cygwin or not, i.e. before we determined
whether we need to handle `PATH` specially or not. That's a Catch-22.

Therefore, and because it is much cleaner anyway, use the
`$::tcl_platform(os)` value which is guaranteed to start with `CYGWIN_`
when running a Cygwin variant of Tcl/Tk, instead of executing `cygpath
--windir`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

diff --git a/git-gui.sh b/git-gui.sh
index 0cf625ca01ca35ab77dfd2648419bf1d6a22baba..0fe60f80cc256f2eefadec819a440eadf482d89a 100755 (executable)
--- a/git-gui.sh
+++ b/git-gui.sh
@@ -269,16 +269,8 @@ proc is_Windows {} {
 proc is_Cygwin {} {
        global _iscygwin
        if {$_iscygwin eq {}} {
-               if {$::tcl_platform(platform) eq {windows}} {
-                       if {[catch {set p [exec cygpath --windir]} err]} {
-                               set _iscygwin 0
-                       } else {
-                               set _iscygwin 1
-                               # Handle MSys2 which is only cygwin when MSYSTEM is MSYS.
-                               if {[info exists ::env(MSYSTEM)] && $::env(MSYSTEM) ne "MSYS"} {
-                                       set _iscygwin 0
-                               }
-                       }
+               if {[string match "CYGWIN_*" $::tcl_platform(os)]} {
+                       set _iscygwin 1
                } else {
                        set _iscygwin 0
                }