[meld] Try showing GUI message for failed requirements check (bgo#776453)

[Kai Willadsen <kaiw src gnome org>]

commit c2c20fefe03861105c1330253a525f281cd47107
Author: Vasily Galkin <galkin-vv ya ru>
Date:   Sat Jan 7 05:37:24 2017 +0300

    Try showing GUI message for failed requirements check (bgo#776453)
    
    The messagebox is trying to be shown with both Gtk and tkinter, because
    for windows platfrom python almost always has tkinter, but can lack Gtk,
    and for linux the situation is opposite.
    Nevertheless both methods are platform-independent.

 bin/meld |   41 ++++++++++++++++++++++++++++++++++++++---
 1 files changed, 38 insertions(+), 3 deletions(-)
---
diff --git a/bin/meld b/bin/meld
index bc6a4df..4b4c5f6 100755
--- a/bin/meld
+++ b/bin/meld
@@ -129,6 +129,42 @@ except WindowsError as e:
     print("Couldn't bind the translation domain. Some translations won't work.")
     print(e)
 
+def show_error_and_exit(error_text):
+    """
+    Show error in a robust way: always print to stdout and try to
+    display gui message via gtk or tkinter (first available).
+    Empty toplevel window is used as message box parent since
+    parentless message box cause toolkit and windowing system problems.
+    This function is both python 2 and python 3 compatible since it is used
+    to display wrong python version.
+    """
+    print(error_text)
+    try:
+        import gi
+        gi.require_version("Gtk", "3.0")
+        from gi.repository import Gtk
+        toplevel = Gtk.Window(title = "Meld")
+        toplevel.show()
+        Gtk.MessageDialog(
+            toplevel, 0, Gtk.MessageType.ERROR,
+            Gtk.ButtonsType.CLOSE, error_text).run()
+    except Exception:
+        # Although tkinter is imported here, it isn't meld's dependency:
+        # if found it is used to show GUI error about lacking true dependecies.
+        try:
+            if sys.version_info < (3, 0):
+                from Tkinter import Tk, Toplevel
+                from tkMessageBox import showerror
+            else:
+                from tkinter import Tk, Toplevel
+                from tkinter.messagebox import showerror
+            toplevel = Tk(className = "Meld")
+            toplevel.wait_visibility()
+            showerror("Meld", error_text, parent = toplevel)
+        except Exception:
+            # Displaying with tkinter failed too, just exit.
+            pass
+    sys.exit(1)
 
 def check_requirements():
 
@@ -139,11 +175,10 @@ def check_requirements():
 
     def missing_reqs(mod, ver, exception=None):
         if isinstance(exception, ImportError):
-            print(_("Cannot import: ") + mod + "\n" + str(e))
+            show_error_and_exit(_("Cannot import: ") + mod + "\n" + str(e))
         else:
             modver = mod + " " + ".".join(map(str, ver))
-            print(_("Meld requires %s or higher.") % modver)
-        sys.exit(1)
+            show_error_and_exit(_("Meld requires %s or higher.") % modver)
 
     if sys.version_info[:2] < pyver:
         missing_reqs("Python", pyver)