Windows: Fix error on first run of calibre after install/upgrade

Apparently python's zipimport on windows is not thread safe. And the
register default programs code was running ina  thread at the same time
as the calibre gui was initializing causing imports to fail.

Workaround it by ensuring that no imports happen in the register default
programs thread.

Fixes #1897314 [Execution problem from the installer](https://bugs.launchpad.net/calibre/+bug/1897314)

src/calibre/utils/winreg/default_programs.py

@@ -5,7 +5,7 @@
 __license__ = 'GPL v3'
 __copyright__ = '2015, Kovid Goyal <kovid at kovidgoyal.net>'
 
-import os, sys, time, ctypes
+import os, sys, time, ctypes, traceback
 from ctypes.wintypes import HLOCAL, LPCWSTR
 from threading import Thread
 
@@ -14,6 +14,7 @@ import winerror
 from calibre import guess_type, prints
 from calibre.constants import is64bit, isportable, isfrozen, __version__, DEBUG, plugins
 from calibre.utils.winreg.lib import Key, HKEY_CURRENT_USER, HKEY_LOCAL_MACHINE
+from calibre.utils.lock import singleinstance
 from polyglot.builtins import iteritems, itervalues
 
 # See https://msdn.microsoft.com/en-us/library/windows/desktop/cc144154(v=vs.85).aspx
@@ -102,6 +103,12 @@ def cap_path(data):
     return r'Software\calibre\%s\Capabilities' % data['capability_name']
 
 
+def pre_import_extensions():
+    for program in default_programs():
+        ext_map = {ext.lower():guess_type('file.' + ext.lower())[0] for ext in extensions(program)}
+        ext_map = {ext:mt for ext, mt in iteritems(ext_map) if mt}
+
+
 def register():
     base = os.path.dirname(sys.executable)
 
@@ -160,17 +167,19 @@ class Register(Thread):
     def __init__(self, prefs):
         Thread.__init__(self, name='RegisterDP')
         self.prefs = prefs
+        pre_import_extensions()
         self.start()
 
     def run(self):
+        # make sure no imports happen in this thread as python's zipimport
+        # machinery is not thread safe and main GUI importing is happening
+        # in parallel
         try:
             self.do_register()
         except Exception:
-            import traceback
             traceback.print_exc()
 
     def do_register(self):
-        from calibre.utils.lock import singleinstance
         try:
             check_allowed()
         except NotAllowed:
@@ -179,11 +188,11 @@ class Register(Thread):
             if self.prefs.get('windows_register_default_programs', None) != __version__:
                 self.prefs['windows_register_default_programs'] = __version__
                 if DEBUG:
-                    st = time.time()
+                    st = time.monotonic()
                     prints('Registering with default programs...')
                 register()
                 if DEBUG:
-                    prints('Registered with default programs in %.1f seconds' % (time.time() - st))
+                    prints('Registered with default programs in %.1f seconds' % (time.monotonic() - st))
 
     def __enter__(self):
         return self
@@ -252,7 +261,6 @@ def friendly_app_name(prog_id=None, exe=None):
     try:
         return plugins['winutil'][0].friendly_name(prog_id, exe)
     except Exception:
-        import traceback
         traceback.print_exc()
 
 
@@ -275,7 +283,6 @@ def find_programs(extensions):
                 try:
                     app_desc, prog_id_map = get_prog_id_map(base, key_path)
                 except Exception:
-                    import traceback
                     traceback.print_exc()
                     continue
                 for ext in extensions: