Make singleinstance() more robust on OS X

It now handles EINTR and also does not hide all errors in opening the
file or calling lockf()

An error is now reported to the user in a nice error dialog before
aborting startup.

src/calibre/gui2/main.py

@@ -459,8 +459,14 @@ def main(args=sys.argv):
         app, opts, args = init_qt(args)
     except AbortInit:
         return 1
+    try:
         from calibre.utils.lock import singleinstance
         si = singleinstance(singleinstance_name)
+    except Exception:
+        error_dialog(None, _('Cannot start calibre'), _(
+            'Failed to start calibre, single instance locking failed. Click "Show Details" for more information'),
+                     det_msg=traceback.format_exc(), show=True)
+        return 1
     if si and opts.shutdown_running_calibre:
         return 0
     if si:

src/calibre/utils/lock.py

@@ -233,12 +233,15 @@ else:
         @param name: The name to lock.
         @type name: string
         '''
+        from calibre.utils.ipc import eintr_retry_call
         path = singleinstance_path(name)
-        try:
         f = open(path, 'w')
-            fcntl.lockf(f.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)
+        try:
+            eintr_retry_call(fcntl.lockf, f.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)
             atexit.register(_clean_lock_file, f)
             return True
-        except EnvironmentError:
-            pass
+        except IOError as err:
+            if err.errno == errno.EAGAIN:
+                return False
+            raise
         return False