Fix imex race condition, (#2255)

fix #2254: if the DB was closed without calling stop_io() and then an interrupt arrives (e.g. incoming message), the db was corrupted.

* Add result.log() for logging with less boilerplate code

* Bugfix: Resultify housekeeping() to make it abort if the db is closed instead of just deleting everything

* Require the UI to call dc_stop_io() before backup export

* Prepare a bit better for closed-db: Resultify get_uidvalidity and get_uid_next and let job::load_next() wait until the db is open

About the bug (before this PR):
if the DB was closed without calling stop_io() and then an interrupt arrives (e.g. incoming message):
- I don't know if it downloads the message, but of course at some point the process of receiving the message will fail
- In my test, DC is just in the process of moving a message when the imex starts, but then can't delete the job or update the msg server_uid
- Then, when job::load_next() is called, no job can be loaded. That's why it calls `load_housekeeping_job()`. As `load_housekeeping_job()` can't load the time of the last housekeeping, it assumes we never ran housekeeping and returns a new Housekeeping job, which is immediately executed.
- housekeeping can't find any blobs referenced in the db and therefore deletes almost all blobs.

python/src/deltachat/_build.py

@@ -150,6 +150,7 @@ def extract_defines(flags):
                 | DC_CHAT
                 | DC_PROVIDER
                 | DC_KEY_GEN
+                | DC_IMEX
             )         # End of prefix matching
             _[\w_]+   # Match the suffix, e.g. _RSA2048 in DC_KEY_GEN_RSA2048
         )             # Close the capturing group, this contains

python/src/deltachat/account.py

@@ -412,23 +412,23 @@ class Account(object):
 
         Note that the account does not have to be started.
         """
-        return self._export(path, imex_cmd=1)
+        return self._export(path, imex_cmd=const.DC_IMEX_EXPORT_SELF_KEYS)
 
     def export_all(self, path):
         """return new file containing a backup of all database state
         (chats, contacts, keys, media, ...). The file is created in the
         the `path` directory.
 
-        Note that the account does not have to be started.
+        Note that the account has to be stopped; call stop_io() if necessary.
         """
-        export_files = self._export(path, 11)
+        export_files = self._export(path, const.DC_IMEX_EXPORT_BACKUP)
         if len(export_files) != 1:
             raise RuntimeError("found more than one new file")
         return export_files[0]
 
     def _export(self, path, imex_cmd):
         with self.temp_plugin(ImexTracker()) as imex_tracker:
-            lib.dc_imex(self._dc_context, imex_cmd, as_dc_charpointer(path), ffi.NULL)
+            self.imex(path, imex_cmd)
             return imex_tracker.wait_finish()
 
     def import_self_keys(self, path):
@@ -438,7 +438,7 @@ class Account(object):
 
         Note that the account does not have to be started.
         """
-        self._import(path, imex_cmd=2)
+        self._import(path, imex_cmd=const.DC_IMEX_IMPORT_SELF_KEYS)
 
     def import_all(self, path):
         """import delta chat state from the specified backup `path` (a file).
@@ -446,13 +446,16 @@ class Account(object):
         The account must be in unconfigured state for import to attempted.
         """
         assert not self.is_configured(), "cannot import into configured account"
-        self._import(path, imex_cmd=12)
+        self._import(path, imex_cmd=const.DC_IMEX_IMPORT_BACKUP)
 
     def _import(self, path, imex_cmd):
         with self.temp_plugin(ImexTracker()) as imex_tracker:
-            lib.dc_imex(self._dc_context, imex_cmd, as_dc_charpointer(path), ffi.NULL)
+            self.imex(path, imex_cmd)
             imex_tracker.wait_finish()
 
+    def imex(self, path, imex_cmd):
+        lib.dc_imex(self._dc_context, imex_cmd, as_dc_charpointer(path), ffi.NULL)
+
     def initiate_key_transfer(self):
         """return setup code after a Autocrypt setup message
         has been successfully sent to our own e-mail address ("self-sent message").
@@ -577,6 +580,28 @@ class Account(object):
             raise ValueError("account not configured, cannot start io")
         lib.dc_start_io(self._dc_context)
 
+    def maybe_network(self):
+        """This function should be called when there is a hint
+        that the network is available again,
+        e.g. as a response to system event reporting network availability.
+        The library will try to send pending messages out immediately.
+
+        Moreover, to have a reliable state
+        when the app comes to foreground with network available,
+        it may be reasonable to call the function also at that moment.
+
+        It is okay to call the function unconditionally when there is
+        network available, however, calling the function
+        _without_ having network may interfere with the backoff algorithm
+        and will led to let the jobs fail faster, with fewer retries
+        and may avoid messages being sent out.
+
+        Finally, if the context was created by the dc_accounts_t account manager
+        (currently not implemented in the Python bindings),
+        use dc_accounts_maybe_network() instead of this function
+        """
+        lib.dc_maybe_network(self._dc_context)
+
     def configure(self, reconfigure=False):
         """ Start configuration process and return a Configtracker instance
         on which you can block with wait_finish() to get a True/False success