doc: "Troubleshooting" section + minor changes

docs/troubleshooting.rst

@@ -0,0 +1,60 @@
+.. _troubleshooting:
+
+Troubleshooting
+===============
+
+.. highlight:: python
+
+429 - Too Many Requests
+-----------------------
+
+Instaloader has a logic to keep track of its requests to Instagram and to obey
+their rate limits. Since they are nowhere documented, we try them out
+experimentally. We have a daily cron job running to confirm that Instaloader
+still stays within the rate limits. Nevertheless, the rate control logic assumes
+that
+
+- at one time, Instaloader is the only application that consumes requests. I.e.
+  neither the Instagram browser interface, nor a mobile app, nor another
+  Instaloader instance is running in parallel,
+
+- no requests had been consumed when Instaloader starts.
+
+The latter one implies that restarting or reinstantiating Instaloader often
+within short time is prone to cause a 429. When a request is denied with a 429,
+Instaloader retries the request as soon as the temporary ban is assumed to be
+expired. In case the retry continuously fails for some reason, which should not
+happen in normal conditions, consider adjusting the
+:option:`--max-connection-attempts` option.
+
+**"Too many queries in the last time"** is not an error. It is a notice that the
+rate limit has almost been reached, according to Instaloader's own rate
+accounting mechanism. We regularly adjust this mechanism to match Instagram's
+current rate limiting.
+
+Login error
+-----------
+
+Instaloader's login *should* work fine, both with and without
+Two-Factor-Authentication. It also supports handling the *checkpoint challenge*,
+issued when Instagram suspects authentication activity on your account, by
+pointing the user to an URL to be opened in a browser.
+
+Nevertheless, in :issue:`92` users report problems with logging in. To still use
+Instaloader's logged-in functionality, you may use the following script to
+workaround login problems by importing the session cookies from Firefox and
+bypassing Instaloader's login.
+
+.. literalinclude:: codesnippets/92_import_firefox_session.py
+
+To use this,
+
+#. login to Instagram in Firefox, 
+
+#. execute the snippet,
+
+#. then, ``instaloader -l USERNAME`` should work fine.
+
+If you do not use your default firefox profile, or your operating system has the
+paths differently set up, you may have to alter the ``FIREFOXCOOKIEFILE``
+variable first.