Let --sessionfile default to a more persistent path (#659)

The presence of a sessionfile avoids the usage of the failure-prone login
mechanism.  This commit addresses a suggestion in #615 to store the sessionfile
in a persistent path rather than a path within a temporary directory if no
--sessionfile parameter is given.

The default path is now:

  $XDG_CONFIG_HOME/instaloader/session-USERNAME or
    ~/.config/instaloader/session-USERNAME on Unix,

  %LOCALAPPDATA%\Instaloader\session-USERNAME on Windows.

If no file exists in the new path, Instaloader tries loading from the path
where the sessionfile was stored before this commit, hence it automatically
migrates to the new sessionfile path.

docs/basic-usage.rst

@@ -51,7 +51,7 @@ invoke it with
     instaloader --login=your_username profile [profile ...]
 
 When logging in, Instaloader **stores the session cookies** in a file in your
-temporary directory, which will be reused later the next time :option:`--login`
+home directory, which will be reused later the next time :option:`--login`
 is given.  So you can download private profiles **non-interactively** when you
 already have a valid session cookie file.
 
@@ -291,17 +291,18 @@ Instaloader as Cronjob
 Instaloader is suitable for running as a cronjob to periodically update your
 personal Instagram archive. The :option:`--quiet` option disables user
 interactions and logging of non-error messages. To non-interactively use
-Instaloader logged-in, create a session file, e.g. in your home directory::
+Instaloader logged-in, create a session file::
 
-   instaloader --login=your_username --sessionfile=~/.instaloader-session
+   instaloader --login=your_username
 
-Then use the same parameters in your cronjob to load the session and download
+Then use the same parameter in your cronjob to load the session and download
 the given targets::
 
-   instaloader --login=your_username --sessionfile=~/.instaloader-session --quiet <target> [...]
+   instaloader --login=your_username --quiet <target> [...]
 
-Without :option:`--sessionfile` option, Instaloader saves the session file in
-a path within your temporary directory.
+Instaloader saves the session file to
+``~/.config/instaloader/session-YOUR-USERNAME``. See
+:option:`--sessionfile` option for how to override this path.
 
 Programming Instaloader
 ^^^^^^^^^^^^^^^^^^^^^^^

docs/cli-options.rst

@@ -193,9 +193,8 @@ Instaloader to login.
 
 .. option:: --sessionfile SESSIONFILE, -f SESSIONFILE
 
-   Path for loading and storing session key file.  Defaults to a path within
-   your temporary directory, encoding your local username and your Instagram
-   profile name.
+   Path for loading and storing session key file.  Defaults to
+   ``~/.config/instaloader/session-YOUR-USERNAME``.
 
 .. option:: --password YOUR-PASSWORD, -p YOUR-PASSWORD
 

instaloader/instaloader.py

@@ -27,6 +27,20 @@ from .structures import (Hashtag, Highlight, JsonExportable, Post, PostLocation,
 
 def get_default_session_filename(username: str) -> str:
     """Returns default session filename for given username."""
+    sessionfilename = "session-{}".format(username)
+    if platform.system() == "Windows":
+        # on Windows, use %LOCALAPPDATA%\Instaloader\session-USERNAME
+        localappdata = os.getenv("LOCALAPPDATA")
+        if localappdata is not None:
+            return os.path.join(localappdata, "Instaloader", sessionfilename)
+        # legacy fallback - store in temp dir if %LOCALAPPDATA% is not set
+        return os.path.join(tempfile.gettempdir(), ".instaloader-" + getpass.getuser(), sessionfilename)
+    # on Unix, use ~/.config/instaloader/session-USERNAME
+    return os.path.join(os.getenv("XDG_CONFIG_HOME", os.path.expanduser("~/.config")), "instaloader", sessionfilename)
+
+
+def get_legacy_session_filename(username: str) -> str:
+    """Returns legacy (until v4.4.3) default session filename for given username."""
     dirname = tempfile.gettempdir() + "/" + ".instaloader-" + getpass.getuser()
     filename = dirname + "/" + "session-" + username
     return filename.lower()
@@ -441,6 +455,8 @@ class Instaloader:
         """
         if filename is None:
             filename = get_default_session_filename(username)
+            if not os.path.exists(filename):
+                filename = get_legacy_session_filename(username)
         with open(filename, 'rb') as sessionfile:
             self.context.load_session_from_file(username, sessionfile)
             self.context.log("Loaded session from %s." % filename)