--latest-stamps, like --fast-update, but without the need to keep downloaded files (#1131)

Adds the --latest-stamps command line option, pointing to a file where
the latest time each profile was scraped. On the next run, only posts
newer than that time are downloaded.

Fixes #1122.

docs/basic-usage.rst

@@ -39,6 +39,16 @@ To later **update your local copy** of that profiles, you may run
 If :option:`--fast-update` is given, Instaloader stops when arriving at the
 first already-downloaded picture.
 
+Alternatively, you can use :option:`--latest-stamps` to have Instaloader store
+the time each profile was last downloaded and only download newer media:
+
+::
+
+    instaloader --latest-stamps -- profile [profile ...]
+
+With this option it's possible to move or delete downloaded media and still keep
+the archive updated.
+
 When updating profiles, Instaloader
 automatically **detects profile name changes** and renames the target directory
 accordingly.

docs/cli-options.rst

@@ -146,6 +146,23 @@ Which Posts to Download
    This flag is recommended when you use Instaloader to update your personal
    Instagram archive.
 
+.. option:: --latest-stamps [STAMPSFILE]
+
+   Works similarly to :option:`--fast-update`, but instead of relying on already
+   downloaded media, the time each profile was downloaded is stored, and only
+   media newer than the last download is fetched. This allows updating your
+   personal Instagram archive while emptying the target directories.
+
+   Only works for media associated with a specific profile, and that is returned
+   in chronological order: profile posts, profile stories, profile IGTV posts
+   and profile tagged posts.
+
+   By default, the information is stored in
+   ``~/.config/instaloader/latest-stamps.ini``, but you can specify an
+   alternative location.
+
+   .. versionadded:: 4.8
+
 .. option:: --post-filter filter, --only-if filter
 
    Expression that, if given, must evaluate to True for each post to be

docs/module/structures.rst

@@ -84,3 +84,9 @@ Loading and Saving
 .. autofunction:: get_json_structure
 
 .. autofunction:: save_structure_to_file
+
+LatestStamps
+""""""""""""
+
+.. autoclass:: LatestStamps
+   :no-show-inheritance: