Add standalone prune_orphaned_activities CLI task

This part of pruning can be very expensive and bog down the whole
instance to an unusable sate for a long time. It can thus be desireable
to split it from prune_objects and run it on its own in smaller limited batches.

If the batches are smaller enough and spaced out a bit, it may even be possible
to avoid any downtime. If not, the limit can still help to at least make the
downtime duration somewhat more predictable.

CHANGELOG.md

@@ -106,6 +106,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Akkoma API is now documented
 - ability to auto-approve follow requests from users you are already following
 - The SimplePolicy MRF can now strip user backgrounds from selected remote hosts
+- New standalone `prune_orphaned_activities` mix task with configurable batch limit
 
 ## Changed
 - OTP builds are now built on erlang OTP26

docs/docs/administration/CLI_tasks/database.md

@@ -53,6 +53,28 @@ This will prune remote posts older than 90 days (configurable with [`config :ple
 - `--prune-orphaned-activities` - Also prune orphaned activities afterwards. Activities are things like Like, Create, Announce, Flag (aka reports)... They can significantly help reduce the database size.
 - `--vacuum` - Run `VACUUM FULL` after the objects are pruned. This should not be used on a regular basis, but is useful if your instance has been running for a long time before pruning.
 
+## Prune orphaned activities from the database
+
+This will prune activities which are no longer referenced by anything.
+Such activities might be the result of running `prune_objects` without `--prune-orphaned-activities`.
+The same notes and warnings apply as for `prune_objects`.
+
+=== "OTP"
+
+    ```sh
+    ./bin/pleroma_ctl database prune_orphaned_activities [option ...]
+    ```
+
+=== "From Source"
+
+    ```sh
+    mix pleroma.database prune_orphaned_activities [option ...]
+    ```
+
+### Options
+
+- `--limit n` - Only delete up to `n` activities in each query making up this job, i.e. if this job runs two queries at most `2n` activities will be deleted. Running this task repeatedly in limited batches can help maintain the instance’s responsiveness while still freeing up some space.
+
 ## Create a conversation for all existing DMs
 
 Can be safely re-run

lib/mix/tasks/pleroma/database.ex

@@ -20,7 +20,14 @@ defmodule Mix.Tasks.Pleroma.Database do
   @shortdoc "A collection of database related tasks"
   @moduledoc File.read!("docs/docs/administration/CLI_tasks/database.md")
 
-  def prune_orphaned_activities() do
+  def prune_orphaned_activities(limit \\ 0) when is_number(limit) do
+    limit_arg =
+      if limit > 0 do
+        "LIMIT #{limit}"
+      else
+        ""
+      end
+
     # Prune activities who link to a single object
     """
     delete from public.activities
@@ -34,6 +41,7 @@ def prune_orphaned_activities() do
       and o.id is null
       and a2.id is null
       and u.id is null
+      #{limit_arg}
     )
     """
     |> Repo.query([], timeout: :infinity)
@@ -51,6 +59,7 @@ def prune_orphaned_activities() do
       having max(o.data ->> 'id') is null
       and max(a2.data ->> 'id') is null
       and max(u.ap_id) is null
+      #{limit_arg}
     )
     """
     |> Repo.query([], timeout: :infinity)
@@ -98,6 +107,33 @@ def run(["update_users_following_followers_counts"]) do
     )
   end
 
+  def run(["prune_orphaned_activities" | args]) do
+    {options, [], []} =
+      OptionParser.parse(
+        args,
+        strict: [
+          limit: :integer
+        ]
+      )
+
+    start_pleroma()
+
+    limit = Keyword.get(options, :limit, 0)
+
+    log_message = "Pruning orphaned activities"
+
+    log_message =
+      if limit > 0 do
+        log_message <> ", limiting deletion to #{limit} rows"
+      else
+        log_message
+      end
+
+    Logger.info(log_message)
+
+    prune_orphaned_activities(limit)
+  end
+
   def run(["prune_objects" | args]) do
     {options, [], []} =
       OptionParser.parse(