Migrations: delete contexts with BaseMigrator

Due to the lengthiness of this task, the migration has been adapted into
a BaseMigrator migration, running in the background instead.

lib/pleroma/application.ex

@@ -238,7 +238,8 @@ defmodule Pleroma.Application do
 
   defp background_migrators do
     [
-      Pleroma.Migrators.HashtagsTableMigrator
+      Pleroma.Migrators.HashtagsTableMigrator,
+      Pleroma.Migrators.ContextObjectsDeletionMigrator
     ]
   end
 

lib/pleroma/data_migration.ex

@@ -42,4 +42,5 @@ defmodule Pleroma.DataMigration do
   end
 
   def populate_hashtags_table, do: get_by_name("populate_hashtags_table")
+  def delete_context_objects, do: get_by_name("delete_context_objects")
 end

lib/pleroma/migrators/context_objects_deletion_migrator.ex

@@ -0,0 +1,139 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Migrators.ContextObjectsDeletionMigrator do
+  defmodule State do
+    use Pleroma.Migrators.Support.BaseMigratorState
+
+    @impl Pleroma.Migrators.Support.BaseMigratorState
+    defdelegate data_migration(), to: Pleroma.DataMigration, as: :delete_context_objects
+  end
+
+  use Pleroma.Migrators.Support.BaseMigrator
+
+  alias Pleroma.Migrators.Support.BaseMigrator
+  alias Pleroma.Object
+
+  @doc "This migration removes objects created exclusively for contexts, containing only an `id` field."
+
+  @impl BaseMigrator
+  def feature_config_path, do: [:features, :delete_context_objects]
+
+  @impl BaseMigrator
+  def fault_rate_allowance, do: Config.get([:delete_context_objects, :fault_rate_allowance], 0)
+
+  @impl BaseMigrator
+  def perform do
+    data_migration_id = data_migration_id()
+    max_processed_id = get_stat(:max_processed_id, 0)
+
+    Logger.info("Deleting context objects from `objects` (from oid: #{max_processed_id})...")
+
+    query()
+    |> where([object], object.id > ^max_processed_id)
+    |> Repo.chunk_stream(100, :batches, timeout: :infinity)
+    |> Stream.each(fn objects ->
+      object_ids = Enum.map(objects, & &1.id)
+
+      results = Enum.map(object_ids, &delete_context_object(&1))
+
+      failed_ids =
+        results
+        |> Enum.filter(&(elem(&1, 0) == :error))
+        |> Enum.map(&elem(&1, 1))
+
+      chunk_affected_count =
+        results
+        |> Enum.filter(&(elem(&1, 0) == :ok))
+        |> length()
+
+      for failed_id <- failed_ids do
+        _ =
+          Repo.query(
+            "INSERT INTO data_migration_failed_ids(data_migration_id, record_id) " <>
+              "VALUES ($1, $2) ON CONFLICT DO NOTHING;",
+            [data_migration_id, failed_id]
+          )
+      end
+
+      _ =
+        Repo.query(
+          "DELETE FROM data_migration_failed_ids " <>
+            "WHERE data_migration_id = $1 AND record_id = ANY($2)",
+          [data_migration_id, object_ids -- failed_ids]
+        )
+
+      max_object_id = Enum.at(object_ids, -1)
+
+      put_stat(:max_processed_id, max_object_id)
+      increment_stat(:iteration_processed_count, length(object_ids))
+      increment_stat(:processed_count, length(object_ids))
+      increment_stat(:failed_count, length(failed_ids))
+      increment_stat(:affected_count, chunk_affected_count)
+      put_stat(:records_per_second, records_per_second())
+      persist_state()
+
+      # A quick and dirty approach to controlling the load this background migration imposes
+      sleep_interval = Config.get([:delete_context_objects, :sleep_interval_ms], 0)
+      Process.sleep(sleep_interval)
+    end)
+    |> Stream.run()
+  end
+
+  @impl BaseMigrator
+  def query do
+    # Context objects have no activity type, and only one field, `id`.
+    # Only those context objects are without types.
+    from(
+      object in Object,
+      where: fragment("(?)->'type' IS NULL", object.data),
+      select: %{
+        id: object.id
+      }
+    )
+  end
+
+  @spec delete_context_object(integer()) :: {:ok | :error, integer()}
+  defp delete_context_object(id) do
+    result =
+      %Object{id: id}
+      |> Repo.delete()
+      |> elem(0)
+
+    {result, id}
+  end
+
+  @impl BaseMigrator
+  def retry_failed do
+    data_migration_id = data_migration_id()
+
+    failed_objects_query()
+    |> Repo.chunk_stream(100, :one)
+    |> Stream.each(fn object ->
+      with {res, _} when res != :error <- delete_context_object(object.id) do
+        _ =
+          Repo.query(
+            "DELETE FROM data_migration_failed_ids " <>
+              "WHERE data_migration_id = $1 AND record_id = $2",
+            [data_migration_id, object.id]
+          )
+      end
+    end)
+    |> Stream.run()
+
+    put_stat(:failed_count, failures_count())
+    persist_state()
+
+    force_continue()
+  end
+
+  defp failed_objects_query do
+    from(o in Object)
+    |> join(:inner, [o], dmf in fragment("SELECT * FROM data_migration_failed_ids"),
+      on: dmf.record_id == o.id
+    )
+    |> where([_o, dmf], dmf.data_migration_id == ^data_migration_id())
+    |> order_by([o], asc: o.id)
+  end
+end