fix: [torrust#526] DB migration for SQLite. Backup secondary tables

The table `torrust_torrents` has 8 secondary tables that reference it
with a foreign key:

```sql
FOREIGN KEY("torrent_id") REFERENCES "torrust_torrents"("torrent_id") ON DELETE CASCADE,
```

The migration fixed in this commit creates a new table in order to alter
it and drops the old one. However, when you drop the old table, SQLite
uses an implicit DELETE query for the `torrust_torrents` table which
triggers a DELETE ON CASCADE, deleting all the related records in
secondary tables.

The secondary tables are:

- torrust_torrent_files
- torrust_torrent_announce_urls
- torrust_torrent_info
- torrust_torrent_tracker_stats
- torrust_torrent_tag_links
- torrust_torrent_info_hashes
- torrust_torrent_http_seeds
- torrust_torrent_nodes

These tables store the torrent file fiel together with the master
`torrust_torrents`.

migrations/sqlite3/20240305120015_torrust_add_independent_root_hash_field.sql

@@ -1,6 +1,68 @@
--- add field `root_hash` and make `pieces` nullable
+PRAGMA foreign_keys = off;
+
+-- Step 1: backup secondary tables. They will be truncated because of the DELETE ON CASCADE
+CREATE TEMPORARY TABLE IF NOT EXISTS "torrust_torrent_files_backup" (
+	"file_id"	INTEGER NOT NULL,
+	"torrent_id"	INTEGER NOT NULL,
+	"md5sum"	TEXT DEFAULT NULL,
+	"length"	BIGINT NOT NULL,
+	"path"	TEXT DEFAULT NULL
+);
+INSERT INTO torrust_torrent_files_backup SELECT * FROM torrust_torrent_files;
+
+CREATE TEMPORARY TABLE IF NOT EXISTS "torrust_torrent_announce_urls_backup" (
+	"announce_url_id"	INTEGER NOT NULL,
+	"torrent_id"	INTEGER NOT NULL,
+	"tracker_url"	TEXT NOT NULL
+);
+INSERT INTO torrust_torrent_announce_urls_backup SELECT * FROM torrust_torrent_announce_urls;
+
+CREATE TEMPORARY TABLE IF NOT EXISTS "torrust_torrent_info_backup" (
+	"torrent_id"	INTEGER NOT NULL,
+	"title"	VARCHAR(256) NOT NULL UNIQUE,
+	"description"	TEXT DEFAULT NULL
+);
+INSERT INTO torrust_torrent_info_backup SELECT * FROM torrust_torrent_info;
+
+CREATE TEMPORARY TABLE IF NOT EXISTS "torrust_torrent_tracker_stats_backup" (
+	"torrent_id"	INTEGER NOT NULL,
+	"tracker_url"	VARCHAR(256) NOT NULL,
+	"seeders"	INTEGER NOT NULL DEFAULT 0,
+	"leechers"	INTEGER NOT NULL DEFAULT 0
+);
+INSERT INTO torrust_torrent_tracker_stats_backup SELECT * FROM torrust_torrent_tracker_stats;
+
+CREATE TEMPORARY TABLE IF NOT EXISTS "torrust_torrent_tag_links_backup" (
+	"torrent_id"	INTEGER NOT NULL,
+	"tag_id"	INTEGER NOT NULL
+);
+INSERT INTO torrust_torrent_tag_links_backup SELECT * FROM torrust_torrent_tag_links;
+
+CREATE TEMPORARY TABLE IF NOT EXISTS "torrust_torrent_info_hashes_backup" (
+	"info_hash"	TEXT NOT NULL,
+	"canonical_info_hash"	TEXT NOT NULL,
+	"original_is_known"	BOOLEAN NOT NULL
+);
+INSERT INTO torrust_torrent_info_hashes_backup SELECT * FROM torrust_torrent_info_hashes;
+
+CREATE TEMPORARY TABLE IF NOT EXISTS "torrust_torrent_http_seeds_backup" (
+	"http_seed_id"	INTEGER NOT NULL,
+	"torrent_id"	INTEGER NOT NULL,
+	"seed_url"	TEXT NOT NULL
+);
+INSERT INTO torrust_torrent_http_seeds_backup SELECT * FROM torrust_torrent_http_seeds;
+
+CREATE TEMPORARY TABLE IF NOT EXISTS "torrust_torrent_nodes_backup" (
+	"node_id"	INTEGER NOT NULL,
+	"torrent_id"	INTEGER NOT NULL,
+	"node_ip"	TEXT NOT NULL,
+	"node_port"	INTEGER NOT NULL
+);
+INSERT INTO torrust_torrent_nodes_backup SELECT * FROM torrust_torrent_nodes;
+
+-- Step 2: Add field `root_hash` and make `pieces` nullable
 CREATE TABLE
-    "torrust_torrents_new" (
+    IF NOT EXISTS "torrust_torrents_new" (
         "torrent_id" INTEGER NOT NULL,
         "uploader_id" INTEGER NOT NULL,
         "category_id" INTEGER,
@@ -23,7 +85,7 @@ CREATE TABLE
         PRIMARY KEY ("torrent_id" AUTOINCREMENT)
     );
 
--- Step 2: Copy data from the old table to the new table
+-- Step 3: Copy data from the old table to the new table
 INSERT INTO
     torrust_torrents_new (
         torrent_id,
@@ -69,9 +131,30 @@ SELECT
 FROM
     torrust_torrents;
 
--- Step 3: Drop the old table
+-- Step 4: Drop the old table
 DROP TABLE torrust_torrents;
 
--- Step 4: Rename the new table to the original name
-ALTER TABLE torrust_torrents_new
-RENAME TO torrust_torrents;
+-- Step 5: Rename the new table to the original name
+ALTER TABLE torrust_torrents_new RENAME TO torrust_torrents;
+
+-- Step 6: Repopulate secondary tables from backup tables
+INSERT INTO torrust_torrent_files SELECT * FROM torrust_torrent_files_backup;
+INSERT INTO torrust_torrent_announce_urls SELECT * FROM torrust_torrent_announce_urls_backup;
+INSERT INTO torrust_torrent_info SELECT * FROM torrust_torrent_info_backup;
+INSERT INTO torrust_torrent_tracker_stats SELECT * FROM torrust_torrent_tracker_stats_backup;
+INSERT INTO torrust_torrent_tag_links SELECT * FROM torrust_torrent_tag_links_backup;
+INSERT INTO torrust_torrent_info_hashes SELECT * FROM torrust_torrent_info_hashes_backup;
+INSERT INTO torrust_torrent_http_seeds SELECT * FROM torrust_torrent_http_seeds_backup;
+INSERT INTO torrust_torrent_nodes SELECT * FROM torrust_torrent_nodes_backup;
+
+-- Step 7: Drop temporary secondary table backups
+DROP TABLE torrust_torrent_files_backup;
+DROP TABLE torrust_torrent_announce_urls_backup;
+DROP TABLE torrust_torrent_info_backup;
+DROP TABLE torrust_torrent_tracker_stats_backup;
+DROP TABLE torrust_torrent_tag_links_backup;
+DROP TABLE torrust_torrent_info_hashes_backup;
+DROP TABLE torrust_torrent_http_seeds_backup;
+DROP TABLE torrust_torrent_nodes_backup;
+
+PRAGMA foreign_keys = on;