Refuse a full restore when the table prefix differs

CHANGELOG.md

@@ -24,6 +24,8 @@ All-in-One onto a plain Nextcloud with PostgreSQL).
 - A full restore no longer refuses when apps are missing: it restores their data and lists the
   apps to install afterwards. `nextcloud-aio` is never required, so a backup of Nextcloud
   All-in-One can be restored onto a plain Nextcloud.
+- A full restore onto an installation with a different table prefix (`dbtableprefix`) is refused
+  before it starts, with what to change, instead of restoring into tables nobody uses.
 - All problems are listed at once, with the app each missing table probably belongs to. For
   backups made before this version, `--skip-missing-tables` leaves out tables whose structure is
   unknown.

lib/Db/DbDumper.php

@@ -145,6 +145,11 @@ final class DbDumper {
 	 */
 	public function restore(Repository $repo, array $manifest, ?array &$report = null, ?callable $heartbeat = null, bool $skipMissing = false): int {
 		$report = ['skipped' => [], 'restored' => [], 'created' => [], 'missing' => []];
+		$source = self::dumpPrefix($manifest);
+		if ($source !== null && $source !== $this->prefix) {
+			// Every table would look missing and be created under the old prefix, unused.
+			throw new \RuntimeException("The backup's tables use the prefix '$source', this installation '{$this->prefix}'. Install Nextcloud again with 'dbtableprefix' => '$source' in config/config.php before occ maintenance:install, and restore again.");
+		}
 		$missing = $this->missingTables($manifest);
 		$unknown = array_values(array_filter($missing, fn (string $t) => !isset($manifest['tables'][$t]['schema'])));
 		if ($unknown !== [] && !$skipMissing) {
@@ -241,6 +246,22 @@ final class DbDumper {
 		return $total;
 	}
 
+	/**
+	 * The table prefix of the instance the dump was made on (from its appconfig table), or null
+	 * when the dump has none.
+	 *
+	 */
+	public static function dumpPrefix(array $manifest): ?string {
+		/** @var list<string> $tables table names */
+		$tables = array_keys($manifest['tables'] ?? []);
+		foreach ($tables as $name) {
+			if (str_ends_with($name, 'appconfig') && !str_ends_with($name, '_ex_appconfig')) {
+				return substr($name, 0, -strlen('appconfig'));
+			}
+		}
+		return null;
+	}
+
 	/**
 	 * Tables in the dump that do not exist on this instance.
 	 *

lib/Service/RestoreFullService.php

@@ -63,19 +63,25 @@ final class RestoreFullService {
 		$enabled = array_flip($this->appManager->getEnabledApps());
 		$missingApps = array_values(array_diff(array_keys($meta['apps'] ?? []), array_keys($enabled), self::ENVIRONMENT_APPS));
 		$missingTables = [];
+		$prefixMismatch = null;
 		if (isset($meta['db'])) {
 			$manifest = json_decode($repo->getObject($meta['db']), true, 512, JSON_THROW_ON_ERROR);
 			$prefix = $this->config->getSystemValueString('dbtableprefix', 'oc_');
+			$sourcePrefix = DbDumper::dumpPrefix($manifest);
+			if ($sourcePrefix !== null && $sourcePrefix !== $prefix) {
+				$prefixMismatch = [$sourcePrefix, $prefix];
+			}
 			$dumper = new DbDumper($this->db->getInner(), $prefix);
 			$missingTables = array_values(array_filter($dumper->missingTables($manifest),
 				fn (string $t) => !isset($manifest['tables'][$t]['schema']) && !str_starts_with($t, $prefix . 'ngb_')));
 		}
 		return [
-			'ok' => $currentMajor === $snapshotMajor && ($missingTables === [] || $skipMissingTables),
+			'ok' => $currentMajor === $snapshotMajor && $prefixMismatch === null && ($missingTables === [] || $skipMissingTables),
 			'currentMajor' => $currentMajor,
 			'snapshotMajor' => $snapshotMajor,
 			'missingApps' => $missingApps,
-			'missingTables' => $missingTables,
+			'missingTables' => $prefixMismatch === null ? $missingTables : [],
+			'prefixMismatch' => $prefixMismatch,
 		];
 	}
 
@@ -224,6 +230,10 @@ final class RestoreFullService {
 		if ($check['currentMajor'] !== $check['snapshotMajor']) {
 			$parts[] = "this installation is Nextcloud {$check['currentMajor']}, the backup is from {$check['snapshotMajor']} (same major version required)";
 		}
+		if (($check['prefixMismatch'] ?? null) !== null) {
+			[$source, $here] = $check['prefixMismatch'];
+			$parts[] = "the backup's tables use the prefix '$source', this installation '$here': install Nextcloud again with 'dbtableprefix' => '$source' in config/config.php before occ maintenance:install, and restore again";
+		}
 		if ($check['missingTables'] !== []) {
 			$prefix = $this->config->getSystemValueString('dbtableprefix', 'oc_');
 			$parts[] = DbDumper::describeMissing($check['missingTables'], $prefix);