Read ahead: a plan that cannot be made leaves the restore to the usual reads; only a whole buffered SFTP packet counts as an answer waiting

lib/Backend/SftpRangeFetcher.php

@@ -150,7 +150,7 @@ final class SftpRangeFetcher {
 			$buffered = $vars['requestBuffer'] ?? null;
 			$channels = $vars['channel_buffers'] ?? null;
 			if ((is_array($buffered) && isset($job->queue[0]) && array_key_exists($job->queue[0][0], $buffered))
-				|| (is_string($vars['packet_buffer'] ?? null) && $vars['packet_buffer'] !== '')
+				|| (is_string($vars['packet_buffer'] ?? null) && self::wholePacket($vars['packet_buffer']))
 				|| (is_array($channels) && array_filter($channels) !== [])) {
 				$ready[] = $c;
 				continue;
@@ -173,6 +173,12 @@ final class SftpRangeFetcher {
 		return $n === false || $n === 0 ? array_keys($this->jobs) : array_keys($read);
 	}
 
+	/** Whether $buffer starts with a whole SFTP packet (length, then that many bytes). */
+	private static function wholePacket(string $buffer): bool {
+		$length = strlen($buffer) >= 4 ? unpack('N', $buffer) : false;
+		return is_array($length) && strlen($buffer) >= 4 + $length[1];
+	}
+
 	private function start(int $c, int $i): void {
 		[$path, $offset, $length] = $this->ranges[$i];
 		$fh = $this->spoolFile();

lib/Repository/Repository.php

@@ -243,18 +243,22 @@ final class Repository {
 			@rmdir($dir);
 		}
 		$plan = new ReadAheadPlan(rtrim($target, '/') . '/.ngb-restore-spool-' . bin2hex(random_bytes(4)));
-		foreach ($this->streamEntries($snapshotId) as $entry) {
-			if ($prefix !== '' && !str_starts_with($entry['p'], $prefix)) {
-				continue;
-			}
-			foreach ($entry['b'] as $id) {
-				try {
-					[$pack, $offset, $length] = $this->index->get($id);
-				} catch (RepositoryException) {
-					continue; // the restore reports it when it gets there
+		try {
+			foreach ($this->streamEntries($snapshotId) as $entry) {
+				if ($prefix !== '' && !str_starts_with($entry['p'], $prefix)) {
+					continue;
+				}
+				foreach ($entry['b'] as $id) {
+					try {
+						[$pack, $offset, $length] = $this->index->get($id);
+					} catch (RepositoryException) {
+						continue; // the restore reports it when it gets there
+					}
+					$plan->add('packs/' . substr($pack, 0, 2) . '/' . $pack, $offset, $length);
 				}
-				$plan->add('packs/' . substr($pack, 0, 2) . '/' . $pack, $offset, $length);
 			}
+		} catch (\Throwable) {
+			return null; // reading ahead is only faster: the restore itself reads the snapshot again and reports what is wrong
 		}
 		if (count($plan->ranges()) < 2 || (!is_dir($plan->spool) && !@mkdir($plan->spool, 0700, true))) {
 			return null;