target:add: show progress and time left while reading the index of an existing repository

lib/Command/TargetAdd.php

@@ -64,11 +64,15 @@ HELP);
 			return 1;
 		}
 		// Adopting an existing repository reads its whole index first, which takes a while on a
-		// large backup or a slow line; say so instead of looking stuck.
-		$output->writeln('Connecting to the location; with existing backups this reads their index first and can take a while ...');
+		// large backup or a slow line; show how far it is instead of looking stuck.
+		$output->writeln('Connecting to the location; with existing backups this reads their index first ...');
 		try {
-			$r = $this->targets->add($input->getArgument('name'), $input->getArgument('backend'), $input->getOption('auth'), $options, (string)$input->getOption('path'));
+			$r = $this->targets->add($input->getArgument('name'), $input->getArgument('backend'), $input->getOption('auth'), $options,
+				(string)$input->getOption('path'), self::progress($output));
 		} catch (\Throwable $e) {
+			if ($output->isDecorated()) {
+				$output->write("\r\033[K"); // clear an unfinished progress line
+			}
 			$output->writeln('<error>' . $e->getMessage() . '</error>');
 			return 1;
 		}
@@ -76,4 +80,34 @@ HELP);
 			$r['target']->getRepositoryId(), $r['created'] ? 'created' : 'found and adopted'));
 		return 0;
 	}
+
+	/**
+	 * Progress of reading the index: on a terminal one line that is updated in place, otherwise
+	 * (a log, a pipe) a line every 30 seconds. The time left is estimated from the rate so far.
+	 *
+	 * @return callable(int, int): void
+	 */
+	private static function progress(OutputInterface $output): callable {
+		$start = microtime(true);
+		$shown = 0.0;
+		$inPlace = $output->isDecorated();
+		return static function (int $done, int $total) use ($output, $start, &$shown, $inPlace): void {
+			$now = microtime(true);
+			if ($done < $total && $now - $shown < ($inPlace ? 1 : 30)) {
+				return;
+			}
+			$shown = $now;
+			$line = sprintf('Reading the index: %s/%s files (%d%%)', number_format($done), number_format($total), intdiv(100 * $done, max(1, $total)));
+			$elapsed = $now - $start;
+			if ($done < $total && $done > 0 && $elapsed >= 5) {
+				$left = (int)ceil($elapsed / (float)$done * (float)($total - $done) / 60.0);
+				$line .= $left <= 1 ? ', less than a minute left' : sprintf(', about %d min left', $left);
+			}
+			if ($inPlace) {
+				$output->write("\r\033[K" . $line . ($done >= $total ? "\n" : ''));
+			} else {
+				$output->writeln($line);
+			}
+		};
+	}
 }

lib/Repository/Repository.php

@@ -81,7 +81,8 @@ final class Repository {
 	}
 
 	/** Open with the master key the server holds (scheduled backups, no passphrase needed). */
-	public static function openWithKey(IBackend $backend, KeyRing $keys, ?IIndexCache $cache = null, ?ICatalogAnchor $anchor = null): self {
+	/** @param ?callable(int, int): void $progress index files downloaded so far, and in all (see BlobIndex::load()) */
+	public static function openWithKey(IBackend $backend, KeyRing $keys, ?IIndexCache $cache = null, ?ICatalogAnchor $anchor = null, ?callable $progress = null): self {
 		$config = self::readConfig($backend);
 		try {
 			$ok = isset($config['check']) && (new StreamCipher($keys))->decryptString((string)base64_decode($config['check'], true), 'config-check') === $config['id'];
@@ -95,7 +96,7 @@ final class Repository {
 		if ($cache !== null) {
 			$repo->index->setCache($cache, $config['id']);
 		}
-		$repo->index->load();
+		$repo->index->load(false, $progress);
 		$repo->loadCatalog($anchor);
 		return $repo;
 	}

lib/Service/TargetService.php

@@ -48,15 +48,16 @@ final class TargetService {
 	 * installation's key), then save it.
 	 *
 	 * @param array<string, mixed> $options
+	 * @param ?callable(int, int): void $progress reading an existing repository's index: files done, and in all
 	 * @return array{target: Target, created: bool}
 	 */
-	public function add(string $name, string $backend, string $auth, array $options, string $basePath): array {
+	public function add(string $name, string $backend, string $auth, array $options, string $basePath, ?callable $progress = null): array {
 		$this->validateName($name);
 		$this->requireUsableBackend($backend);
 		$keys = $this->keys->keyRing();
 		$storageBackend = $this->buildBackend($backend, $auth, $options, $basePath);
 		if ($storageBackend->exists('config')) {
-			$repo = Repository::openWithKey($storageBackend, $keys, $this->indexCache, $this->anchor);
+			$repo = Repository::openWithKey($storageBackend, $keys, $this->indexCache, $this->anchor, $progress);
 			$created = false;
 		} else {
 			$repo = Repository::initWithKey($storageBackend, $keys, $this->keys->wrappedKey(), $this->anchor);