Reduce API memory consumption (#6137)

`echo json_encode(...)` is very memory demanding for large responses, so optimised.
Contributes to https://github.com/FreshRSS/FreshRSS/issues/6136
https://github.com/FreshRSS/FreshRSS/pull/6013#discussion_r1506779881

lib/lib_rss.php

@@ -5,6 +5,24 @@ if (version_compare(PHP_VERSION, FRESHRSS_MIN_PHP_VERSION, '<')) {
 	die(sprintf('FreshRSS error: FreshRSS requires PHP %s+!', FRESHRSS_MIN_PHP_VERSION));
 }
 
+if (!function_exists('array_is_list')) {
+	/**
+	 * Polyfill for PHP <8.1
+	 * https://php.net/array-is-list#127044
+	 * @param array<mixed> $array
+	 */
+	function array_is_list(array $array): bool {
+		$i = -1;
+		foreach ($array as $k => $v) {
+			++$i;
+			if ($k !== $i) {
+				return false;
+			}
+		}
+		return true;
+	}
+}
+
 if (!function_exists('mb_strcut')) {
 	function mb_strcut(string $str, int $start, ?int $length = null, string $encoding = 'UTF-8'): string {
 		return substr($str, $start, $length) ?: '';
@@ -89,6 +107,45 @@ function classAutoloader(string $class): void {
 spl_autoload_register('classAutoloader');
 //</Auto-loading>
 
+/**
+ * Memory efficient replacement of `echo json_encode(...)`
+ * @param array<mixed>|mixed $json
+ * @param int $optimisationDepth Number of levels for which to perform memory optimisation
+ * before calling the faster native JSON serialisation.
+ * Set to negative value for infinite depth.
+ */
+function echoJson($json, int $optimisationDepth = -1): void {
+	if ($optimisationDepth === 0 || !is_array($json)) {
+		echo json_encode($json, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
+		return;
+	}
+	$first = true;
+	if (array_is_list($json)) {
+		echo '[';
+		foreach ($json as $item) {
+			if ($first) {
+				$first = false;
+			} else {
+				echo ',';
+			}
+			echoJson($item, $optimisationDepth - 1);
+		}
+		echo ']';
+	} else {
+		echo '{';
+		foreach ($json as $key => $value) {
+			if ($first) {
+				$first = false;
+			} else {
+				echo ',';
+			}
+			echo json_encode($key, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE), ':';
+			echoJson($value, $optimisationDepth - 1);
+		}
+		echo '}';
+	}
+}
+
 function idn_to_puny(string $url): string {
 	if (function_exists('idn_to_ascii')) {
 		$idn = parse_url($url, PHP_URL_HOST);

p/api/greader.php

@@ -715,8 +715,9 @@ final class GReaderAPI {
 				$response['continuation'] = '' . $entry->id();
 			}
 		}
-
-		echo json_encode($response, JSON_OPTIONS), "\n";
+		unset($entries, $entryDAO, $items);
+		gc_collect_cycles();
+		echoJson($response, 2);	// $optimisationDepth=2 as we are interested in being memory efficient for {"items":[...]}
 		exit();
 	}
 
@@ -805,8 +806,9 @@ final class GReaderAPI {
 			'updated' => time(),
 			'items' => $items,
 		);
-
-		echo json_encode($response, JSON_OPTIONS), "\n";
+		unset($entries, $entryDAO, $items);
+		gc_collect_cycles();
+		echoJson($response, 2);	// $optimisationDepth=2 as we are interested in being memory efficient for {"items":[...]}
 		exit();
 	}