From ed74c43f18998d6e3469203dd71109d519fd4f2e Mon Sep 17 00:00:00 2001
From: wn_ <invalid@email.com>
Date: Tue, 14 Dec 2021 12:06:32 +0000
Subject: Handle the admin user not having any entries in
 'Feeds::_get_global_unread'.

---
 classes/feeds.php | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'classes')

diff --git a/classes/feeds.php b/classes/feeds.php
index a9afb70f2..3c0578782 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -1359,7 +1359,7 @@ class Feeds extends Handler_Protected {
 		$sth->execute([$user_id]);
 		$row = $sth->fetch();
 
-		return $row["count"];
+		return $row["count"] ?? 0;
 	}
 
 	static function _get_cat_title(int $cat_id): string {
-- 
cgit v1.2.3


From ddc81b2c8979904bd042efd0412d93fb7c821903 Mon Sep 17 00:00:00 2001
From: wn_ <invalid@email.com>
Date: Tue, 14 Dec 2021 12:47:25 +0000
Subject: Add a note on why ed74c43f18 was needed.

---
 classes/feeds.php | 1 +
 1 file changed, 1 insertion(+)

(limited to 'classes')

diff --git a/classes/feeds.php b/classes/feeds.php
index 3c0578782..2a4fe4993 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -1359,6 +1359,7 @@ class Feeds extends Handler_Protected {
 		$sth->execute([$user_id]);
 		$row = $sth->fetch();
 
+		// Handle 'SUM()' returning null if there are no articles/results (e.g. admin user with no feeds)
 		return $row["count"] ?? 0;
 	}
 
-- 
cgit v1.2.3


From 0726a9d820855f96dccf873e6279f74479800efb Mon Sep 17 00:00:00 2001
From: wn_ <invalid@email.com>
Date: Tue, 14 Dec 2021 12:50:53 +0000
Subject: Handle another potential 'SUM()' null situation in Feeds.

---
 classes/feeds.php | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

(limited to 'classes')

diff --git a/classes/feeds.php b/classes/feeds.php
index 2a4fe4993..62fd6a5b3 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -955,7 +955,8 @@ class Feeds extends Handler_Protected {
 			$sth->execute([$owner_uid, $feed]);
 			$row = $sth->fetch();
 
-			return $row["count"];
+			// Handle 'SUM()' returning null if there are no results
+			return $row["count"] ?? 0;
 
 		} else if ($n_feed == -1) {
 			$match_part = "marked = true";
-- 
cgit v1.2.3


From 720b31879634f21ea7db85f49d6f07ec7d7344bc Mon Sep 17 00:00:00 2001
From: Andrew Dolgov <noreply@fakecake.org>
Date: Tue, 14 Dec 2021 21:53:45 +0300
Subject:  * fox.form.Select: add several properties allowing it to better 
 imitate other controls like DropDownButton, etc.  * rework several main
 toolbar items to use fox.form.Select instead of other controls  * replace
 HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM with
 HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM2 because of markup change (option
 instead of menuitem)  * PluginHost: add some explicit typecasts to make
 intellephense shut up

---
 classes/feeds.php      |  2 +-
 classes/plugin.php     | 13 +++++++++++++
 classes/pluginhost.php | 28 +++++++++++++++++-----------
 3 files changed, 31 insertions(+), 12 deletions(-)

(limited to 'classes')

diff --git a/classes/feeds.php b/classes/feeds.php
index 62fd6a5b3..503108e41 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -133,7 +133,7 @@ class Feeds extends Handler_Protected {
 		$reply['vfeed_group_enabled'] = $vfeed_group_enabled;
 
 		$plugin_menu_items = "";
-		PluginHost::getInstance()->chain_hooks_callback(PluginHost::HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM,
+		PluginHost::getInstance()->chain_hooks_callback(PluginHost::HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM2,
 			function ($result) use (&$plugin_menu_items) {
 				$plugin_menu_items .= $result;
 			},
diff --git a/classes/plugin.php b/classes/plugin.php
index be8376925..3bced3b04 100644
--- a/classes/plugin.php
+++ b/classes/plugin.php
@@ -647,6 +647,7 @@ abstract class Plugin {
 	}
 
 	/** Allows adding custom elements to headlines Select... dropdown
+	 * @deprecated removed, see Plugin::hook_headline_toolbar_select_menu_item2()
 	 * @param int $feed_id
 	 * @param int $is_cat
 	 * @return string
@@ -658,6 +659,18 @@ abstract class Plugin {
 		return "";
 	}
 
+	/** Allows adding custom elements to headlines Select... select dropdown (<option> format)
+	 * @param int $feed_id
+	 * @param int $is_cat
+	 * @return string
+	 * @see PluginHost::HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM2
+	 */
+	function hook_headline_toolbar_select_menu_item2($feed_id, $is_cat) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
 	/** Invoked when user tries to subscribe to feed, may override information (i.e. feed URL) used afterwards
 	 * @param string $url
 	 * @param string $auth_login
diff --git a/classes/pluginhost.php b/classes/pluginhost.php
index a3a389def..952d4df77 100755
--- a/classes/pluginhost.php
+++ b/classes/pluginhost.php
@@ -189,9 +189,14 @@ class PluginHost {
 	/** @see Plugin::hook_headlines_custom_sort_override() */
 	const HOOK_HEADLINES_CUSTOM_SORT_OVERRIDE = "hook_headlines_custom_sort_override";
 
-	/** @see Plugin::hook_headline_toolbar_select_menu_item() */
+	/** @see Plugin::hook_headline_toolbar_select_menu_item()
+	 * @deprecated removed, see PluginHost::HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM2
+	*/
 	const HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM = "hook_headline_toolbar_select_menu_item";
 
+	/** @see Plugin::hook_headline_toolbar_select_menu_item() */
+	const HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM2 = "hook_headline_toolbar_select_menu_item2";
+
 	/** @see Plugin::hook_pre_subscribe() */
 	const HOOK_PRE_SUBSCRIBE = "hook_pre_subscribe";
 
@@ -270,9 +275,10 @@ class PluginHost {
 	 * @param mixed $args
 	 */
 	function run_hooks(string $hook, ...$args): void {
-		$method = strtolower($hook);
 
-		foreach ($this->get_hooks($hook) as $plugin) {
+		$method = strtolower((string)$hook);
+
+		foreach ($this->get_hooks((string)$hook) as $plugin) {
 			//Debug::log("invoking: " . get_class($plugin) . "->$hook()", Debug::$LOG_VERBOSE);
 
 			try {
@@ -291,9 +297,9 @@ class PluginHost {
 	 * @param mixed $check
 	 */
 	function run_hooks_until(string $hook, $check, ...$args): bool {
-		$method = strtolower($hook);
+		$method = strtolower((string)$hook);
 
-		foreach ($this->get_hooks($hook) as $plugin) {
+		foreach ($this->get_hooks((string)$hook) as $plugin) {
 			try {
 				$result = $plugin->$method(...$args);
 
@@ -315,9 +321,9 @@ class PluginHost {
 	 * @param mixed $args
 	 */
 	function run_hooks_callback(string $hook, Closure $callback, ...$args): void {
-		$method = strtolower($hook);
+		$method = strtolower((string)$hook);
 
-		foreach ($this->get_hooks($hook) as $plugin) {
+		foreach ($this->get_hooks((string)$hook) as $plugin) {
 			//Debug::log("invoking: " . get_class($plugin) . "->$hook()", Debug::$LOG_VERBOSE);
 
 			try {
@@ -336,9 +342,9 @@ class PluginHost {
 	 * @param mixed $args
 	 */
 	function chain_hooks_callback(string $hook, Closure $callback, &...$args): void {
-		$method = strtolower($hook);
+		$method = strtolower((string)$hook);
 
-		foreach ($this->get_hooks($hook) as $plugin) {
+		foreach ($this->get_hooks((string)$hook) as $plugin) {
 			//Debug::log("invoking: " . get_class($plugin) . "->$hook()", Debug::$LOG_VERBOSE);
 
 			try {
@@ -358,7 +364,7 @@ class PluginHost {
 	function add_hook(string $type, Plugin $sender, int $priority = 50): void {
 		$priority = (int) $priority;
 
-		if (!method_exists($sender, strtolower($type))) {
+		if (!method_exists($sender, strtolower((string)$type))) {
 			user_error(
 				sprintf("Plugin %s tried to register a hook without implementation: %s",
 					get_class($sender), $type),
@@ -422,7 +428,7 @@ class PluginHost {
 
 		asort($plugins);
 
-		$this->load(join(",", $plugins), $kind, $owner_uid, $skip_init);
+		$this->load(join(",", $plugins), (int)$kind, $owner_uid, $skip_init);
 	}
 
 	/**
-- 
cgit v1.2.3


From 40b2356be212cfee1d68784714672b7c00114e2d Mon Sep 17 00:00:00 2001
From: Andrew Dolgov <noreply@fakecake.org>
Date: Mon, 20 Dec 2021 07:56:16 +0300
Subject: filters:  * add filter action to ignore feed-provided tags  *
 simplify handling of various filter-provided tags  * bump schema to 146

---
 classes/config.php       |  2 +-
 classes/pref/filters.php | 28 ++++++++++++++++++++--------
 classes/rssutils.php     | 28 +++++++++++++---------------
 3 files changed, 34 insertions(+), 24 deletions(-)

(limited to 'classes')

diff --git a/classes/config.php b/classes/config.php
index 1c65fc76f..2c78b908d 100644
--- a/classes/config.php
+++ b/classes/config.php
@@ -6,7 +6,7 @@ class Config {
 	const T_STRING = 2;
 	const T_INT = 3;
 
-	const SCHEMA_VERSION = 145;
+	const SCHEMA_VERSION = 146;
 
 	/** override default values, defined below in _DEFAULTS[], prefixing with _ENVVAR_PREFIX:
 	 *
diff --git a/classes/pref/filters.php b/classes/pref/filters.php
index 6e6e3d9ee..04178f1a6 100755
--- a/classes/pref/filters.php
+++ b/classes/pref/filters.php
@@ -1,6 +1,15 @@
 <?php
 class Pref_Filters extends Handler_Protected {
 
+	const ACTION_TAG = 4;
+	const ACTION_SCORE = 6;
+	const ACTION_LABEL = 7;
+	const ACTION_PLUGIN = 9;
+	const ACTION_REMOVE_TAG = 10;
+
+	const PARAM_ACTIONS = [self::ACTION_TAG, self::ACTION_SCORE,
+		self::ACTION_LABEL, self::ACTION_PLUGIN, self::ACTION_REMOVE_TAG];
+
 	function csrf_ignore(string $method): bool {
 		$csrf_ignored = array("index", "getfiltertree", "savefilterorder");
 
@@ -274,7 +283,7 @@ class Pref_Filters extends Handler_Protected {
 				}
 			}
 
-			if ($line['action_id'] == 7) {
+			if ($line['action_id'] == self::ACTION_LABEL) {
 				$label_sth = $this->pdo->prepare("SELECT fg_color, bg_color
 					FROM ttrss_labels2 WHERE caption = ? AND
 						owner_uid = ?");
@@ -474,11 +483,7 @@ class Pref_Filters extends Handler_Protected {
 
 			$title = __($row["description"]);
 
-			if ($action["action_id"] == 4 || $action["action_id"] == 6 ||
-				$action["action_id"] == 7)
-				$title .= ": " . $action["action_param"];
-
-			if ($action["action_id"] == 9) {
+			if ($action["action_id"] == self::ACTION_PLUGIN) {
 				list ($pfclass, $pfaction) = explode(":", $action["action_param"]);
 
 				$filter_actions = PluginHost::getInstance()->get_filter_actions();
@@ -491,6 +496,8 @@ class Pref_Filters extends Handler_Protected {
 						}
 					}
 				}
+			} else if (in_array($action["action_id"], self::PARAM_ACTIONS)) {
+				$title .= ": " . $action["action_param"];
 			}
 		}
 
@@ -596,14 +603,19 @@ class Pref_Filters extends Handler_Protected {
 					$action_param = $action["action_param"];
 					$action_param_label = $action["action_param_label"];
 
-					if ($action_id == 7) {
+					if ($action_id == self::ACTION_LABEL) {
 						$action_param = $action_param_label;
 					}
 
-					if ($action_id == 6) {
+					if ($action_id == self::ACTION_SCORE) {
 						$action_param = (int)str_replace("+", "", $action_param);
 					}
 
+					if (in_array($action_id, [self::ACTION_TAG, self::ACTION_REMOVE_TAG])) {
+						$action_param = implode(", ", FeedItem_Common::normalize_categories(
+							explode(",", $action_param)));
+					}
+
 					$asth->execute([$filter_id, $action_id, $action_param]);
 				}
 			}
diff --git a/classes/rssutils.php b/classes/rssutils.php
index b886a060c..7b50343f6 100755
--- a/classes/rssutils.php
+++ b/classes/rssutils.php
@@ -1164,32 +1164,30 @@ class RSSUtils {
 				}
 
 				// check for manual tags (we have to do it here since they're loaded from filters)
-
 				foreach ($article_filters as $f) {
 					if ($f["type"] == "tag") {
+						$entry_tags = array_merge($entry_tags,
+							FeedItem_Common::normalize_categories(explode(",", $f["param"])));
+					}
+				}
 
-						$manual_tags = array_map('trim', explode(",", mb_strtolower($f["param"])));
-
-						foreach ($manual_tags as $tag) {
-							array_push($entry_tags, $tag);
-						}
+				// like boring tags, but filter-based
+				foreach ($article_filters as $f) {
+					if ($f["type"] == "ignore-tag") {
+						$entry_tags = array_diff($entry_tags,
+							FeedItem_Common::normalize_categories(explode(",", $f["param"])));
 					}
 				}
 
 				// Skip boring tags
-
-				$boring_tags = array_map('trim',
-						explode(",", mb_strtolower(
-							get_pref(Prefs::BLACKLISTED_TAGS, $feed_obj->owner_uid))));
-
 				$entry_tags = FeedItem_Common::normalize_categories(
-					array_unique(
-						array_diff($entry_tags, $boring_tags)));
+						array_diff($entry_tags,
+							FeedItem_Common::normalize_categories(explode(",",
+								get_pref(Prefs::BLACKLISTED_TAGS, $feed_obj->owner_uid)))));
 
-				Debug::log("filtered tags: " . implode(", ", $entry_tags), Debug::LOG_VERBOSE);
+				Debug::log("resulting article tags: " . implode(", ", $entry_tags), Debug::LOG_VERBOSE);
 
 				// Save article tags in the database
-
 				if (count($entry_tags) > 0) {
 
 					$tsth = $pdo->prepare("SELECT id FROM ttrss_tags
-- 
cgit v1.2.3


From 7aeaa1b039b269c75e543045dcf8c077b3aa258d Mon Sep 17 00:00:00 2001
From: Andrew Dolgov <noreply@fakecake.org>
Date: Mon, 20 Dec 2021 08:03:30 +0300
Subject: rssutils: rewrite several invocations of (deprecated)
 rewrite_relative_url() to UrlHelper::rewrite_relative()

---
 classes/rssutils.php | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

(limited to 'classes')

diff --git a/classes/rssutils.php b/classes/rssutils.php
index 7b50343f6..e826a32f8 100755
--- a/classes/rssutils.php
+++ b/classes/rssutils.php
@@ -557,7 +557,7 @@ class RSSUtils {
 			Debug::log("language: $feed_language", Debug::LOG_VERBOSE);
 			Debug::log("processing feed data...", Debug::LOG_VERBOSE);
 
-			$site_url = mb_substr(rewrite_relative_url($feed_obj->feed_url, clean($rss->get_link())), 0, 245);
+			$site_url = mb_substr(UrlHelper::rewrite_relative($feed_obj->feed_url, clean($rss->get_link())), 0, 245);
 
 			Debug::log("site_url: $site_url", Debug::LOG_VERBOSE);
 			Debug::log("feed_title: {$rss->get_title()}", Debug::LOG_VERBOSE);
@@ -736,7 +736,7 @@ class RSSUtils {
 
 						// TODO: Just use FeedEnclosure (and modify it to cover whatever justified this)?
 						$e_item = array(
-							rewrite_relative_url($site_url, $e->link),
+							UrlHelper::rewrite_relative($site_url, $e->link),
 							$e->type, $e->length, $e->title, $e->width, $e->height);
 
 						// Yet another episode of "mysql utf8_general_ci is gimped"
@@ -1284,7 +1284,7 @@ class RSSUtils {
 			foreach ($enclosures as $enc) {
 
 				if (preg_match("/(image|audio|video)/", $enc[1])) {
-					$src = rewrite_relative_url($site_url, $enc[0]);
+					$src = UrlHelper::rewrite_relative($site_url, $enc[0]);
 
 					$local_filename = sha1($src);
 
@@ -1310,7 +1310,7 @@ class RSSUtils {
 
 	/* TODO: move to DiskCache? */
 	static function cache_media_url(DiskCache $cache, string $url, string $site_url): void {
-		$url = rewrite_relative_url($site_url, $url);
+		$url = UrlHelper::rewrite_relative($site_url, $url);
 		$local_filename = sha1($url);
 
 		Debug::log("cache_media: checking $url", Debug::LOG_VERBOSE);
@@ -1872,14 +1872,14 @@ class RSSUtils {
 
 				$base = $xpath->query('/html/head/base[@href]');
 				foreach ($base as $b) {
-					$url = rewrite_relative_url($url, $b->getAttribute("href"));
+					$url = UrlHelper::rewrite_relative($url, $b->getAttribute("href"));
 					break;
 				}
 
 				$entries = $xpath->query('/html/head/link[@rel="shortcut icon" or @rel="icon"]');
 				if (count($entries) > 0) {
 					foreach ($entries as $entry) {
-						$favicon_url = rewrite_relative_url($url, $entry->getAttribute("href"));
+						$favicon_url = UrlHelper::rewrite_relative($url, $entry->getAttribute("href"));
 						break;
 					}
 				}
@@ -1887,7 +1887,7 @@ class RSSUtils {
 		}
 
 		if (!$favicon_url)
-			$favicon_url = rewrite_relative_url($url, "/favicon.ico");
+			$favicon_url = UrlHelper::rewrite_relative($url, "/favicon.ico");
 
 		return $favicon_url;
 	}
-- 
cgit v1.2.3


From 6971ca08b24dfb6d96568a3f20d211240bbab59c Mon Sep 17 00:00:00 2001
From: Andrew Dolgov <noreply@fakecake.org>
Date: Thu, 23 Dec 2021 17:32:27 +0300
Subject: remove deprecated LOG_ constants

---
 classes/feeds.php | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

(limited to 'classes')

diff --git a/classes/feeds.php b/classes/feeds.php
index 503108e41..65027cb9e 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -718,7 +718,7 @@ class Feeds extends Handler_Protected {
 						<fieldset>
 							<label>
 							<?= \Controls\select_hash("xdebug", $xdebug,
-									[Debug::$LOG_VERBOSE => "LOG_VERBOSE", Debug::$LOG_EXTENDED => "LOG_EXTENDED"]);
+									[Debug::LOG_VERBOSE => "LOG_VERBOSE", Debug::LOG_EXTENDED => "LOG_EXTENDED"]);
 							?></label>
 						</fieldset>
 
@@ -2134,7 +2134,7 @@ class Feeds extends Handler_Protected {
 			$owner_uid = $row["owner_uid"];
 
 			if (Config::get(Config::FORCE_ARTICLE_PURGE) != 0) {
-				Debug::log("purge_feed: FORCE_ARTICLE_PURGE is set, overriding interval to " . Config::get(Config::FORCE_ARTICLE_PURGE), Debug::$LOG_VERBOSE);
+				Debug::log("purge_feed: FORCE_ARTICLE_PURGE is set, overriding interval to " . Config::get(Config::FORCE_ARTICLE_PURGE), Debug::LOG_VERBOSE);
 				$purge_unread = true;
 				$purge_interval = Config::get(Config::FORCE_ARTICLE_PURGE);
 			} else {
@@ -2143,10 +2143,10 @@ class Feeds extends Handler_Protected {
 
 			$purge_interval = (int) $purge_interval;
 
-			Debug::log("purge_feed: interval $purge_interval days for feed $feed_id, owner: $owner_uid, purge unread: $purge_unread", Debug::$LOG_VERBOSE);
+			Debug::log("purge_feed: interval $purge_interval days for feed $feed_id, owner: $owner_uid, purge unread: $purge_unread", Debug::LOG_VERBOSE);
 
 			if ($purge_interval <= 0) {
-				Debug::log("purge_feed: purging disabled for this feed, nothing to do.", Debug::$LOG_VERBOSE);
+				Debug::log("purge_feed: purging disabled for this feed, nothing to do.", Debug::LOG_VERBOSE);
 				return null;
 			}
 
@@ -2179,10 +2179,10 @@ class Feeds extends Handler_Protected {
 
 			$rows_deleted = $sth->rowCount();
 
-			Debug::log("purge_feed: deleted $rows_deleted articles.", Debug::$LOG_VERBOSE);
+			Debug::log("purge_feed: deleted $rows_deleted articles.", Debug::LOG_VERBOSE);
 
 		} else {
-			Debug::log("purge_feed: owner of $feed_id not found", Debug::$LOG_VERBOSE);
+			Debug::log("purge_feed: owner of $feed_id not found", Debug::LOG_VERBOSE);
 		}
 
 		return $rows_deleted;
-- 
cgit v1.2.3


From 1818fc11a52dee85f5fd28e4853ee266116c44ad Mon Sep 17 00:00:00 2001
From: Andrew Dolgov <noreply@fakecake.org>
Date: Thu, 23 Dec 2021 17:32:44 +0300
Subject: fetch: return HTTP code when no curl_error() is available

---
 classes/urlhelper.php | 2 ++
 1 file changed, 2 insertions(+)

(limited to 'classes')

diff --git a/classes/urlhelper.php b/classes/urlhelper.php
index 9696c16db..9ac7781ef 100644
--- a/classes/urlhelper.php
+++ b/classes/urlhelper.php
@@ -419,6 +419,8 @@ class UrlHelper {
 
 				if (curl_errno($ch) != 0) {
 					self::$fetch_last_error .=  "; " . curl_errno($ch) . " " . curl_error($ch);
+				} else {
+					self::$fetch_last_error = "HTTP Code: $http_code ";
 				}
 
 				self::$fetch_last_error_content = $contents;
-- 
cgit v1.2.3


From c3482fbe6b22885f5619df02387de09be0886cfe Mon Sep 17 00:00:00 2001
From: Andrew Dolgov <noreply@fakecake.org>
Date: Thu, 6 Jan 2022 10:37:03 +0300
Subject: generate a warning if plugin-generated content of HOOK_ARTICLE_BUTTON
 or _LEFT_BUTTON can't be parsed as valid XML

---
 classes/errors.php     | 23 +++++++++++++++++++++++
 classes/feedparser.php |  5 ++---
 classes/feeds.php      |  8 ++++++++
 classes/plugin.php     |  4 ++--
 4 files changed, 35 insertions(+), 5 deletions(-)

(limited to 'classes')

diff --git a/classes/errors.php b/classes/errors.php
index 31be558cf..aa626d017 100644
--- a/classes/errors.php
+++ b/classes/errors.php
@@ -14,4 +14,27 @@ class Errors {
 	static function to_json(string $code, array $params = []): string {
 		return json_encode(["error" => ["code" => $code, "params" => $params]]);
 	}
+
+	static function libxml_last_error() : string {
+		$error = libxml_get_last_error();
+		$error_formatted = "";
+
+		if ($error) {
+			foreach (libxml_get_errors() as $error) {
+				if ($error->level == LIBXML_ERR_FATAL) {
+					// currently only the first error is reported
+					$error_formatted = self::format_libxml_error($error);
+					break;
+				}
+			}
+		}
+
+		return UConverter::transcode($error_formatted, 'UTF-8', 'UTF-8');
+	}
+
+	static function format_libxml_error(LibXMLError $error) : string {
+		return sprintf("LibXML error %s at line %d (column %d): %s",
+			$error->code, $error->line, $error->column,
+			$error->message);
+	}
 }
diff --git a/classes/feedparser.php b/classes/feedparser.php
index 6ce69cc89..3ed0647d2 100644
--- a/classes/feedparser.php
+++ b/classes/feedparser.php
@@ -193,10 +193,9 @@ class FeedParser {
 		}
 	}
 
+	/** @deprecated use Errors::format_libxml_error() instead */
 	function format_error(LibXMLError $error) : string {
-		return sprintf("LibXML error %s at line %d (column %d): %s",
-			$error->code, $error->line, $error->column,
-			$error->message);
+		return Errors::format_libxml_error($error);
 	}
 
 	// libxml may have invalid unicode data in error messages
diff --git a/classes/feeds.php b/classes/feeds.php
index 65027cb9e..2c37d659a 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -254,6 +254,10 @@ class Feeds extends Handler_Protected {
 
 								$line["buttons_left"] .= $button_doc->saveXML($button_doc->firstChild);
 							}
+						} else if ($result) {
+							user_error(get_class($plugin) .
+								" plugin: content provided in HOOK_ARTICLE_LEFT_BUTTON is not valid XML: " .
+								Errors::libxml_last_error() . " $result", E_USER_WARNING);
 						}
 					},
 					$line);
@@ -273,6 +277,10 @@ class Feeds extends Handler_Protected {
 
 								$line["buttons"] .= $button_doc->saveXML($button_doc->firstChild);
 							}
+						} else if ($result) {
+							user_error(get_class($plugin) .
+								" plugin: content provided in HOOK_ARTICLE_BUTTON is not valid XML: " .
+								Errors::libxml_last_error() . " $result", E_USER_WARNING);
 						}
 					},
 					$line);
diff --git a/classes/plugin.php b/classes/plugin.php
index 3bced3b04..39af6a9a1 100644
--- a/classes/plugin.php
+++ b/classes/plugin.php
@@ -98,7 +98,7 @@ abstract class Plugin {
 
 	/* GLOBAL hooks are invoked in global context, only available to system plugins (loaded via .env for all users) */
 
-	/** Adds buttons for article (on the right) - e.g. mail, share, add note.
+	/** Adds buttons for article (on the right) - e.g. mail, share, add note. Generated markup must be valid XML.
 	 * @param array<string,mixed> $line
 	 * @return string
 	 * @see PluginHost::HOOK_ARTICLE_BUTTON
@@ -307,7 +307,7 @@ abstract class Plugin {
 		return [];
 	}
 
-	/** Adds per-article buttons on the left side
+	/** Adds per-article buttons on the left side. Generated markup must be valid XML.
 	 * @param array<string,mixed> $row
 	 * @return string
 	 * @see PluginHost::HOOK_ARTICLE_LEFT_BUTTON
-- 
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