Add canonical meta tag to view issue page

It is possible for issue URLs to have additional parameters in some 
situations. Search engines such as Google will index better if they 
know which are the set of canonical URLs. It reduces load through 
over-indexing, allows the search engine to prioritise the indexing of 
URLs that are correct, and avoids extraneous or non-ideal URLs getting 
into the index.

Fixes #31833, PR #1862

bug_view_inc.php

@@ -110,7 +110,7 @@
 compress_enable();
 
 if( $t_show_page_header ) {
-	layout_page_header( bug_format_summary( $f_issue_id, SUMMARY_CAPTION ), null, 'view-issue-page' );
+	layout_page_header( bug_format_summary( $f_issue_id, SUMMARY_CAPTION ), null, 'view-issue-page', 'view.php?id=' . $f_issue_id );
 	layout_page_begin( 'view_all_bug_page.php' );
 }
 

core/html_api.php

@@ -283,7 +283,7 @@ function html_css_cdn_link( $p_url, $p_hash = '' ) {
  * $p_time is the number of seconds to wait before redirecting.
  * If we have handled any errors on this page return false and don't redirect.
  *
- * @param string  $p_url      The page to redirect: has to be a relative path.
+ * @param string  $p_url      The page to redirect: has to be relative to the install path {@see $g_path}.
  * @param integer $p_time     Seconds to wait for before redirecting.
  * @param boolean $p_sanitize Apply string_sanitize_url to passed URL.
  * @return boolean
@@ -311,6 +311,19 @@ function html_meta_redirect( $p_url, $p_time = null, $p_sanitize = true ) {
 	return true;
 }
 
+/**
+ * Print a canonical meta tag.
+ *
+ * @param string  $p_url      The canonical URL: has to be relative to the install path {@see $g_path}.
+ */
+function html_meta_canonical( $p_url ) {
+	$t_url = config_get_global( 'path' ) . $p_url;
+
+	$t_url = htmlspecialchars( $t_url );
+
+	echo "\t" . '<link rel="canonical" href="' . $t_url . '" />' . "\n";
+}
+
 /**
  * Require a javascript file to be in html page headers
  * @param string $p_script_path Path to javascript file.

core/layout_api.php

@@ -47,16 +47,20 @@
 
 /**
  * Print the page header section
- * @param string $p_page_title   Html page title.
- * @param string $p_redirect_url URL to redirect to if necessary.
- * @param string $p_page_id      The page id.
+ * @param string $p_page_title    Html page title.
+ * @param string $p_redirect_url  URL to redirect to if necessary: has to be relative to the install path {@see $g_path}.
+ * @param string $p_page_id       The page id.
+ * @param string $p_canonical_url Canonical URL if necessary: has to be relative to the install path {@see $g_path}.
  * @return void
  */
-function layout_page_header( $p_page_title = null, $p_redirect_url = null, $p_page_id = null ) {
+function layout_page_header( $p_page_title = null, $p_redirect_url = null, $p_page_id = null, $p_canonical_url = null ) {
 	layout_page_header_begin( $p_page_title );
 	if( $p_redirect_url !== null ) {
 		html_meta_redirect( $p_redirect_url );
 	}
+	if( $p_canonical_url !== null ) {
+		html_meta_canonical( $p_canonical_url );
+	}
 
 	layout_page_header_end( $p_page_id );
 }