Fix GH-17900 and GH-8084

Calling the constructor twice has no real world benefit.
Block it to fix these two issues.
We also clean up the constructor code a bit:
- `in_ctor` implies `object` exist.
- We surround the instance check with ZEND_DEBUG to avoid a runtime
  penalty.

Closes GH-17900.
Closes GH-8084.
Closes GH-17908.

NEWS

@@ -56,6 +56,10 @@ PHP                                                                        NEWS
   . IntlDateFormatter::setTimeZone()/datefmt_set_timezone() throws an exception
     with uninitialised classes or clone failure. (David Carlier)
 
+- MySQLi:
+  . Fixed bugs GH-17900 and GH-8084 (calling mysqli::__construct twice).
+    (nielsdos)
+
 - MySQLnd:
   . Added mysqlnd.collect_memory_statistics to ini quick reference.
     (hauk92)

UPGRADING

@@ -44,6 +44,10 @@ PHP 8.5 UPGRADE NOTES
   . ldap_get_option() and ldap_set_option() now throw a ValueError when
     passing an invalid option.
 
+- MySQLi:
+  . Calling the mysqli constructor on an already-constructed object
+    is now no longer possible and throws an Error.
+
 - PCNTL:
   . pcntl_exec() now throws ValueErrors when entries of the $args parameter
     contain null bytes.

ext/mysqli/mysqli_api.c

@@ -969,10 +969,6 @@ void php_mysqli_init(INTERNAL_FUNCTION_PARAMETERS, bool is_method)
 	MYSQLI_RESOURCE *mysqli_resource;
 	MY_MYSQL *mysql;
 
-	if (zend_parse_parameters_none() == FAILURE) {
-		RETURN_THROWS();
-	}
-
 	if (is_method && (Z_MYSQLI_P(getThis()))->ptr) {
 		return;
 	}
@@ -1004,6 +1000,7 @@ void php_mysqli_init(INTERNAL_FUNCTION_PARAMETERS, bool is_method)
 /* {{{ Initialize mysqli and return a resource for use with mysql_real_connect */
 PHP_FUNCTION(mysqli_init)
 {
+	ZEND_PARSE_PARAMETERS_NONE();
 	php_mysqli_init(INTERNAL_FUNCTION_PARAM_PASSTHRU, false);
 }
 /* }}} */

ext/mysqli/mysqli_nonapi.c

@@ -72,7 +72,14 @@ void mysqli_common_connect(INTERNAL_FUNCTION_PARAMETERS, bool is_real_connect, b
 	}
 #endif
 
-	if (getThis() && !ZEND_NUM_ARGS() && in_ctor) {
+	if (in_ctor && !ZEND_NUM_ARGS()) {
+		ZEND_PARSE_PARAMETERS_NONE();
+
+		if (UNEXPECTED(Z_MYSQLI_P(object)->ptr)) {
+			zend_throw_error(NULL, "Cannot call constructor twice");
+			return;
+		}
+
 		php_mysqli_init(INTERNAL_FUNCTION_PARAM_PASSTHRU, in_ctor);
 		return;
 	}
@@ -84,6 +91,11 @@ void mysqli_common_connect(INTERNAL_FUNCTION_PARAMETERS, bool is_real_connect, b
 			RETURN_THROWS();
 		}
 
+		if (UNEXPECTED(in_ctor && Z_MYSQLI_P(object)->ptr)) {
+			zend_throw_error(NULL, "Cannot call constructor twice");
+			return;
+		}
+
 		if (object) {
 			ZEND_ASSERT(instanceof_function(Z_OBJCE_P(object), mysqli_link_class_entry));
 			mysqli_resource = (Z_MYSQLI_P(object))->ptr;
@@ -325,6 +337,7 @@ PHP_METHOD(mysqli, __construct)
 /* {{{ Initialize mysqli and return a resource for use with mysql_real_connect */
 PHP_METHOD(mysqli, init)
 {
+	ZEND_PARSE_PARAMETERS_NONE();
 	php_mysqli_init(INTERNAL_FUNCTION_PARAM_PASSTHRU, true);
 }
 /* }}} */

ext/mysqli/tests/gh17900.phpt

@@ -0,0 +1,16 @@
+--TEST--
+GH-17900 (Assertion failure ext/mysqli/mysqli_prop.c)
+--EXTENSIONS--
+mysqli
+--FILE--
+<?php
+mysqli_report(MYSQLI_REPORT_OFF);
+$mysqli = new mysqli();
+try {
+    $mysqli->__construct('doesnotexist');
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+Cannot call constructor twice

ext/mysqli/tests/mysqli_incomplete_initialization.phpt

@@ -12,8 +12,8 @@ $mysqli->close();
 
 ?>
 --EXPECTF--
-Fatal error: Uncaught Error: mysqli object is not fully initialized in %s:%d
+Fatal error: Uncaught Error: Cannot call constructor twice in %s:%d
 Stack trace:
-#0 %s(%d): mysqli->close()
+#0 %s(%d): mysqli->__construct('doesnotexist')
 #1 {main}
   thrown in %s on line %d