Assertion failure in zend_hash_del with HashTable refcount inconsistency

Description

The following code:

<?php
$c = new ReflectionClass('stdClass');
ob_start(fn() => serialize($GLOBALS), 1);
$array = [$c];
unset($array[$x]);

php: /path/to/php-src/Zend/zend_hash.c:1556: zend_hash_del: Assertion `(zend_gc_refcount(&(ht)->gc) == 1) || ((ht)->u.flags & (1<<6))' failed.
Aborted

Commit

5472cac806a

Configurations

CC="clang" CXX="clang++" CFLAGS="-fsanitize=address -g -O0" CXXFLAGS="-fsanitize=address -g -O0" ./configure --enable-debug --enable-address-sanitizer --disable-shared --with-pic

PHP Version

PHP 8.6.0-dev (cli) (built: Jan  7 2026 13:33:43) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.6.0-dev, Copyright (c) Zend Technologies
    with Zend OPcache v8.6.0-dev, Copyright (c), by Zend Technologies

Operating System

Ubuntu 22.04

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=6, no_tid=0) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (threadid=<optimized out>, signo=6) at ./nptl/pthread_kill.c:89
#2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:100
#3  0x00007ffff6845e2e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4  0x00007ffff6828888 in __GI_abort () at ./stdlib/abort.c:77
#5  0x00007ffff68287f0 in __assert_fail_base (fmt=<optimized out>, assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at ./assert/assert.c:118
#6  0x000055555e21ac5e in zend_hash_del (ht=0x7bfff284c6c0, key=0x7c2ff4de1cf0) at /home/dcarlier/Contribs/php-src/Zend/zend_hash.c:1556
#7  0x000055555dd5c38d in ZEND_UNSET_DIM_SPEC_CV_CV_TAILCALL_HANDLER (execute_data=0x7bfff2807020, opline=0x7bfff2893268) at Zend/zend_vm_execute.h:111422
#8  0x000055555d46d93f in execute_ex (ex=0x7bfff2807020) at Zend/zend_vm_execute.h:116212
#9  0x000055555d46f9bf in zend_execute (op_array=0x7bfff2889000, return_value=0x0) at Zend/zend_vm_execute.h:121924
#10 0x000055555e690e40 in zend_execute_script (type=8, retval=0x0, file_handle=0x7fffffffc300) at /home/dcarlier/Contribs/php-src/Zend/zend.c:1981
#11 0x000055555cb45f93 in php_execute_script_ex (primary_file=0x7fffffffc300, retval=0x0) at /home/dcarlier/Contribs/php-src/main/main.c:2645
#12 0x000055555cb46e79 in php_execute_script (primary_file=0x7fffffffc300) at /home/dcarlier/Contribs/php-src/main/main.c:2685
#13 0x000055555e6a270f in do_cli (argc=2, argv=0x7c2ff4de1ba0) at /home/dcarlier/Contribs/php-src/sapi/cli/php_cli.c:951
#14 0x000055555e69e6b2 in main (argc=2, argv=0x7c2ff4de1ba0) at /home/dcarlier/Contribs/php-src/sapi/cli/php_cli.c:1362

@arnaud-lb

Looks like more of #20018.

unset() separates $array
$x emits an undefined variable warning
This emits output and calls serialize($GLOBALS).
Which throws an exception because ReflectionClass is not serializable
Which stores the cloned $GLOBALS in the exception args
Which holds a reference to $array
Eventually asserting because RC != 1

@arnaud-lb I'll let it up to you whether this is unique enough to keep.

I agree that this falls in #20018. It's a bit different than user error handlers, but it's still related to side effects of errors.

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Assertion failure in zend_hash_del with HashTable refcount inconsistency #20855

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Assertion failure in zend_hash_del with HashTable refcount inconsistency #20855

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devnexen commented on Jan 7, 2026

iluuu1994 commented on Jan 7, 2026

arnaud-lb commented on Jan 7, 2026

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