GDB分析PHP连接Memcached导致coredump问题
最近一项目压力比较大，502超多，PHP进程coredump比较频繁
于是学习了下coredump的调试
讲下PHP coredump的调试，coredump先准备代码比较好，因为PHP是开源的，这点好办。我的是存在/usr/local/src//usr/local/src/php-5.3.10/ 下
第一种当然是通用的调试
gdb /usr/local/php/sbin/php-fpm -c /data/core_files/core-php-fpm-28436-1370008094
第二种是PHP特有的调试
获得一个PHP正在运行的进程
#ps -ef|grep php-fpm|head #gdb --pid=$PID >source /usr/local/src//usr/local/src/php-5.3.10/.gdbinit
这种方式能调试一些PHP特有的东西，比如包括参数的函数调用栈的信息.
先讲通用方式调试
(gdb) bt #0 0x00007ffed0603a75 in raise () from /lib/libc.so.6 #1 0x00007ffed06075c0 in abort () from /lib/libc.so.6 #2 0x00007ffed342ef7c in memcached_io_slurp (ptr=0x2ef9f50) at libmemcached/io.cc:510 #3 0x00007ffed3431da6 in memcached_quit_server (ptr=0x2ef9f50, io_death=false) at libmemcached/quit.cc:95 #4 0x00007ffed3431eed in send_quit (ptr=0x2d34e90) at libmemcached/quit.cc:137 #5 0x00007ffed342ff39 in _free (ptr=0x2d34e90, release_st=true) at libmemcached/memcached.cc:130 #6 0x00007ffed3430397 in memcached_free (ptr=0x2d34e90) at libmemcached/memcached.cc:284 #7 0x00000000005d00e0 in php_memc_destroy (m_obj=0x2e3bf60, persistent=0 '\000') at /usr/local/src/php-5.3.10/ext/memcached/php_memcached.c:2519 #8 0x00000000005d145f in php_memc_free_storage (i_obj=0x2dea580) at /usr/local/src/php-5.3.10/ext/memcached/php_memcached.c:2530 #9 0x000000000081817a in zend_objects_store_del_ref_by_handle_ex (handle=, handlers= ) at /usr/local/src/php-5.3.10/Zend/zend_objects_API.c:220 #10 0x0000000000818193 in zend_objects_store_del_ref (zobject=0x2dea620) at /usr/local/src/php-5.3.10/Zend/zend_objects_API.c:172 #11 0x00000000007e9fb5 in _zval_dtor (zval_ptr= ) at /usr/local/src/php-5.3.10/Zend/zend_variables.h:35 #12 _zval_ptr_dtor (zval_ptr= ) at /usr/local/src/php-5.3.10/Zend/zend_execute_API.c:447 #13 0x00000000008020eb in zend_hash_destroy (ht=0x2d21e20) at /usr/local/src/php-5.3.10/Zend/zend_hash.c:529 #14 0x00000000007f5416 in _zval_dtor_func (zvalue=0x2d21df0) at /usr/local/src/php-5.3.10/Zend/zend_variables.c:43 #15 0x00000000007e9fb5 in _zval_dtor (zval_ptr= ) at /usr/local/src/php-5.3.10/Zend/zend_variables.h:35 #16 _zval_ptr_dtor (zval_ptr= ) at /usr/local/src/php-5.3.10/Zend/zend_execute_API.c:447 #17 0x00000000008023f3 in zend_hash_clean (ht=0x2d21cb0) at /usr/local/src/php-5.3.10/Zend/zend_hash.c:561 #18 0x00000000007ee2cb in zend_cleanup_op_array_data (function= ) at /usr/local/src/php-5.3.10/Zend/zend_opcode.c:133 #19 zend_cleanup_function_data (function= ) at /usr/local/src/php-5.3.10/Zend/zend_opcode.c:140 #20 0x0000000000801ea4 in zend_hash_reverse_apply (ht=0x29e5c30, apply_func=0x7ee2a0 ) at /usr/local/src/php-5.3.10/Zend/zend_hash.c:757 #21 0x00000000007ea2f3 in shutdown_executor () at /usr/local/src/php-5.3.10/Zend/zend_execute_API.c:296 #22 0x00000000007f6563 in zend_deactivate () at /usr/local/src/php-5.3.10/Zend/zend.c:891 #23 0x00000000007a47a0 in php_request_shutdown (dummy= ) at /usr/local/src/php-5.3.10/main/main.c:1661 #24 0x0000000000884442 in main (argc= , argv= ) at /usr/local/src/php-5.3.10/sapi/fpm/fpm/fpm_main.c:1886 (gdb) f 2 #2 0x00007ffed342ef7c in memcached_io_slurp (ptr=0x2ef9f50) at libmemcached/io.cc:510 510 libmemcached/io.cc: No such file or directory. in libmemcached/io.cc
为了调试memcached，会碰到了提示说 libmemcached/io.cc不存在，于是也把libmemcached的源码也下载回来。
注意版本得配对，否则容易造成找代码行数找不准，就白瞎了。
warning: Source file is more recent than executable.
看版本用 -i来看
#/usr/local/php/sbin/php-fpm -i|grep libmemcached libmemcached version => 1.0.4 #wget https://launchpad.net/libmemcached/1.0/1.0.4/+download/libmemcached-1.0.4.tar.gz
把代码解压到php源码目录下 ，如果出错可能是路径不对
继续回来gdb
(gdb) f 2 #2 0x00007ffed342ef7c in memcached_io_slurp (ptr=0x2ef9f50) at libmemcached/io.cc:510 510 in libmemcached/io.cc
看到是libmemcached/io.cc 的510行报错
#vim libmemcached/io.cc +510
看到代码如下：
memcached_return_t memcached_io_slurp(memcached_server_write_instance_st ptr)
{
assert_msg(ptr, "Programmer error, invalid memcached_server_write_instance_st");
assert(memcached_is_udp(ptr->root) == false);
if (ptr->fd == INVALID_SOCKET)
{
assert_msg(int(ptr->state) <= int(MEMCACHED_SERVER_STATE_ADDRINFO), "Invalid socket state"); #line 510
return MEMCACHED_CONNECTION_FAILURE;
}
ssize_t data_read;这是一个无效的连接？ 除非说是网络非常繁忙，或压力很大。
往下看看吧
(gdb) f 3
#3 0x00007ffed3431da6 in memcached_quit_server (ptr=0x2ef9f50, io_death=false) at libmemcached/quit.cc:95
warning: Source file is more recent than executable.
95 while (memcached_continue(rc_slurp= memcached_io_slurp(ptr))) {} ;是在quit函数中，进代码看看
WATCHPOINT_ASSERT(rc == MEMCACHED_SUCCESS or rc == MEMCACHED_FETCH_NOTFINISHED);
/* read until socket is closed, or there is an error
* closing the socket before all data is read
* results in server throwing away all data which is
* not read
*
* In .40 we began to only do this if we had been doing buffered
* requests of had replication enabled.
*/
if (ptr->root->flags.buffer_requests or ptr->root->number_of_replicas)
{
memcached_return_t rc_slurp;
while (memcached_continue(rc_slurp= memcached_io_slurp(ptr))) {} ; #line 95
WATCHPOINT_ASSERT(rc_slurp == MEMCACHED_CONNECTION_FAILURE);
}
/*
* memcached_io_read may call memcached_quit_server with io_death if
* it encounters problems, but we don't care about those occurences.
* The intention of that loop is to drain the data sent from the
* server to ensure that the server processed all of the data we
* sent to the server.
*/
ptr->server_failure_counter= 0;
}
memcached_io_close(ptr);
} slurp的意思是：大量读取
看看注释的意思：等待socket 正常关闭或没有读完数据但是出现了错误引起的异常关闭，结果是服务器扔掉了所有没有读完的数据。
95行的代码是一个条件循环，条件是memcached_io_slurp 的结果
继续分析函数 memcached_io_slurp
发现是等待读完buffer的过程
总体的意思是说服务端开启了buffer，PHP发送了退出请求，所以memcached扩展也被要求退出，然而buffer可能没有读完，所以memcached等待buffer读完，而php等待memcached退出后才能退出
(gdb) f 7 #7 0x00000000005d00e0 in php_memc_destroy (m_obj=0x2e3bf60, persistent=0 '\000') at /usr/local/src/php-5.3.10/ext/memcached/php_memcached.c:2519 2519 in /usr/local/src/php-5.3.10/ext/memcached/php_memcached.c 一路往后f (gdb) f 24 #24 0x0000000000884442 in main (argc=, argv= ) at /usr/local/src/php-5.3.10/sapi/fpm/fpm/fpm_main.c:1886 1886 php_request_shutdown((void *) 0);
看到是 fpm请求退出进程 php_request_shutdown
fastcgi_request_done:
if (primary_script) {
efree(primary_script);
}
if (request_body_fd != -1) {
close(request_body_fd);
}
request_body_fd = -2;
if (EG(exit_status) == 255) {
if (CGIG(error_header) && *CGIG(error_header)) {
sapi_header_line ctr = {0};
ctr.line = CGIG(error_header);
ctr.line_len = strlen(CGIG(error_header));
sapi_header_op(SAPI_HEADER_REPLACE, &ctr TSRMLS_CC);
}
}
fpm_request_end(TSRMLS_C);
fpm_log_write(NULL TSRMLS_CC);
STR_FREE(SG(request_info).path_translated);
SG(request_info).path_translated = NULL;
php_request_shutdown((void *) 0); #line 1886
if (exit_status == 0) {
exit_status = EG(exit_status);
}
requests++;
if (max_requests && (requests == max_requests)) {
fcgi_finish_request(&request, 1);
if (max_requests != 1) {
/* no need to return exit_status of the last request */
exit_status = 0;好像还没有结果。虽然还没查到具体的原因，大概问题归结在memcached扩展和memcached服务器通讯的时候，数据还未传完，
PHP进程就要结束，导致coredump。
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