趁着年轻

前几天东政同学反馈说Linode服务器快卡死了，今天有时间排查了一下具体原因，最终原因稍微有点悲壮：file_get_contents没有设置超时时间，加上我用的php5.2关于curl的代码有个bug，于是导致PHP进程进入死循环。

今天下午又发现系统负载很高，于是上去看了一下，发现一大坨PHP进程没有退出，占用了很多CPU，如图：

问题进程：

后面运行的脚本是我的RSS定时更新任务，看来PHP代码什么地方有问题，于是strace -p 14043看了一下：

select(5, [4], [4], [], {15, 0}) = 1 (out [4], left {14, 999996})
poll([{fd=4, events=POLLIN|POLLPRI}], 1, 0) = 0 (Timeout)
clock_gettime(CLOCK_MONOTONIC, {4582888, 760370017}) = 0
clock_gettime(CLOCK_MONOTONIC, {4582888, 760468615}) = 0
clock_gettime(CLOCK_MONOTONIC, {4582888, 760565053}) = 0
select(5, [4], [4], [], {15, 0}) = 1 (out [4], left {14, 999997})

在４号fd上面死循环了，于是看看FD是什么：ll /proc/14043/fd

lrwx------ 1 wuhaiwen wuhaiwen 64  7月 21 11:00 4 -> socket:[53176380]

再看了一下原来是在请求CSDN的一个网页的时候死循环了，但不知道什么地方请求的，想到GDB一下php进程看看，bt显示：

(gdb) bt
#0 0x00007f6721f8f013 in __select_nocancel () at ../sysdeps/unix/syscall-template.S:82
#1 0x0000000000481952 in php_curl_stream_read (stream=0x2280650,
buf=0x22ea5d0 "2Fwww.laruence.com%2Ftag%2F%25e6%25ad%25a3%25e5%2588%2599%27+class%3D%27tag-link-191%27+title%3D%273+topics%27+style%3D%27font-size%3A+9.0243902439pt%3B%27%3E%E6%AD%A3%E5%88%99%3C%2Fa%3E%3C%2Ftags%3E\""..., count=8192) at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/ext/curl/streams.c:169
#2 0x00000000006738f9 in php_stream_fill_read_buffer (stream=0x2280650, size=4283) at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/main/streams/streams.c:554
#3 0x0000000000673c39 in _php_stream_read (stream=0x2280650,
buf=0x2301fd5 "f='http://www.laruence.com/tag/json' class='tag-link-79' title='3 topics' style='font-size: 9.0243902439pt;'>json</a>\n<a href='http://www.laruence.com/tag/module' class='tag-link-43' title='2 topics' "..., size=4283) at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/main/streams/streams.c:600
#4 0x0000000000674c51 in _php_stream_copy_to_mem (src=0x2280650, buf=0x7fff376ed898, maxlen=<optimized out>, persistent=0)
at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/main/streams/streams.c:1267
#5 0x00000000005fdb85 in zif_file_get_contents (ht=<optimized out>, return_value=0x2223da0, return_value_ptr=<optimized out>, this_ptr=<optimized out>, return_value_used=<optimized out>)
at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/ext/standard/file.c:565
#6 0x00000000006c2a59 in zend_do_fcall_common_helper_SPEC (execute_data=0x7fff376edc60) at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/Zend/zend_vm_execute.h:200
#7 0x00000000006c239f in execute (op_array=0x1f26730) at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/Zend/zend_vm_execute.h:92
·············
#16 0x0000000000730d8e in main (argc=4, argv=0x7fff376f2468) at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/sapi/cli/php_cli.c:1133

看一下当前PHP执行的脚步是什么：

</pre>
(gdb) p *op_array
 $4 = {type = 2 '\002', function_name = 0x1e54278 "getContent", scope = 0x1f8e850, fn_flags = 257, prototype = 0x0, num_args = 2, required_num_args = 1, arg_info = 0x1fd5e20,
 pass_rest_by_reference = 0 '\000', return_reference = 0 '\000', refcount = 0x1fd3ab8, opcodes = 0x1fddcc8, last = 28, size = 28, vars = 0x1fd3cc0, last_var = 6, size_var = 16, T = 15,
 brk_cont_array = 0x0, last_brk_cont = 0, current_brk_cont = 4294967295, try_catch_array = 0x0, last_try_catch = 0, static_variables = 0x0, start_op = 0x0, backpatch_count = 0,
 done_pass_two = 1 '\001', uses_this = 0 '\000', filename = 0x1fd3b58 "/home/wuhaiwen/webroot/kulvrss/libs/Myrss/Model/UrlContenter.php", line_start = 9, line_end = 30, doc_comment = 0x0,
 doc_comment_len = 0, reserved = {0x0, 0x0, 0x0, 0x0}}
<pre>

找到了问题代码位置，原来是一个file_get_contents($url)调用，没有设置超时时间，于是PHP卡死在网络请求了。于是用stream_context_create 设置超时时间搞定。

到这里 似乎问题解决了，但是，为什么没有设置超时时间就导致php进程占用CPU，系统负载那么高？按理说应该等待I/O才是呀？看上面CPU情况，完全是进入了死循环的节奏。

根据上面的bt堆栈，首先看倒数第二个函数的调用：

#1 0x0000000000481952 in php_curl_stream_read (stream=0x2280650, 
buf=0x22ea5d0 "2Fwww.laruence.com%2Ftag%2F%25e6%25ad%25a3%25e5%2588%2599%27+class%3D%27tag-link-191%27+title%3D%273+topics%27+style%3D%27font-size%3A+9.0243902439pt%3B%27%3E%E6%AD%A3%E5%88%99%3C%2Fa%3E%3C%2Ftags%3E\""..., count=8192) at /home/wuhaiwen/install/php-env/src/php/php-5.2.8/ext/curl/streams.c:169

看一下代码，我用的事5.2.8版本的PHP，比较老。代码如下：

static size_t php_curl_stream_read(php_stream *stream, char *buf, size_t count TSRMLS_DC)
{
        php_curl_stream *curlstream = (php_curl_stream *) stream->abstract;
        size_t didread = 0;

        if (curlstream->readbuffer.readpos >= curlstream->readbuffer.writepos && curlstream->pending) {
//········
                do {
                        /* get the descriptors from curl */
                        curl_multi_fdset(curlstream->multi, &curlstream->readfds, &curlstream->writefds, &curlstream->excfds, &curlstream->maxfd);

                        /* if we are in blocking mode, set a timeout */
                        tv.tv_usec = 0;
                        tv.tv_sec = 15; /* TODO: allow this to be configured from the script */

                        /* wait for data */
                        switch (select(curlstream->maxfd + 1, &curlstream->readfds, &curlstream->writefds, &curlstream->excfds, &tv)) {
                                case -1:
                                        /* error */
                                        return 0;
                                case 0:
                                        /* no data yet: timed-out */
                                        return 0;
                                default:
                                        /* fetch the data */
                                        do {
                                                curlstream->mcode = curl_multi_perform(curlstream->multi, &curlstream->pending);
                                        } while (curlstream->mcode == CURLM_CALL_MULTI_PERFORM);
                        }
                } while (curlstream->readbuffer.readpos >= curlstream->readbuffer.writepos && curlstream->pending > 0);

        }
//··········
        return didread;
}

GDB进去发现，代码一直在里面的do-while里面循环了！心想curl_multi_fdset怎么不用先FD_ZERO 清空FD呢？一般做法都是会先清空的。

莫非是PHP的bug, 于是网上找了一下发现了这个Pierrick-Charron的commit，确实是一个bug, 其实curl_multi_fdset 的文档开头写了的：

This function extracts file descriptor information from a given multi_handle. libcurl returns its fd_set sets. The application can use these to select() on, but be sure to FD_ZERO them before calling this function as curl_multi_fdset(3) only adds its own descriptors,

好吧，最后用GDB验证一下，我在上面的do下面，curl_multi_fdset调用之前，手动将fd清空，看看能否退出循环：

(gdb) print FD_ZERO(&curlstream->readfds)
No symbol "FD_ZERO" in current context.

FD_ZERO竟然没有，不管了，其本来是个宏定义，展开就行：#define FD_ZERO(p)      bzero((char *)(p), sizeof(*(p)))

直接用call修改curl_muti_fdset的三个参数数组如下：

(gdb) call bzero((char *)(&curlstream->readfds), sizeof(*(&curlstream->readfds)))
$5 = 17055392
(gdb) call bzero((char *)(&curlstream->writefds),sizeof(*(&curlstream->writefds)))
$6 = 17055520
(gdb) call bzero((char *)(&curlstream->excfds), sizeof(*(&curlstream->excfds)))
$7 = 17055648

然后GDB单步执行，如期的由于curlstream->pending变为0，从而退出了循环，回到php_stream_fill_read_buffer的大函数了

到此基本结束。有问题的PHP版本应该是5.2. 具体没有细看，读者可以参考下上面的这个提交改动或者直接看自己的版本代码是否有问题。