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php5.2的curl-bug 服務器被php進程卡死問題排查

編輯：PHP綜合

前幾天東政同學反饋說Linode服務器快卡死了，今天有時間排查了一下具體原因，最終原因稍微有點悲壯：file_get_contents沒有設置超時時間，加上我用的php5.2關於curl的代碼有個bug，於是導致PHP進程進入死循環。

今天下午又發現系統負載很高，於是上去看了一下，發現一大坨PHP進程沒有退出，占用了很多CPU，如圖：

問題進程：

後面運行的腳本是我的RSS定時更新任務，看來PHP代碼什麼地方有問題，於是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號fd上面死循環了，於是看看FD是什麼：ll /proc/14043/fd

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

再看了一下原來是在請求CSDN的一個網頁的時候死循環了，但不知道什麼地方請求的，想到GDB一下php進程看看，bt顯示：

(gdb) bt
#0 0x00007f6721f8f013 in __select_nocancel () at ../sysdeps/unix/syscall-template.S:82
#1 0×0000000000481952 in php_curl_stream_read (stream=0×2280650,
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=0×2280650, 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=0×2280650,
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=0×2280650, 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

看一下當前PHP執行的腳步是什麼：

(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}}

找到了問題代碼位置，原來是一個file_get_contents($url)調用，沒有設置超時時間，於是PHP卡死在網絡請求了。於是用stream_context_create 設置超時時間搞定。

到這裡似乎問題解決了，但是，為什麼沒有設置超時時間就導致php進程占用CPU，系統負載那麼高？按理說應該等待I/O才是呀？看上面CPU情況，完全是進入了死循環的節奏。

根據上面的bt堆棧，首先看倒數第二個函數的調用：

#1 0×0000000000481952 in php_curl_stream_read (stream=0×2280650,
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

看一下代碼，我用的事5.2.8版本的PHP，比較老。代碼如下：

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進去發現，代碼一直在裡面的do-while裡面循環了！心想curl_multi_fdset怎麼不用先FD_ZERO 清空FD呢？一般做法都是會先清空的。

莫非是PHP的bug, 於是網上找了一下發現了這個Pierrick-Charron的commit，確實是一個bug, 其實curl_multi_fdset 的文檔開頭寫了的：

復制代碼代碼如下:
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，最後用GDB驗證一下，我在上面的do下面，curl_multi_fdset調用之前，手動將fd清空，看看能否退出循環：

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

FD_ZERO竟然沒有，不管了，其本來是個宏定義，展開就行：#define FD_ZERO(p) bzero((char *)(p), sizeof(*(p)))

直接用call修改curl_muti_fdset的三個參數數組如下：

復制代碼代碼如下:
(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

然後GDB單步執行，如期的由於curlstream->pending變為0，從而退出了循環，回到php_stream_fill_read_buffer的大函數了

到此基本結束。有問題的PHP版本應該是5.2. 具體沒有細看，讀者可以參考下上面的這個提交改動或者直接看自己的版本代碼是否有問題。