前幾天東政同學(xué)反饋說Linode服務(wù)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(shè)置超時時間，加上我用的php5.2關(guān)于curl的代碼有個bug，于是導(dǎo)致PHP進程進入死循環(huán)。

今天下午又發(fā)現(xiàn)系統(tǒng)負(fù)載很高，于是上去看了一下，發(fā)現(xiàn)一大坨PHP進程沒有退出，占用了很多CPU，如圖：

問題進程：

后面運行的腳本是我的RSS定時更新任務(wù)，看來PHP代碼什么地方有問題，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號fd上面死循環(huán)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]

再看了一下原來是在請求CSDN的一個網(wǎng)頁的時候死循環(huán)了，但不知道什么地方請求的，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 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(dāng)前PHP執(zhí)行的腳步是什么：

(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(diào)用，沒有設(shè)置超時時間，于是PHP卡死在網(wǎng)絡(luò)請求了。于是用stream_context_create 設(shè)置超時時間搞定。

到這里 似乎問題解決了，但是，為什么沒有設(shè)置超時時間就導(dǎo)致php進程占用CPU，系統(tǒng)負(fù)載那么高？按理說應(yīng)該等待I/O才是呀？看上面CPU情況，完全是進入了死循環(huán)的節(jié)奏。

根據(jù)上面的bt堆棧，首先看倒數(shù)第二個函數(shù)的調(diào)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(fā)現(xiàn)，代碼一直在里面的do-while里面循環(huán)了！心想curl_multi_fdset怎么不用先FD_ZERO 清空FD呢？一般做法都是會先清空的。

莫非是PHP的bug, 于是網(wǎng)上找了一下發(fā)現(xiàn)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：

好吧，最后用GDB驗證一下，我在上面的do下面，curl_multi_fdset調(diào)用之前，手動將fd清空，看看能否退出循環(huán)：

(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(shù)數(shù)組如下：

然后GDB單步執(zhí)行，如期的由于curlstream->pending變?yōu)?，從而退出了循環(huán)，回到php_stream_fill_read_buffer的大函數(shù)了

到此基本結(jié)束。有問題的PHP版本應(yīng)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。