I got 502 and error_log print upstream sent invalid status "0"

[neoesque]

Hi everyone,

I have a php-fpm backend. And sometimes I got nginx return 502. After I
check the error_log I found each 502s are caused by upstream sent
invalid status "0" while reading response header from upstream

The error_log are in the form "upstream sent invalid status "0" while
reading response header from upstream client: 127.0.0.1, server: xxxx,
request: "xxxxx HTTP/1.0", upstream:
"fastcgi://unix:/dev/shm/api-php.socket:", host: "xxxx", referrer:
"xxxx""

Does anyone have same experience? I have no idea after search Google
'upstream sent invalid status "0"'

I think it is caused by PHP. But why does it sent status '0'? 

Yours Sincerely.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,224638,224638#msg-224638