Hello,

We're using cURL to do some asynchronous programming in PHP.

The code fires off a multi-curl request to get some data from a remote
server. Once the request is sent i.e. curl_multi_exec() (PHP's wrapper
around curl_multi_perform() ) returns CURLM_OK, the code continues its
processing and once it is all done and ready, it checks the multi-curl
request for a response.

We're seeing some unusual behavior that might be a bug. We're
currently on 7.19.7-26.el6_1.2, but I checked the code and the same
behavior is in the latest version as well.

If the curl request tries to connect to a host that is slow to respond
to the initial connect, then curl_multi_perform() returns CURLM_OK,
even though the connection is not completed and the request not sent.

Here's a couple of sample straces showing both conditions. The PHP code is:

// Set up the multi-curl
do {
    $cme = curl_multi_exec($this->mc_handle, $this->mc_running);
} while ($cme === CURLM_CALL_MULTI_PERFORM);
// For purposes of the test, sleep. This is where other code would execute
sleep(2);

Connection to localhost where connect() is super fast:
1372871818.596930 socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 61 <0.000024>
1372871818.596990 fcntl(61, F_GETFL) = 0x2 (flags O_RDWR) <0.000023>
1372871818.597044 fcntl(61, F_SETFL, O_RDWR|O_NONBLOCK) = 0 <0.000015>
1372871818.597086 connect(61, {sa_family=AF_INET, sin_port=htons(80),
sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation now
in progress) <0.000186>
1372871818.597315 poll([{fd=61, events=POLLOUT|POLLWRNORM}], 1, 0) = 1
([{fd=61, revents=POLLOUT|POLLWRNORM}]) <0.000017>
1372871818.597370 getsockopt(61, SOL_SOCKET, SO_ERROR, [0], [4]) = 0 <0.000016>
1372871818.597902 socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 62 <0.000020>
1372871818.597960 fcntl(62, F_GETFL) = 0x2 (flags O_RDWR) <0.000016>
1372871818.598005 fcntl(62, F_SETFL, O_RDWR|O_NONBLOCK) = 0 <0.000018>
1372871818.598060 connect(62, {sa_family=AF_INET, sin_port=htons(80),
sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation now
in progress) <0.000057>
1372871818.598162 poll([{fd=62, events=POLLOUT|POLLWRNORM}], 1, 0) = 1
([{fd=62, revents=POLLOUT|POLLWRNORM}]) <0.000016>
1372871818.598212 getsockopt(62, SOL_SOCKET, SO_ERROR, [0], [4]) = 0 <0.000015>
1372871818.598665 sendto(61, "GET /rpc_test.php?sleep=1
HTTP/1.1\r\nAccept: */*\r\nHost: www.example.com\r\n\r\n", 75,
MSG_NOSIGNAL, NULL, 0) = 75 <0.000159>
1372871818.599054 sendto(62, "GET /rpc_test.php?sleep=1
HTTP/1.1\r\nAccept: */*\r\nHost: www.example.com\r\n\r\n", 75,
MSG_NOSIGNAL, NULL, 0) = 75 <0.000072>
1372871818.601071 nanosleep({2, 0}, 0x7fff7ce2f4c0) = 0 <2.000130>

Connecting to a slow remote host:
1372871825.806179 connect(61, {sa_family=AF_INET, sin_port=htons(80),
sin_addr=inet_addr("10.255.58.37")}, 16) = -1 EINPROGRESS (Operation
now in progress) <0.000195>
1372871825.806417 poll([{fd=61, events=POLLOUT|POLLWRNORM}], 1, 0) = 0
(Timeout) <0.000017>
1372871825.806919 socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 62 <0.000024>
1372871825.806976 fcntl(62, F_GETFL) = 0x2 (flags O_RDWR) <0.000016>
1372871825.807020 fcntl(62, F_SETFL, O_RDWR|O_NONBLOCK) = 0 <0.000016>
1372871825.807064 connect(62, {sa_family=AF_INET, sin_port=htons(80),
sin_addr=inet_addr("10.255.58.37")}, 16) = -1 EINPROGRESS (Operation
now in progress) <0.000173>
1372871825.807278 poll([{fd=62, events=POLLOUT|POLLWRNORM}], 1, 0) = 0
(Timeout) <0.000021>
1372871825.807688 poll([{fd=61, events=POLLOUT|POLLWRNORM}], 1, 0) = 0
(Timeout) <0.000016>
1372871825.807824 poll([{fd=62, events=POLLOUT|POLLWRNORM}], 1, 0) = 0
(Timeout) <0.000016>
1372871825.809776 nanosleep({2, 0}, 0x7fff04b17530) = 0 <2.000174>

For the slow remote host, the sleep is getting triggered even though
the GET request was not yet sent.

Reading through the code in lib/multi.c, in the
CURLM_STATE_WAITCONNECT block, if connected is false, then the result
should be CURLM_CALL_MULTI_PERFORM in order to keep calling poll until
the connections are established and the requests flushed out.

Here's the diff on the 7.19.7 branch that I think will fix the issue.
The patch for 7.31.0 would be exactly the same:
diff --git a/lib/multi.c b/lib/multi.c
index 48df928..dc5ec48 100644
--- a/lib/multi.c
+++ b/lib/multi.c
@@ -1077,6 +1077,7 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
         break;
       }

+ result = CURLM_CALL_MULTI_PERFORM;
       if(connected) {
         if(!protocol_connect) {
           /* We have a TCP connection, but 'protocol_connect' may be false
@@ -1095,8 +1096,6 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
           /* after the connect has completed, go WAITDO or DO */
           multistate(easy, multi->pipelining_enabled?
                      CURLM_STATE_WAITDO:CURLM_STATE_DO);
-
- result = CURLM_CALL_MULTI_PERFORM;
         }
       }
       break;

Is there anything I'm missing, or another way to accomplish this?

Thanks,
Keyur.
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Received on 2013-07-05