On Fri, Jul 5, 2013 at 5:26 AM, Evgeny Turnaev <turnaev.e_at_gmail.com> wrote:
> Hi Keyur,
> First of i am not really expert in curl. But reading through
> documentation i can't find evidence that curl_multi_perform should
> return CURLM_CALL_MULTI_PERFORM untill connection succeeds. Connection
> can take very long time. Why don't you change your application code so
> that it selects (poll or whatever) on curl sockets and when there is
> action - call curl_multi_perform again. I suppose this is a typical
> usage. Someone please correct me if i am wrong.
>

Hi Evgeny,

Thanks for the response.

The issue is the inconsistency of the response code.

In the localhost case, when curl_multi_perform returns CURLM_OK, the
TCP connection is done, the HTTP connection is done, and the request
has been sent over. If I call a select() on the FDs returned by
curl_multi_fdset(), I'm actually waiting for a response for the
server. I don't want this because I want to execute other code while
the async request is being procesed.

Whereas in the slow-remote-host case, even the TCP connection is not
done when CURLM_OK is returned by curl_multi_perform(). So when I call
a select(), I am still waiting for the TCP connect to go through.

There is no way for my code to know which of the two scenarios is it
currently experiencing.

Once curl_multi_perform() has a TCP connection it returns
CURML_CALL_MULTI_PERFORM until the request is sent over. It seems like
a miss to not return the same when the TCP connection has not yet gone
through.

Hope that makes sense.

Thanks,
Keyur.

> 2013/7/5 Keyur Govande <keyurgovande_at_gmail.com>:
>> 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