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Infinite loop

From: Jakub Zakrzewski <>
Date: Tue, 12 Jan 2016 11:18:26 +0000

Hi All,

I have a very weird problem and I'm having hard time trying to find the cause.

My program makes frequent calls to a sftp server to get the list of files (like once a minute or so). The sftp server is quite slow and overloaded. Every once in a while (according to my colleage once or twice a month) it goes into infinite loop. Weird thing is it affects all the instances that have tried to connect to this server at around this time so I suspect that a weird (or simply unexpected) answer from the server is causing this. The bad thing is this is a production server with release versions of everything, so debugging is a nightmare. The frequency of the problem occuring doesn't help also.

This is what I know so far:
sftp software: something that runs on linux, I'm trying to get this info

$ curl.exe --version
curl 7.44.0 (Windows) libcurl/7.44.0 OpenSSL/1.0.2d zlib/ libssh2/1.6.0
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtsp scp sftp smtp smtps telnet tftp
Features: SSPI Kerberos SPNEGO NTLM SSL libz

The callstacks from two dumps made from hanging processes (unfortunately, the variable values are mangled and make no sense; probably due to this being an optimezed release build):
        ntdll.dll!NtDeviceIoControlFile() Unknown
         mswsock.dll!WSPSelect() Unknown
         ws2_32.dll!select() Unknown
         libcurl.dll!Curl_socket_check(unsigned __int64 readfd0=1477468750437493, unsigned __int64 readfd1=634792935808, unsigned __int64 writefd=0, long timeout_ms=-865151808) Line 328 C
        libcurl.dll!ssh_block_statemach(connectdata * conn=0x00000093cc9b8101, bool duringconnect=128) Line 2713 C
         libcurl.dll!Curl_disconnect(connectdata * conn=0x00000093cc6ed4c0, bool dead_connection=128) Line 2764 C
         libcurl.dll!Curl_done(connectdata * * connp=0x0000000000000000, CURLcode status=-865151808, bool premature=72) Line 6088 C
         libcurl.dll!multi_runsingle(Curl_multi * multi=0x00000093cc6ed4c0, timeval now={...}, SessionHandle * data=0x00000093cc860a00) Line 1079 C
         libcurl.dll!curl_multi_perform(void * multi_handle=0x0005b8d800026521, int * running_handles=0x0000000000000000) Line 1811 C
         libcurl.dll!easy_transfer(void * multi=0x00000093cc6ed4c0) Line 715 C
         libcurl.dll!easy_perform(SessionHandle * data=0x0000000000000001, bool events=240) Line 803 C

        Network.dll!CURLProgressCallback(void * clientp=0x000a7f8000095c83, double dltotal=1.976262583365e-322#DEN, double dlnow=2.149869607130e-314#DEN, double ultotal=2.149869607130e-314#DEN, double ulnow=0.00000000000000000) Line 253 C++
         libcurl.dll!Curl_pgrsUpdate(connectdata * conn=0x000a410000095c83) Line 388 C
         libcurl.dll!ssh_block_statemach(connectdata * conn=0x0000000105787e01, bool duringconnect=false) Line 2721 C
         libcurl.dll!Curl_disconnect(connectdata * conn=0x000000010577a380, bool dead_connection=false) Line 2764 C
         libcurl.dll!Curl_done(connectdata * * connp=0x0000000000000000, CURLcode status=91726720, bool premature=120) Line 6088 C
        libcurl.dll!multi_runsingle(Curl_multi * multi=0x000000010577a380, timeval now={...}, SessionHandle * data=0x00000001056bae00) Line 1079 C
         libcurl.dll!curl_multi_perform(void * multi_handle=0x0007280800027fd9, int * running_handles=0x0000000000000000) Line 1811 C
         libcurl.dll!easy_transfer(void * multi=0x000000010577a380) Line 715 C
         libcurl.dll!easy_perform(SessionHandle * data=0x0000000000000001, bool events=240) Line 803 C

In our code there's no retry whatsoever - we call the easy_perform and return the result.

The affected processes take as much CPU as they can, so it's an active loop.

There's a timeout set (via CURLOPT_TIMEOUT) but it apparently has no effect.

There's no network communication between my program and the sftp server - it just loops without contacting the server.

I have the two affected processes running at the moment. I have no idea how long I can keep that state (they already run for a couple of days).

Does anyone have an idea what can I do next?

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Received on 2016-01-12