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curl_multi: resumed transfer is not completed although all data was received #3994

sstruchtrup opened this issue Jun 7, 2019 · 5 comments


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I did this

I am using libcurl with the multi interface (and libev) to receive data via http. Received data will be written to a relatively slow flash memory. curl_easy_pause() is used for flow control.

I noticed stuck transfers when the transfer is paused on the last invocation on the write callback, i.e. when the last chunk (in a non-http sense) of data is received. Once the transfer is resumed, it does not complete, although all data was received.

I observed the problem both when using curl_easy_pause(easy, CURLPAUSE_ALL) (consuming the data) from within the callback and as well when returning CURL_WRITEFUNC_PAUSE from the callback (without consuming the data).

Restarting the transfer will deliver the remaining data, in case it was not consumed (by returning CURL_WRITEFUNC_PAUSE). But in any case the transfer is not completed, that is CURLMSG_DONE is not received. It looks it will eventually be delivered, likely once the server closes the connection.

The application is single-threaded.

I will submit a example to demonstrate the problem, based on the evhiperfifo.c example, modified to pause the transfer on the first invocation and restart it after one second.

Fetching a resource with a short body (single callback invocation) will demonstrate the problem, e.g.

I expected the following

The transfer shall complete once all data was received. CURLMSG_DONE should be delivered by curl_multi_info_read().

curl/libcurl version

[curl -V output]

curl 7.64.0 (x86_64-pc-linux-gnu) libcurl/7.64.0 OpenSSL/1.1.1b zlib/1.2.11 libidn2/2.0.5 libpsl/0.20.2 (+libidn2/2.0.5) libssh/0.8.6/openssl/zlib nghttp2/1.36.0 librtmp/2.3
Release-Date: 2019-02-06
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP HTTP2 UnixSockets HTTPS-proxy PSL

I tested current curl master as well as curl 7.40 and 7.29/7.30, all seem to show this problem.

operating system

Linux / Ubuntu 18.04

sstruchtrup pushed a commit to sstruchtrup/curl that referenced this issue Jun 7, 2019
Fetch an URL with a short body, e.g.
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Modified example in sstruchtrup/curl@e62b9f6

Output for

event_cb  w 0x557d9dae8a80 revents 1
* Mark bundle as not supporting multiuse
* HTTP 1.1 or later with persistent connection
< HTTP/1.1 301 Moved Permanently
< Location:
< Content-Type: text/html; charset=UTF-8
< Date: Fri, 07 Jun 2019 14:41:44 GMT
< Expires: Sun, 07 Jul 2019 14:41:44 GMT
< Cache-Control: public, max-age=2592000
< Server: gws
< Content-Length: 219
< X-XSS-Protection: 0
< X-Frame-Options: SAMEORIGIN
Received 219 bytes
*** pausing transfer ***
* Paused 219 bytes in buffer for type 01
Progress: (219/219)
Progress: (219/219)
sock_cb e 0x557d9daf1fc8 s 6 what 4 cbp 0x7ffd6efa9590 sockp 0x557d9dae8a60
socket callback: s=6 e=0x557d9daf1fc8 what=REMOVE
*** timeout, restarting transfer ***
Received 219 bytes

The transfer is complete, all 219 bytes were consumed, but no CURLMSG_DONE is received.

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Using the curl multi interface without an external event loop, e.g. via curl_multi_wait() does not show this behaviour.

@sstruchtrup sstruchtrup changed the title curl_multi: unpaused transfer is not completed although all data was received curl_multi: resumed transfer is not completed although all data was received Jun 7, 2019
bagder added a commit that referenced this issue Jun 8, 2019
... so that timeouts or other state machine actions get going again
after a changing pause state. For example, if the last delivery was
paused there's no pending socket activity.

Reported-by: sstruchtrup on github
Fixes #3994
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bagder commented Jun 9, 2019

I could reproduce the problem and the fix in #4001 made it work for me. It'd be great if you could see if it does for you as well?

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I can confirm that it solves it for me as well, both for the sample code as well as for the actual application.
Thanks for the quick fix!

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bagder commented Jun 9, 2019

Lovely, thanks for confirming!

@bagder bagder closed this as completed in 680f141 Jun 9, 2019
@lock lock bot locked as resolved and limited conversation to collaborators Sep 7, 2019
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2 participants