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multi_wait() ignores Curl_poll() errors #8921

piru opened this issue May 26, 2022 · 3 comments

multi_wait() ignores Curl_poll() errors #8921

piru opened this issue May 26, 2022 · 3 comments


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piru commented May 26, 2022

multi_wait() ignores the potential error from Curl_poll() at


Line 1315 in d67f41a

If Curl_poll() returns immediately the function will then return CURLM_OK right away without performing any waiting. This will lead curl_multi_poll()/curl_multi_wait() to return immediately as well. Effectively this will lead to a busyloop as there is no status returned that would lead to terminating a loop of a typical curl_multi_perform() + curl_multi_poll() app, at least if the app doesn't bail out once the connections are completed (still_running == 0 doesn't lead to loop termination).

I did this

Had condition where poll() (or select() if using poll() emulation) return a permanent error without waiting.

I expected the following

curl_multi_poll() / curl_multi_wait() to return an error if Curl_poll() fails.

The question is what error to return though as none of the CURLM_ errors seem to fit the bill.

curl/libcurl version


operating system

This is a generic issue, but it is likely more likely to get triggered on specific platforms.

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bagder commented May 27, 2022

I don't think all poll/select errors are permanent though, are they? Should all underlying errors make curl_multi_poll()/wait() return error?

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piru commented May 27, 2022

       EFAULT fds  points  outside the process's accessible address space.  The array given as
              argument was not contained in the calling program's address space.

       EINTR  A signal occurred before any requested event; see signal(7).

       EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

       EINVAL (ppoll()) The timeout value expressed in *ip is invalid (negative).

       ENOMEM Unable to allocate memory for kernel data structures.

Indeed, EINTR probably should not be considered fatal.

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piru commented May 27, 2022

lib/easy.c wait_or_timeout() seems to have the same issue btw


Line 509 in a77d14d

pollrc = Curl_poll(fds, numfds, ev->ms);

lib/asyn-ares.c waitperform() as well. waitperform() result is not checked, which seems to be wrong as well.

nfds = Curl_poll(pfd, num, timeout_ms);

bagder added a commit that referenced this issue Jun 5, 2022
@bagder bagder closed this as completed in 5912da2 Jun 8, 2022
t-8ch added a commit to t-8ch/curl that referenced this issue Jul 3, 2022
The same was done for select() in 5912da2
but poll() was missed.

Downstream report:
See curl#8921 and curl#8961
jay pushed a commit that referenced this issue Jul 4, 2022
The same was done for select() in 5912da2 but poll() was missed.

Reported-by: Alexandre Bury (gyscos at archlinux)

Ref: #8921
Ref: #8961
Ref: 5912da25#r77584294

Closes #9091
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2 participants