pitrou commented on issue #35318:
URL: https://github.com/apache/arrow/issues/35318#issuecomment-1552745718
Ok, I took a brief tour through the libcurl source code:
* The "Unrecoverable error in select/poll" error is generated in
`curl_multi_wait` if `Curl_poll` returns -1
* `Curl_poll` (which, understably, is a wrapper around `poll` on Unix)
returns 1 in three situations:
1. `nfds` is non-zero and `poll` returns an error that's not EINTR
2. `nfds` is zero and the given timeout is negative
3. `nfds` is zero and `poll` returns an error _including EINTR_
Let's dive a bit into google-cloud-cpp. There are two similar functions
named `WaitForHandles` (`CurlImpl::WaitForHandles` and
`CurlDownloadRequest::WaitForHandles). Both call `curl_multi_wait` with zero
extra file descriptors and a hard-coded positive timeout. This eliminates the
"negative timeout" situation above.
We are left with an error returned from `poll`. According to the Linux man
page, these can be:
```
EFAULT fds points outside the process's accessible address space. The
array given as argument was not contained in the calling pro‐
gram's address space.
EINTR A signal occurred before any requested event; see signal(7).
EINVAL The nfds value exceeds the RLIMIT_NOFILE value.
ENOMEM Unable to allocate memory for kernel data structures.
```
We can eliminate EFAULT as `curl_poll` ensures the fds point to accessible
memory.
EINVAL is extremely unlikely given a limit of 1048576 open files in
https://github.com/apache/arrow/issues/35318#issuecomment-1552161651 .
ENOMEM cannot be ruled out, but I guess exhaustion of kernel data space
would manifest randomly in other ways?
This leaves us with EINTR, which can happen in the case that
`curl_multi_wait` [doesn't find any file
descriptors](https://github.com/curl/curl/blob/a9f8fe28481fef7c28d85b4a12a3a35521408eaf/lib/multi.c#L1185-L1207)
to wait for.
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