Random "couldn't resolve host" failures in curl_multi

Hello!

I'm experiencing a pretty random failure of curl_multi to grab some of
the handles added to it.

I create 12 handles for http:// URLs, all using the same target machine,
just different files. I add them to curl_multi. And as soon as I
curl_multi_exec, sometimes they all work, and sometimes only exactly
first 8 get fetched, others don't even show up in curl_multi_select
active count.

After making a proper error-checking setup with curl_multi_info_read, I
found that some of the handles get a "couldn't resolve host name"
message queued, right on the very first call to curl_multi_exec.
However, the error is not recorded in the child handles - the failed
ones return 0 for curl_errno.

Also, it's always the final handles that fail - n first handles work,
n+1 till the end fail to resolve, with n sometimes being all 12,
sometimes 8, a few times 4, but I also caught it on 11.

This is in cURL 7.62 under PHP 7.2.21.

Pseudocode to illustrate this would go:

m = curl_multi_init()
for i = 1 to 12 {
     handles[i] = curl_init ("http://the.same.host.com/file" + i)
     curl_multi_add_handle (m, handles[i])
}
status = curl_multi_exec (m, still_running) // status is 0,
still_running is 8, because...
info = curl_multi_info_read (m, queue) // info contains "couldn't
resolve host" for handle 9, queue shows 3 as there are also failures for
10, 11 and 12
keep_calling_curl_multi_exec_and_select_as_usual() //
error = curl_errno (handle[9]) // error is 0
info = curl_getinfo (handle[9]) // pretty much empty structure,
basically all 0s

So here are my 2 questions:
- what could be causing this to happen randomly?
- why isn't the error stored in the child handle?

Regards,

-- 
Adam "Sinus" Skawiński
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