I did this

Ran the example, passed URL to the fifo

I expected the following

Whenever the last transfer finishes, I'd expect to see
"last transfer done, kill timeout"
and the mainloop waits for the next fifo call, without timer events

What I saw

Sometimes the message above is missing, and the mainloop keeps calling the timer after the last transfer is done.

curl/libcurl version

curl 7.66.0 (x86_64-redhat-linux-gnu) libcurl/7.66.0 OpenSSL/1.1.1d-fips zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.2.0) libssh/0.9.0/openssl/zlib nghttp2/1.39.2
Release-Date: 2019-09-11
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: AsynchDNS brotli GSS-API HTTP2 HTTPS-proxy IDN IPv6 Kerberos Largefile libz Metalink NTLM NTLM_WB PSL SPNEGO SSL TLS-SRP UnixSockets

operating system

Linux fc31 5.3.7-301.fc31.x86_64 #1 SMP Mon Oct 21 19:18:58 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Extra info:

In the good case, the end of the sequence is generally:
REMAINING: 1
Progress: http://www.example.com (62/62)
socket callback: s=6 e=0x54a1e20 what=REMOVE
multi_timer_cb: Setting timeout to -1 ms
REMAINING: 0
DONE: http://www.example.com => (0)
last transfer done, kill timeout

In the "bad" case, it's:
REMAINING: 1
socket callback: s=6 e=0x5482b60 what=REMOVE
Progress: http://www.example.com (62/62)
REMAINING: 0
DONE: http://www.leahnim.org => (0)
REMAINING: 0
REMAINING: 0
...

Call chain in the good case seems to be:
curl_multi_socket_action(real socket)-> { write_callback ; socket_callback remove action } -> timer_callback -1

In the bad case:
curl_multi_socket_action(TIMEOUT) -> {write_callback ; socket_callback remove action }

i.e. the timer callback isn't called.