test652 may go into busy loop

[icing]

I did this

On the master branch, I ran:

curl> make -j20 && (cd tests && make -j20 && CURL_DBG_SOCK_WBLOCK=90 ./runtests.pl -a -v 652)

which runs until timeout and uses 100% cpu.

I expected the following

The test case to work or at least fail on its own.

curl/libcurl version

curl 8.10.0-DEV (x86_64-apple-darwin23.5.0) libcurl/8.10.0-DEV wolfSSL/5.7.0 zlib/1.2.12 brotli/1.1.0 zstd/1.5.6 c-ares/1.28.1 libidn2/2.3.7 libpsl/0.21.5 libssh2/1.11.0 nghttp2/1.62.1 ngtcp2/1.6.0 nghttp3/1.4.0 librtmp/2.3
Release-Date: [unreleased]
Protocols: dict file ftp ftps gopher gophers http https imap imaps ipfs ipns ldap ldaps mqtt pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp ws wss
Features: alt-svc AsynchDNS brotli Debug GSS-API HSTS HTTP2 HTTP3 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM PSL SPNEGO SSL threadsafe TrackMemory UnixSockets zstd

operating system

Darwin xxx 23.5.0 Darwin Kernel Version 23.5.0: Wed May 1 20:09:52 PDT 2024; root:xnu-10063.121.3~5/RELEASE_X86_64 x86_64

[icing]

Fixed by #14532.