cf-socket, simulate slow/blocked receives in debug

[icing]

Add 2 env variables for non-UDP sockets:

1. CURL_DBG_SOCK_RBLOCK: percentage of receive calls that randomly should return EAGAIN
2. CURL_DBG_SOCK_RMAX: max amount of bytes read from socket

similar to the existing CURL_DBG_SOCK_WBLOCK and CURL_DBG_SOCK_WPARTIAL. Documentation to be added once Jay's doc PR has landed.

Why?

Tests were run locally on a macOS dev machine with different value for the variables. Only one var was used in tests to isolate problems.

CURL_DBG_SOCK_RBLOCK

X percent of socket recv() calls return CURLE_AGAIN right away.

Interesting for cases where a subsequent read would normally be successful, but now is not.

CURL_DBG_SOCK_RBLOCK	Failures
0	-
10	823 869 907 2302
50	823 869 907 2302
90	823 869 907 1474 2302

CURL_DBG_SOCK_RMAX

Calls to socket recv() with a buffer limit the length of the buffer to this value.

Interesting for cases where the network is slow or where the server sends in strange chunks or where a middle box is involved that buffers differently. Note that low values just trigger edge cases in taking in data deterministically. The failures could happen on "real" data as well, only with lower probability.

CURL_DBG_SOCK_RMAX	Failures	Protocol
500	-
250	-
125	-
120	-
118	-
117	571	RTSP
116	571	RTSP
115	571	RTSP
110	571	RTSP
100	571	RTSP
75	571	RTSP
50	571	RTSP
48	571	RTSP
45	571	RTSP
44	571	RTSP
43	571 3100	RTSP
37	571 3100	RTSP
25	571 3100	RTSP
23	568 571 572 2302 3100	RTSP WebSockets
20	568 571 572 577 3100	RTSP WebSockets
12	567 568 569 570 571 572 577 1540 3100	RTSP WebSockets HTTP
6	457 567 568 569 570 571 572 577 1540 3100	RTSP WebSockets HTTP
5	567 577 571 568 569 570 572 1540 3100	RTSP WebSockets HTTP
4	567 569 571 568 570 572 577 981 982 1540 2302 3100	RTSP WebSockets HTTP
3	232 457 567 568 569 570 571 572 577 980 983 1540 2302 3100	RTSP WebSockets HTTP
2	457 567 568 569 570 571 577 572 981 982 1190 1192 1194 1452 1540 2204 2302 3018 3100	RTSP WebSockets HTTP IMAP POP3
1	232 457 567 568 570 569 571 572 577 980 981 982 983 1190 1191 1192 1193 1194 1195 1198 1199 1452 1474 1540 1916 1917 2201 2204 2302 3017 3018 3100	RTSP WebSockets HTTP IMAP POP3 MQTT TELNET

command run:

make -j20 && (cd tests && make -j20 && ENV_VAR=xxx ./runtests.pl -a -j20)

curl build used:

curl 8.4.0-DEV (x86_64-apple-darwin22.6.0) libcurl/8.4.0-DEV OpenSSL/3.0.9 zlib/1.2.11 brotli/1.0.9 zstd/1.5.5 c-ares/1.19.1 libidn2/2.3.4 libssh2/1.11.0 nghttp2/1.56.0 ngtcp2/0.19.1 nghttp3/0.15.0 librtmp/2.3
Release-Date: [unreleased]
Protocols: dict file ftp ftps gopher gophers http https imap imaps 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 SPNEGO SSL threadsafe TLS-SRP TrackMemory UnixSockets zstd