sslutil: set context security level for legacy tls testing (issue6760) stable

sslutil: set context security level for legacy tls testing (issue6760)

Current versions of OpenSSL do not allow the use of TLS <1.2 when the
library's security level is >=1 (1 being the default on most distributions).

Setting the security level in addition to the minimum protocol is therefore
necessary for the legacy protocol tests.

This is done here ONLY when testing, when:
- explicitly setting the cipher string, or
- using the "--insecure" flag, or
- using the "devel.serverexactprotocol" testing option.

See: https://github.com/openssl/openssl/blob/master/NEWS.md#major-changes-between-openssl-30-and-openssl-310-14-mar-2023

--- a/mercurial/sslutil.py	Thu Apr 13 11:28:48 2023 +0200
+++ b/mercurial/sslutil.py	Wed Apr 12 17:28:39 2023 +0200
@@ -122,7 +122,7 @@
     if ui.insecureconnections:
         minimumprotocol = b'tls1.0'
         if not ciphers:
-            ciphers = b'DEFAULT'
+            ciphers = b'DEFAULT:@SECLEVEL=0'
 
     s[b'minimumprotocol'] = minimumprotocol
     s[b'ciphers'] = ciphers
@@ -627,7 +627,7 @@
     # In tests, allow insecure ciphers
     # Otherwise, use the list of more secure ciphers if found in the ssl module.
     if exactprotocol:
-        sslcontext.set_ciphers('DEFAULT')
+        sslcontext.set_ciphers('DEFAULT:@SECLEVEL=0')
     elif util.safehasattr(ssl, b'_RESTRICTED_SERVER_CIPHERS'):
         sslcontext.options |= getattr(ssl, 'OP_CIPHER_SERVER_PREFERENCE', 0)
         # pytype: disable=module-attr

--- a/tests/test-https.t	Thu Apr 13 11:28:48 2023 +0200
+++ b/tests/test-https.t	Wed Apr 12 17:28:39 2023 +0200
@@ -356,9 +356,9 @@
 
 Clients talking same TLS versions work
 
-  $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.0 --config hostsecurity.ciphers=DEFAULT id https://localhost:$HGPORT/
+  $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.0 --config hostsecurity.ciphers="DEFAULT:@SECLEVEL=0" id https://localhost:$HGPORT/
   5fed3813f7f5
-  $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.1 --config hostsecurity.ciphers=DEFAULT id https://localhost:$HGPORT1/
+  $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.1 --config hostsecurity.ciphers="DEFAULT:@SECLEVEL=0" id https://localhost:$HGPORT1/
   5fed3813f7f5
   $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.2 id https://localhost:$HGPORT2/
   5fed3813f7f5
@@ -400,7 +400,7 @@
 The per-host config option overrides the default
 
   $ P="$CERTSDIR" hg id https://localhost:$HGPORT/ \
-  > --config hostsecurity.ciphers=DEFAULT \
+  > --config hostsecurity.ciphers="DEFAULT:@SECLEVEL=0" \
   > --config hostsecurity.minimumprotocol=tls1.2 \
   > --config hostsecurity.localhost:minimumprotocol=tls1.0
   5fed3813f7f5