Comment 13 for bug 1971489

[jgh: edited to hide identifying info]

Created attachment 1160 [details]
First shot at a fix.

Hello,

exim (4.92 rc2) built against gnutls >= 3.6.5 supports TLS 1.3. However while incoming connections seem to work, outgoing TLS 1.3 is totally broken, mails are stuck in the queue.
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16:09:51 23494 cipher: TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256
16:09:51 23494 Have channel bindings cached for possible auth usage.
16:09:51 23494 SMTP>> EHLO <elided>
16:09:51 23494 cmd buf flush 22 bytes
16:09:51 23494 tls_write(0x7ffe02af0cd0, 22)
16:09:51 23494 gnutls_record_send(SSL, 0x7ffe02af0cd0, 22)
16:09:51 23494 outbytes=22
16:09:51 23494 Calling gnutls_record_recv(0x559451c5fe30, 0x7ffe02aefcd0, 4096)
16:09:51 23494 GnuTLS<3>: ASSERT: ../../lib/buffers.c[get_last_packet]:1171
16:09:51 23494 GnuTLS<3>: ASSERT: ../../lib/buffers.c[_gnutls_handshake_io_recv_int]:1431
16:09:51 23494 GnuTLS<3>: ASSERT: ../../lib/record.c[_gnutls_recv_in_buffers]:1560
16:09:51 23494 GnuTLS<3>: ASSERT: ../../lib/record.c[_gnutls_recv_int]:1759
16:09:51 23494 tls_read: err from gnutls_record_recv(
16:09:51 23494 LOG: MAIN
16:09:51 23494 H=<elided> [<elided>] TLS error on connection (recv): Resource temporarily unavailable, try again.
16:09:51 23494 SMTP(No such file or directory)<<
16:09:51 23494 tls_close(): shutting down TLS
16:09:51 23494 GnuTLS<3>: ASSERT: ../../lib/buffers.c[_gnutls_io_write_flush]:696
16:09:51 23494 SMTP(close)>>
16:09:51 23494 LOG: MAIN
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(Thanks, Jeremy Harris, for help with this and providing a test server to connect to.)

The error seems to be caused by exim regarding gnutls_record_recv() returning a
value <0 as a fatal error, while GNUTLS_E_AGAIN will happen in a normal TLS1.3 connection, due to the changed handshake.
See https://gitlab.com/gnutls/gnutls/issues/644#note_123363338

Attached patch seems to work for, Jeremy said he had tested something similarily, so we better use his version.