Fix SSL handshake over-reading in STARTTLS by youknowone · Pull Request #7417 · RustPython/RustPython

During STARTTLS handshake, sock_recv(16KB) could consume
application data that arrived alongside handshake records.
The consumed data ended up in rustls's internal buffer where
select() could not detect it, causing asyncore-based servers
to miss readable events and the peer to time out.

Use MSG_PEEK to find the TLS record boundary, then recv()
only one complete record. Remaining data stays in the kernel
TCP buffer, visible to select(). This matches OpenSSL's
default no-read-ahead behaviour.

Fixes flaky test_poplib (TestPOP3_TLSClass) failures.

During STARTTLS handshake, sock_recv(16KB) could consume
application data that arrived alongside handshake records.
The consumed data ended up in rustls's internal buffer where
select() could not detect it, causing asyncore-based servers
to miss readable events and the peer to time out.

Use MSG_PEEK to find the TLS record boundary, then recv()
only one complete record. Remaining data stays in the kernel
TCP buffer, visible to select(). This matches OpenSSL's
default no-read-ahead behaviour.

Fixes flaky test_poplib (TestPOP3_TLSClass) failures.