[gnutls-devel] GnuTLS | GNUTLS_ALPN_MANDATORY is enforced in server mode only (#1595)

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Robert Pluim created an issue: https://gitlab.com/gnutls/gnutls/-/issues/1595



## Description of problem:

gnutls_alpn_set_protocols is documented to accept GNUTLS_ALPN_MANDATORY to mean "Require ALPN negotiation", but that flag is only checked when running in server mode, not in client mode.

## Version of gnutls used:

3.7.9

## Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)

Debian

## How reproducible:

Always

Steps to Reproduce:

1. Patch gnutls/src/cli.c to pass GNUTLS_ALPN_MANDATORY in the call to gnutls_alpn_set_protocols

2. Run the modified gnutls-cli: `gnutls-cli --http --port 6666 --alpn=h2` against a gnutls-serv without any alpn flags specified

## Actual results:

Connection succeeds

## Expected results:

*** Fatal error: No common application protocol could be negotiated.

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