Table of Contents /'su.la/ for the suffix of capsule in Spanish, Cápsula. Git A Gemini protocol server written in Scryer Prolog . Requirements # sula depends on a patched Scryer Prolog which can be found here (branch js/fixes ). The required patches are: A native '$copy_stream'/2 builtin used for streaming binary file bodies to TLS clients without materialising the contents on the Prolog heap. A fix to library(pio) ’s buffer_prepare_for_n/5 so that lazy reads from process pipes (and other streams whose at_end_of_stream/1 never reports true) terminate on EOF instead of spinning. A non-blocking poll loop in socket_server_accept/4 that checks Scryer’s INTERRUPT flag, so SIGINT becomes a catchable '$interrupt_thrown' exception instead of being trapped behind a blocking syscall. A port to rustls . A modification of tls_server_negotiate to include the optional client certificate. Build and install the patched Scryer: git clone https://git.sagredo.dev/scryer-prolog -b js/fixes cd scryer-prolog cargo install --path . openssl(1) must also be on PATH — it’s invoked at startup to read the CN from the configured identity certificate and verify it matches the configured hostname. Running # ./sula.pl --addr HOST:PORT --hostname NAME --content DIR --certs DIR sula.pl is a polyglot script: bash detects scryer-prolog on PATH and execs it with sula:run, halt as the entry goal. Example: ./sula.pl \ --addr 127.0.0.1:1965 \ --hostname gmi.example.dev \ --content ./site \ --certs . CLI options # All options accept any order. Anything unrecognised is silently dropped. Option Meaning Default --addr HOST:PORT Bind address and port for the listening socket 127.0.0.1:1965 --hostname NAME Expected CN of the certificate. Startup aborts on mismatch localhost --content DIR Root directory for served files ./site --certs DIR Directory containing cert.pem and key.pem . Stopping the server # Ctrl+C triggers a clean shutdown: the listening socket is closed, the top-level catch logs Shutting down , and the process exits 0. Features # TLS via rustls , PKCS#12 identity files. Hostname verification: at startup, cert_is_for_hostname/2 shells out to openssl x509 and asserts the cert’s CN matches --hostname . Content negotiation by extension via mime/2 , populated at startup from /etc/mime.types (parsed by a DCG in mime.pl ). text/gemini is added for .gmi . Text responses sent via format/3 ; binary responses streamed in native code through copy_stream/2 (file → TLS socket, no Prolog heap traffic). Per-connection error handling: TLS handshake failures and mid-stream client disconnects are logged and the loop continues. Other errors re-throw and surface at the top level. Graceful shutdown on SIGINT via the patched Scryer socket_server_accept/4 . Layout # sula.pl Polyglot launcher + main sula module (run/0, request loop). config.pl CLI parsing (DCG) and config accessors (cert/1, addr/1, ...). cert.pl Certificate loading + hostname-vs-CN check. mime.pl /etc/mime.types parser (DCG) and mime/2 facts. request.pl Request line reader. gemini_uri.pl Gemini URI DCG (gemini://host[:port]/path[?query]). ip.pl IP address recognition (rejected as Gemini hosts). response.pl Response status code DCG. log.pl Tagged log_msg/3. banner.pl Reads banner.txt and emits it line-by-line via display_banner/1. Planned features # Use key and cert instead of identity.p12 Client certificates Load configuration from a configuration file Save and load users Run CGI scripts All status codes Rate limiting Virtual hosting File logging Multi-threading or kind of? Hot reload?