4 files changed, 175 insertions, 2 deletions

diff --git a/src/bin/client/connection.rs b/src/bin/client/connection.rs
index 7516836..6bc9b09 100644
--- a/src/bin/client/connection.rs
+++ b/src/bin/client/connection.rs
@@ -8,6 +8,7 @@ use std::thread;
 use std::time::{Duration, Instant};
 use zagan_bet::client::database;
 use zagan_bet::client::ui::first_contact_lines;
+use zagan_bet::version::{is_newer, parse_hello, VERSION};
 
 static UNSAFE_MODE: Mutex<bool> = Mutex::new(false);
 
@@ -69,16 +70,35 @@ pub fn connect(
     let mut tls = tls;
     tls.set_read_timeout(Some(Duration::from_secs(10)))?;
 
+    // Version lockstep: the first line after TLS is the handshake, and no
+    // credentials are sent until the server accepts it.
     let start = Instant::now();
-    tls.write_line("PING")?;
+    tls.write_line(&format!("HELLO {VERSION}"))?;
     let mut response = String::new();
-    tls.read_line(&mut response)?;
+    if tls.read_line(&mut response)? == 0 {
+        return Err("server closed the connection during HELLO".into());
+    }
     let elapsed = start.elapsed();
+    let response = response.trim();
+
+    if let Some(message) = response.strip_prefix("ERROR") {
+        return Err(message.trim().to_string().into());
+    }
+    let Some(hello) = parse_hello(response) else {
+        return Err(format!("unexpected HELLO reply: {response}").into());
+    };
+
     println!(
         "\n\rConnected to {} with ping delay: {:.1}ms (TLS)",
         addr,
         elapsed.as_secs_f64() * 1000.0
     );
+    if is_newer(&hello.latest, VERSION) {
+        println!(
+            "A newer version is available: {} ({})",
+            hello.latest, hello.url
+        );
+    }
 
     // Credentials are stored per server identity, so any address presenting
     // this certificate key can log in with the same stored account.
diff --git a/src/lib.rs b/src/lib.rs
index 51afafc..f1de5d7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,3 +14,4 @@ pub mod roulette;
 pub mod server;
 pub mod slots;
 pub mod tofu;
+pub mod version;
diff --git a/src/server/core.rs b/src/server/core.rs
index 89264f3..3cd6469 100644
--- a/src/server/core.rs
+++ b/src/server/core.rs
@@ -6,6 +6,7 @@ use std::sync::Arc;
 use crate::blackjack::BlackjackState;
 use crate::crash::CrashState;
 use crate::server::database;
+use crate::version::{download_url, hello_line, Version, VERSION};
 use crate::server::games::blackjack::{BlackjackGame, BlackjackHand};
 use crate::server::games::crash::{Clock, CrashGame, CrashRound, RealClock};
 use crate::server::games::registry::GameRegistry;
@@ -15,6 +16,7 @@ use crate::server::games::traits::{GameError, GamePhase, GameState};
 #[derive(Debug, Default)]
 pub struct ConnectionState {
     user: Option<String>,
+    greeted: bool,
 }
 
 /// Settle reservations left behind by a previous process.
@@ -32,6 +34,11 @@ impl ConnectionState {
     pub fn user(&self) -> Option<&str> {
         self.user.as_deref()
     }
+
+    /// True once the client has completed the version handshake.
+    pub fn greeted(&self) -> bool {
+        self.greeted
+    }
 }
 
 pub struct ServerCore {
@@ -91,7 +98,14 @@ impl ServerCore {
 
     pub fn handle_message(&mut self, state: &mut ConnectionState, msg: &str) -> String {
         let parts: Vec<&str> = msg.splitn(5, ' ').collect();
+
+        // Nothing but the handshake is accepted on a fresh connection.
+        if parts[0] != "HELLO" && !state.greeted {
+            return "ERROR HELLO required".to_string();
+        }
+
         match parts[0] {
+            "HELLO" => self.handle_hello(state, &parts),
             "GAME_LIST" => {
                 let mut games = self.registry.list_games();
                 games.sort();
@@ -193,6 +207,27 @@ impl ServerCore {
         }
     }
 
+    fn handle_hello(&mut self, state: &mut ConnectionState, parts: &[&str]) -> String {
+        if parts.len() != 2 {
+            return "ERROR Invalid HELLO format. Usage: HELLO <version>".to_string();
+        }
+
+        let (Some(client), Some(server)) = (Version::parse(parts[1]), Version::parse(VERSION)) else {
+            return "ERROR Invalid version".to_string();
+        };
+
+        if !client.compatible_with(server) {
+            return format!(
+                "ERROR Version mismatch: client {}, server {VERSION}. Get {VERSION} at {}",
+                parts[1],
+                download_url()
+            );
+        }
+
+        state.greeted = true;
+        hello_line(VERSION, VERSION, &download_url())
+    }
+
     fn handle_register(&mut self, username: &str, credential: &str) -> String {
         if database::get_hashed_password(&self.conn, username).is_some() {
             return "ERROR User already exists".to_string();
diff --git a/src/version.rs b/src/version.rs
new file mode 100644
index 0000000..c1dcee5
--- /dev/null
+++ b/src/version.rs
@@ -0,0 +1,117 @@
+//! Version compatibility and the `HELLO` handshake.
+//!
+//! Client and server must share MAJOR.MINOR; only the patch component may
+//! differ. The first line after TLS is `HELLO <version>`, and nothing else is
+//! accepted until the server has answered it.
+
+use std::env;
+
+/// Version this build reports. The client and the server share it.
+pub const VERSION: &str = env!("CARGO_PKG_VERSION");
+
+/// Where players download new releases when the server advertises one and the
+/// operator has not set `DOWNLOAD_URL`.
+pub const DEFAULT_DOWNLOAD_URL: &str = "https://zagan.2137697.xyz/";
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Version {
+    pub major: u64,
+    pub minor: u64,
+    pub patch: u64,
+}
+
+impl Version {
+    /// Parse `MAJOR.MINOR.PATCH`, tolerating a leading `v` and a pre-release or
+    /// build suffix (`4.0.0-rc1`, `4.0.0+build`).
+    pub fn parse(raw: &str) -> Option<Self> {
+        let raw = raw.trim();
+        let raw = raw.strip_prefix('v').unwrap_or(raw);
+
+        // Drop a pre-release or build suffix, but not an empty one.
+        let (core, suffix) = match raw.find(['-', '+']) {
+            Some(index) => (&raw[..index], Some(&raw[index + 1..])),
+            None => (raw, None),
+        };
+        if suffix == Some("") {
+            return None;
+        }
+
+        let mut parts = core.split('.');
+        let major = parts.next()?.parse().ok()?;
+        let minor = parts.next()?.parse().ok()?;
+        let patch = parts.next()?.parse().ok()?;
+        if parts.next().is_some() {
+            return None;
+        }
+
+        Some(Self {
+            major,
+            minor,
+            patch,
+        })
+    }
+
+    pub fn compatible_with(self, other: Self) -> bool {
+        self.major == other.major && self.minor == other.minor
+    }
+}
+
+/// True when both versions parse and share MAJOR.MINOR.
+pub fn compatible(client: &str, server: &str) -> bool {
+    match (Version::parse(client), Version::parse(server)) {
+        (Some(client), Some(server)) => client.compatible_with(server),
+        _ => false,
+    }
+}
+
+/// True when `latest` is strictly newer than `current`. Unparseable versions
+/// are never "newer", so a broken reply cannot nag the player.
+pub fn is_newer(latest: &str, current: &str) -> bool {
+    match (Version::parse(latest), Version::parse(current)) {
+        (Some(latest), Some(current)) => latest > current,
+        _ => false,
+    }
+}
+
+/// The server line that accepts a handshake.
+pub fn hello_line(version: &str, latest: &str, url: &str) -> String {
+    format!("HELLO {version} latest={latest} url={url}")
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct HelloReply {
+    pub version: String,
+    pub latest: String,
+    pub url: String,
+}
+
+/// Parse `HELLO <version> latest=<version> url=<url>`.
+pub fn parse_hello(line: &str) -> Option<HelloReply> {
+    let rest = line.trim().strip_prefix("HELLO ")?;
+    let mut tokens = rest.split_whitespace();
+    let version = tokens.next()?.to_string();
+
+    let mut latest = None;
+    let mut url = None;
+    for token in tokens {
+        if let Some(value) = token.strip_prefix("latest=") {
+            latest = Some(value.to_string());
+        } else if let Some(value) = token.strip_prefix("url=") {
+            url = Some(value.to_string());
+        }
+    }
+
+    Some(HelloReply {
+        version,
+        latest: latest?,
+        url: url?,
+    })
+}
+
+/// Download page advertised by the server. `DOWNLOAD_URL` wins when set.
+pub fn download_url() -> String {
+    env::var("DOWNLOAD_URL")
+        .ok()
+        .filter(|value| !value.trim().is_empty())
+        .unwrap_or_else(|| DEFAULT_DOWNLOAD_URL.to_string())
+}