gatewayd: main, serve --config with its start checks

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/gatewayd/src/main.rs

@@ -1,4 +1,93 @@
-fn main() {
-    eprintln!("gatewayd: not implemented until M4");
-    std::process::exit(2);
+//! `gatewayd serve --config <path>`: the Mattermost channel. It loads its configuration and its
+//! token, prepares its directory, then serves until it must stop (exit 1).
+
+use std::os::unix::fs::DirBuilderExt;
+use std::path::Path;
+use std::process::ExitCode;
+use std::sync::Arc;
+use std::sync::atomic::AtomicBool;
+
+use gatewayd::config::{Config, MATTERMOST_TOKEN};
+use gatewayd::secrets;
+use gatewayd::serve::{START_FAILED, Tuning, run};
+
+fn main() -> ExitCode {
+    // `args_os`: the config path need not be UTF-8, and `args` would panic on one that is not.
+    let args: Vec<std::ffi::OsString> = std::env::args_os().skip(1).collect();
+    let words: Vec<Option<&str>> = args.iter().map(|a| a.to_str()).collect();
+    match (words.as_slice(), args.get(2)) {
+        ([Some("serve"), Some("--config"), _], Some(path)) => serve(Path::new(path)),
+        _ => {
+            eprintln!("usage: gatewayd serve --config <path>");
+            ExitCode::from(2)
+        }
+    }
+}
+
+fn serve(path: &Path) -> ExitCode {
+    // Each failure prints its message with `eprintln!` and returns `ExitCode::from(1)`.
+    // 1. `let config = match Config::load(path) { ... }`: an Err(e) prints
+    //    "gatewayd: {e}\n{START_FAILED}".
+    // 2. `let source = match config.token_source() { ... }`: an Err(why) prints
+    //    "gatewayd: {}: {why}\n{START_FAILED}" with `path.display()`.
+    // 3. `let loaded = match secrets::load(MATTERMOST_TOKEN, &source, &|k| std::env::var_os(k))`:
+    //    an Err(e) prints "gatewayd: {e}" (the error carries its own pointer).
+    //    Then `if let Some(warning) = &loaded.warning { eprintln!("{warning}"); }`.
+    // 4. The state file's directory:
+    //    `let dir = config.state_path().parent().map(Path::to_path_buf).unwrap_or_default();`
+    //    `std::fs::DirBuilder::new().recursive(true).mode(0o700).create(&dir)`: an Err(e) prints
+    //    "gatewayd: cannot prepare {}: {e}\n{START_FAILED}" with `dir.display()`.
+    // 5. `let log: gatewayd::serve::Log = Arc::new(|line: &str| eprintln!("{line}"));`
+    //    `let stop = run(config, loaded.secret, Tuning::default(), log, &AtomicBool::new(false));`
+    //    `eprintln!("{stop}");` and `ExitCode::from(1)`.
+    let config = match Config::load(path) {
+        Ok(config) => config,
+        Err(e) => {
+            eprintln!("gatewayd: {e}\n{START_FAILED}");
+            return ExitCode::from(1);
+        }
+    };
+    let source = match config.token_source() {
+        Ok(source) => source,
+        Err(why) => {
+            eprintln!("gatewayd: {}: {why}\n{START_FAILED}", path.display());
+            return ExitCode::from(1);
+        }
+    };
+    let loaded = match secrets::load(MATTERMOST_TOKEN, &source, &|k| std::env::var_os(k)) {
+        Ok(loaded) => loaded,
+        Err(e) => {
+            eprintln!("gatewayd: {e}");
+            return ExitCode::from(1);
+        }
+    };
+    if let Some(warning) = &loaded.warning {
+        eprintln!("{warning}");
+    }
+    let dir = config
+        .state_path()
+        .parent()
+        .map(Path::to_path_buf)
+        .unwrap_or_default();
+    if let Err(e) = std::fs::DirBuilder::new()
+        .recursive(true)
+        .mode(0o700)
+        .create(&dir)
+    {
+        eprintln!(
+            "gatewayd: cannot prepare {}: {e}\n{START_FAILED}",
+            dir.display()
+        );
+        return ExitCode::from(1);
+    }
+    let log: gatewayd::serve::Log = Arc::new(|line: &str| eprintln!("{line}"));
+    let stop = run(
+        config,
+        loaded.secret,
+        Tuning::default(),
+        log,
+        &AtomicBool::new(false),
+    );
+    eprintln!("{stop}");
+    ExitCode::from(1)
 }