feat: support user preview configuration and document UI

rust/main.rs

@@ -7,7 +7,7 @@ use std::{
     fs::{self, OpenOptions},
     io,
     os::unix::fs::{MetadataExt, OpenOptionsExt},
-    path::PathBuf,
+    path::{Path, PathBuf},
 };
 
 use clap::Parser;
@@ -34,8 +34,8 @@ const BUILD_TIME: &str = env!("LATCHD_BUILD_TIME");
 )]
 struct Cli {
     /// Configuration TOML path.
-    #[arg(short, long, default_value = "/etc/latchd/config.toml")]
-    config: PathBuf,
+    #[arg(short, long)]
+    config: Option<PathBuf>,
     /// Optional variable TOML path.
     #[arg(short = 'v', long)]
     variables: Option<PathBuf>,
@@ -79,17 +79,7 @@ fn run() -> Result<(), Error> {
         println!("version={BUILD_VERSION}\ncommit={BUILD_COMMIT}\nepoch={BUILD_TIME}");
         return Ok(());
     }
-    let config_path = if cli.config.exists() {
-        Some(cli.config.as_path())
-    } else {
-        if cli.config.as_os_str() != "/etc/latchd/config.toml" {
-            return Err(Error::Config(format!(
-                "configuration does not exist: {}",
-                cli.config.display()
-            )));
-        }
-        None
-    };
+    let config_path = resolve_config_path(cli.config.as_deref(), cli.preview)?;
     if let Some(path) = cli.variables.as_deref() {
         if !path.exists() {
             return Err(Error::Config(format!(
@@ -100,7 +90,7 @@ fn run() -> Result<(), Error> {
     }
     let variables_path = cli.variables.as_deref();
     if !cli.preview {
-        if let Some(path) = config_path {
+        if let Some(path) = config_path.as_deref() {
             config::validate_source(path, "configuration")?;
         }
         if let Some(path) = variables_path {
@@ -109,7 +99,7 @@ fn run() -> Result<(), Error> {
     }
     let mut config = Config::default();
     if config_path.is_some() || variables_path.is_some() {
-        config = config::load(config_path, variables_path)?;
+        config = config::load(config_path.as_deref(), variables_path)?;
     }
     if let Some(tty) = cli.tty {
         if tty == 0 {
@@ -148,7 +138,7 @@ fn run() -> Result<(), Error> {
                 "already inside a session; use --preview".into(),
             ));
         }
-        if unsafe { libc::geteuid() } != 0 {
+        if !running_as_root() {
             return Err(Error::Config("latchd must run as root".into()));
         }
         validate_terminal()?;
@@ -212,6 +202,33 @@ fn run() -> Result<(), Error> {
     Ok(())
 }
 
+fn resolve_config_path(explicit: Option<&Path>, preview: bool) -> Result<Option<PathBuf>, Error> {
+    if let Some(path) = explicit {
+        if path.exists() {
+            return Ok(Some(path.to_owned()));
+        }
+        return Err(Error::Config(format!(
+            "configuration does not exist: {}",
+            path.display()
+        )));
+    }
+
+    let system_path = Path::new(config::SYSTEM_CONFIG_PATH);
+    if !preview {
+        return Ok(system_path.exists().then(|| system_path.to_owned()));
+    }
+
+    if let Some(path) = config::user_config_path().filter(|path| path.exists()) {
+        return Ok(Some(path));
+    }
+    Ok(system_path.exists().then(|| system_path.to_owned()))
+}
+
+fn running_as_root() -> bool {
+    // SAFETY: `geteuid` has no pointer arguments or memory preconditions.
+    unsafe { libc::geteuid() == 0 }
+}
+
 fn setup_logging(path: &std::path::Path) -> Result<(), Error> {
     const MAX_LOG_BYTES: u64 = 64 * 1024 * 1024;
     let parent = path