config: lazyily produce ssh domain list

I noticed that in debug builds, `wezterm set-working-directory` had
high/variable latency, and I traced part of it to the ssh config parsing
logic.

Make that lazily evaluate the ssh config.

refs: #3402

config/src/config.rs

@@ -325,8 +325,8 @@ pub struct Config {
     #[dynamic(default = "UnixDomain::default_unix_domains")]
     pub unix_domains: Vec<UnixDomain>,
 
-    #[dynamic(default = "SshDomain::default_domains")]
-    pub ssh_domains: Vec<SshDomain>,
+    #[dynamic(default)]
+    pub ssh_domains: Option<Vec<SshDomain>>,
 
     #[dynamic(default)]
     pub ssh_backend: SshBackend,
@@ -828,6 +828,17 @@ impl Config {
         Self::load_with_overrides(&wezterm_dynamic::Value::default())
     }
 
+    /// It is relatively expensive to parse all the ssh config files,
+    /// so we defer producing the default list until someone explicitly
+    /// asks for it
+    pub fn ssh_domains(&self) -> Vec<SshDomain> {
+        if let Some(domains) = &self.ssh_domains {
+            domains.clone()
+        } else {
+            SshDomain::default_domains()
+        }
+    }
+
     pub fn update_ulimit(&self) -> anyhow::Result<()> {
         #[cfg(unix)]
         {
@@ -1127,8 +1138,10 @@ impl Config {
         for d in &self.unix_domains {
             check_domain(&d.name, "unix domain")?;
         }
-        for d in &self.ssh_domains {
-            check_domain(&d.name, "ssh domain")?;
+        if let Some(domains) = &self.ssh_domains {
+            for d in domains {
+                check_domain(&d.name, "ssh domain")?;
+            }
         }
         for d in &self.exec_domains {
             check_domain(&d.name, "exec domain")?;

wezterm-gui/src/main.rs

@@ -250,7 +250,7 @@ fn client_domains(config: &config::ConfigHandle) -> Vec<ClientDomainConfig> {
         domains.push(ClientDomainConfig::Unix(unix_dom.clone()));
     }
 
-    for ssh_dom in &config.ssh_domains {
+    for ssh_dom in config.ssh_domains().into_iter() {
         if ssh_dom.multiplexing == SshMultiplexing::WezTerm {
             domains.push(ClientDomainConfig::Ssh(ssh_dom.clone()));
         }
@@ -341,7 +341,7 @@ fn update_mux_domains(config: &ConfigHandle) -> anyhow::Result<()> {
         mux.add_domain(&domain);
     }
 
-    for ssh_dom in &config.ssh_domains {
+    for ssh_dom in config.ssh_domains().into_iter() {
         if ssh_dom.multiplexing != SshMultiplexing::None {
             continue;
         }