ssh: allow chdir to fail when spawning via ssh. use login shell.

Similar to 3b9be25161, but relax for
the ssh session when assume_shell="Posix".

Augument how we run the shell in this case as well, so that we make
an effort to run it as a login shell.

refs: #2092
refs: #2076

mux/src/ssh.rs

@@ -226,27 +226,45 @@ impl RemoteSshDomain {
             cmd: &CommandBuilder,
             env: &HashMap<String, String>,
         ) -> anyhow::Result<String> {
-            let mut env_cmd = vec!["env".to_string()];
+            // "Soft" chdir: if it doesn't exist then it doesn't matter
-            if let Some(dir) = dir {
+            let cd_cmd = if let Some(dir) = dir {
-                env_cmd.push("-C".to_string());
+                format!("cd {};", shell_words::quote(&dir))
-                env_cmd.push(dir.clone());
             } else if let Some(dir) = cmd.get_cwd() {
                 let dir = dir.to_str().context("converting cwd to string")?;
-                env_cmd.push("-C".to_string());
+                format!("cd {};", shell_words::quote(&dir))
-                env_cmd.push(dir.to_string());
+            } else {
-            }
+                String::new()
+            };
+
+            let mut env_cmd = vec!["env".to_string()];
 
             for (k, v) in env {
                 env_cmd.push(format!("{}={}", k, v));
             }
 
             let cmd = if cmd.is_default_prog() {
-                "$SHELL".to_string()
+                // We'd like to spawn a login shell, but since we are invoking env
+                // we end up in a regular shell.
+                // This guff tries to find a reasonably portable way to execute
+                // the shell as a login shell.
+                // Per: <https://unix.stackexchange.com/a/666850/123914>
+                // the most portable way is to use perl, but in case perl is not
+                // installed, zsh, bash and ksh all support `exec -a`.
+                // Other shells may support `exec -a` but there isn't a simple
+                // way to test for them, so we assume that if we have one of those
+                // three that we can use it, otherwise we fall back to just running
+                // the shell directly.
+                let login_shell = "command -v perl > /dev/null && \
+                  exec perl -e 'use File::Basename; $shell = basename($ENV{SHELL}); exec {$ENV{SHELL}} \"-$shell\"'; \
+                  case \"$SHELL\" in */zsh|*/bash|*/ksh ) exec -a \"-$(basename $SHELL)\" $SHELL ;; esac ; \
+                  exec $SHELL";
+
+                format!("$SHELL -c {}", shell_words::quote(login_shell))
             } else {
                 cmd.as_unix_command_line()?
             };
 
-            Ok(shell_words::join(env_cmd) + " " + &cmd)
+            Ok(cd_cmd + &shell_words::join(env_cmd) + " " + &cmd)
         }
 
         let command_line = match (cmd.is_default_prog(), self.dom.assume_shell, command_dir) {