Check session valididty on kak -l and support kak -clear for clearing the dead ones

Fixes #689

README.asciidoc

@@ -224,7 +224,13 @@ Just running *kak* launch a new kak session with a client on local terminal.
        acting as a remote control.
  * `-f <keys>`: Work as a filter, read every file given on the command line
        and stdin if piped in, and apply given keys on each.
- * `-l`: list existing sessions
+ * `-ui <userinterface>`: use given user interface, `<userinterface>` can be
+    - `ncurses`: default terminal user interface
+    - `dummy`: empty user interface not displaying anything
+    - `json`: json-rpc based user interface that writes json on stdout and
+        read keystrokes as json on stdin.
+ * `-l`: list existing sessions, and check the dead ones
+ * `-clear`: clear dead sessions socket files
 
 At startup, if `-n` is not specified, Kakoune will try to source the file
 `../share/kak/kakrc` relative to the kak binary. This kak file will then try

src/main.cc

@@ -731,7 +731,8 @@ int main(int argc, char* argv[])
                    { "f", { true,  "act as a filter, executing given keys on given files" } },
                    { "q", { false, "in filter mode, be quiet about errors applying keys" } },
                    { "ui", { true, "set the type of user interface to use (ncurses, dummy, or json)" } },
-                   { "l", { false, "list existing sessions" } } }
+                   { "l", { false, "list existing sessions" } },
+                   { "clear", { false, "clear dead sessions" } } }
     };
     try
     {
@@ -741,12 +742,26 @@ int main(int argc, char* argv[])
 
         ParametersParser parser(params, param_desc);
 
-        if (parser.get_switch("l"))
+        const bool list_sessions = (bool)parser.get_switch("l");
+        const bool clear_sessions = (bool)parser.get_switch("clear");
+        if (list_sessions or clear_sessions)
         {
-            for (auto& file : list_files(format("/tmp/kakoune/{}/", getpwuid(geteuid())->pw_name)))
+            StringView username = getpwuid(geteuid())->pw_name;
-                write_stdout(format("{}\n", file));
+            for (auto& session : list_files(format("/tmp/kakoune/{}/", username)))
+            {
+                const bool valid = check_session(session);
+                if (list_sessions)
+                    write_stdout(format("{}{}\n", session, valid ? "" : " (dead)"));
+                if (not valid and clear_sessions)
+                {
+                    char socket_file[128];
+                    format_to(socket_file, "/tmp/kakoune/{}/{}", username, session);
+                    unlink(socket_file);
+                }
+            }
             return 0;
         }
+
         if (auto session = parser.get_switch("p"))
         {
             for (auto opt : { "c", "n", "s", "d", "e" })

src/remote.cc

@@ -436,6 +436,14 @@ static int connect_to(StringView session)
     return sock;
 }
 
+bool check_session(StringView session)
+{
+    int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+    auto close_sock = on_scope_end([sock]{ close(sock); });
+    sockaddr_un addr = session_addr(session);
+    return connect(sock, (sockaddr*)&addr, sizeof(addr.sun_path)) != -1;
+}
+
 RemoteClient::RemoteClient(StringView session, std::unique_ptr<UserInterface>&& ui,
                            const EnvVarMap& env_vars, StringView init_command)
     : m_ui(std::move(ui))

src/remote.hh

@@ -59,6 +59,8 @@ private:
     Vector<std::unique_ptr<Accepter>> m_accepters;
 };
 
+bool check_session(StringView session);
+
 }
 
 #endif // remote_hh_INCLUDED