Draft: Resolve "Unix Socket Server schreiben"

Closes #3 (closed)

src/net/unix_socket_server.cpp

 #include "net/unix_socket_server.hpp"
 
+#include <filesystem>
+#include <sstream>
+
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
@@ -12,6 +15,14 @@
 
 namespace rmrf::net {
 
+std::string get_usocket_path(const socketaddr& addr) {
+    if (addr.family() != AF_UNIX) {
+        return std::string();
+    }
+
+    return std::string(((sockaddr_un*) addr.ptr())->sun_path);
+}
+
 auto_fd construct_server_fd(const socketaddr& addr) {
     if (addr.family() != AF_UNIX) {
         throw netio_exception("Expected a UNIX socket file path.");
@@ -25,12 +36,28 @@ auto_fd construct_server_fd(const socketaddr& addr) {
     }
 
     if (auto error = bind(socket_fd.get(), addr.ptr(), addr.size()); error != 0) {
-        throw netio_exception("Failed to bind to socket " + addr.str());
+        std::stringstream ss;
+        ss << "Failed to bind to socket " + addr.str() << ".";
+        const auto p = std::filesystem::path(get_usocket_path(addr));
+
+        if (std::filesystem::exists(p)) {
+            ss << " Reason: The file already exists.";
+
+            if (std::filesystem::is_socket(p)) {
+                ss << " It is also a socket. Is the application already running?";
+            }
+        } else {
+            ss << " Is there buffer space avaiable and do you have the permission to create a socket there?";
+        }
+
+        throw netio_exception(ss.str());
     }
 
     make_socket_nonblocking(socket_fd);
 
     if (listen(socket_fd.get(), 5) == -1) {
+        // We already created the socket and close won't remove the file thus we need to unlink it.
+        unlink(get_usocket_path(addr).c_str());
         throw netio_exception("Failed to enable listening mode for raw socket");
     }
 
@@ -40,11 +67,16 @@ auto_fd construct_server_fd(const socketaddr& addr) {
 unix_socket_server::unix_socket_server(
     const socketaddr& socket_identifier,
     async_server_socket::accept_handler_type client_listener_
-) : async_server_socket{construct_server_fd(socket_identifier)} {
+) : async_server_socket{construct_server_fd(socket_identifier)}, socket_path{get_usocket_path(socket_identifier)} {
     this->set_accept_handler(client_listener_);
 }
 
-unix_socket_server::~unix_socket_server() {}
+unix_socket_server::~unix_socket_server() {
+    if (this->socket_path.length() > 0) {
+        // We're ignoring potential errors as there's nothing we can do about it anyway.
+        unlink(this->socket_path.c_str());
+    }
+}
 
 std::shared_ptr<connection_client> unix_socket_server::await_raw_socket_incomming(const auto_fd& server_socket) {
     auto client_socket = auto_fd{accept(server_socket.get(), nullptr, nullptr)};