dnfdaemon: API to cancel current running transaction

New API `Goal.cancel()` cancels the transaction that was initiated by
`do_transaction()`. The transaction can only be canceled during the
package download phase. Once the RPM transaction has begun, cancellation
is no longer permitted.

dnf5daemon-server/callbacks.cpp

@@ -66,6 +66,9 @@ void * DownloadCB::add_new_download(void * user_data, const char * description,
 }
 
 int DownloadCB::progress(void * user_cb_data, double total_to_download, double downloaded) {
+    if (session.get_cancel_download() == Session::CancelDownload::REQUESTED) {
+        return ReturnCode::ERROR;
+    }
     try {
         if (is_time_to_print()) {
             auto signal = create_signal_download(dnfdaemon::SIGNAL_DOWNLOAD_PROGRESS, user_cb_data);

dnf5daemon-server/dbus/interfaces/org.rpm.dnf.v0.Goal.xml

@@ -94,6 +94,18 @@ along with libdnf.  If not, see <https://www.gnu.org/licenses/>.
         <arg name="options" type="a{sv}" direction="in" />
     </method>
 
+    <!--
+        cancel:
+        @success: true if the cancellation was sucessfully requested
+        @error_msg: error message if the cancellation was refused
+
+        Cancel the transaction that was initiated by `do_transaction()`. The transaction can only be canceled during the package download phase. Once the RPM transaction has begun, cancellation is no longer permitted.
+    -->
+    <method name="cancel">
+        <arg name="success" type="b" />
+        <arg name="error_msg" type="s" />
+    </method>
+
 </interface>
 
 </node>

dnf5daemon-server/services/goal/goal.cpp

@@ -104,6 +104,16 @@ void Goal::dbus_register() {
         [this](sdbus::MethodCall call) -> void {
             session.get_threads_manager().handle_method(*this, &Goal::do_transaction, call, session.session_locale);
         });
+    dbus_object->registerMethod(
+        dnfdaemon::INTERFACE_GOAL,
+        "cancel",
+        "",
+        {},
+        "bs",
+        {"success", "error_msg"},
+        [this](sdbus::MethodCall call) -> void {
+            session.get_threads_manager().handle_method(*this, &Goal::cancel, call, session.session_locale);
+        });
 }
 
 sdbus::MethodReply Goal::resolve(sdbus::MethodCall & call) {
@@ -269,6 +279,7 @@ sdbus::MethodReply Goal::do_transaction(sdbus::MethodCall & call) {
     if (!session.check_authorization(dnfdaemon::POLKIT_EXECUTE_RPM_TRANSACTION, call.getSender())) {
         throw std::runtime_error("Not authorized");
     }
+    session.set_cancel_download(Session::CancelDownload::NOT_REQUESTED);
 
     // read options from dbus call
     dnfdaemon::KeyValueMap options;
@@ -281,6 +292,7 @@ sdbus::MethodReply Goal::do_transaction(sdbus::MethodCall & call) {
         // TODO(mblaha): signalize reboot?
     } else {
         session.download_transaction_packages();
+        session.set_cancel_download(Session::CancelDownload::NOT_ALLOWED);
 
         std::string comment;
         if (options.find("comment") != options.end()) {
@@ -306,3 +318,31 @@ sdbus::MethodReply Goal::do_transaction(sdbus::MethodCall & call) {
     auto reply = call.createReply();
     return reply;
 }
+
+sdbus::MethodReply Goal::cancel(sdbus::MethodCall & call) {
+    bool success{true};
+    std::string error_msg;
+
+    switch (session.get_cancel_download()) {
+        case Session::CancelDownload::NOT_REQUESTED:
+            session.set_cancel_download(Session::CancelDownload::REQUESTED);
+            break;
+        case Session::CancelDownload::REQUESTED:
+            success = false;
+            error_msg = "Cancellation was already requested.";
+            break;
+        case Session::CancelDownload::NOT_ALLOWED:
+            success = false;
+            error_msg = "Cancellation is not allowed after the RPM transaction has started.";
+            break;
+        case Session::CancelDownload::NOT_RUNNING:
+            success = false;
+            error_msg = "No transaction is running.";
+            break;
+    }
+
+    auto reply = call.createReply();
+    reply << success;
+    reply << error_msg;
+    return reply;
+}

dnf5daemon-server/services/goal/goal.hpp

@@ -36,6 +36,7 @@ class Goal : public IDbusSessionService {
     sdbus::MethodReply get_transaction_problems_string(sdbus::MethodCall & call);
     sdbus::MethodReply get_transaction_problems(sdbus::MethodCall & call);
     sdbus::MethodReply do_transaction(sdbus::MethodCall & call);
+    sdbus::MethodReply cancel(sdbus::MethodCall & call);
 };
 
 #endif

dnf5daemon-server/session.cpp

@@ -313,6 +313,7 @@ void Session::store_transaction_offline() {
     std::filesystem::create_directories(dest_dir);
     base->get_config().get_destdir_option().set(dest_dir);
     download_transaction_packages();
+    set_cancel_download(Session::CancelDownload::NOT_ALLOWED);
 
     const auto & offline_datadir = installroot / libdnf5::offline::DEFAULT_DATADIR.relative_path();
     std::filesystem::create_directories(offline_datadir);

dnf5daemon-server/session.hpp

@@ -52,6 +52,7 @@ class IDbusSessionService {
 
 class Session {
 public:
+    enum class CancelDownload { NOT_RUNNING, NOT_REQUESTED, REQUESTED, NOT_ALLOWED };
     Session(
         std::vector<std::unique_ptr<libdnf5::Logger>> && loggers,
         sdbus::IConnection & connection,
@@ -93,6 +94,11 @@ class Session {
     /// prepare the current transaction to run during the next reboot
     void store_transaction_offline();
 
+    /// Getter for download cancel request flag.
+    CancelDownload get_cancel_download() { return cancel_download.load(); }
+    /// Setter for download cancel request flag.
+    void set_cancel_download(CancelDownload value) { cancel_download.store(value); }
+
 private:
     sdbus::IConnection & connection;
     std::unique_ptr<libdnf5::Base> base;
@@ -110,6 +116,7 @@ class Session {
     std::mutex key_import_mutex;
     std::condition_variable key_import_condition;
     std::map<std::string, KeyConfirmationStatus> key_import_status{};  // map key_id: confirmation status
+    std::atomic<CancelDownload> cancel_download{CancelDownload::NOT_RUNNING};
 };
 
 #endif