As a JavaScript engine developer at Igalia I don’t find myself writing much plain C code anymore. I’m either writing JS or TypeScript, or hacking on large compiler codebases in C++1, or writing ECMAScript specification language. Frankly, that is fine with me. C’s time may not be over yet, but I wouldn’t be sad if I never had to write another line of it. (Hopefully this post conveys why.)
However, while working on modernizing an app written in C for the GNOME platform, that I hack on in my spare time, I wanted to copy a folder recursively using the GIO async APIs. Like cp -R at the shell, but without freezing up your GUI while it works.
C’s callback style for async programming, combined with lack of capturing variables in closures, is like going back to the dark ages if you’ve gotten used to languages with async/await style or even C++’s lambdas. I would’ve avoided writing this if I could, but apparently no one else had done it publicly on the internet that I could find.2 So here it is for your enjoyment.
typedef struct {
GFile *dest_folder;
GQueue *files_to_copy;
GQueue *folders_to_copy;
GFileCopyFlags flags;
} CopyRecursiveClosure;
/* Pre-declare so we can read them in the order they are executed: */
static void on_recursive_make_dir_finish(GFile *file, GAsyncResult *res, GTask *data);
static void on_recursive_file_enumerate_finish(GFile* file, GAsyncResult *res, GTask *data);
static void on_recursive_file_next_files_finish(GFileEnumerator *children, GAsyncResult *res, GTask *data);
static void copy_file_queue_async(GTask *task);
static void on_recursive_file_copy_finish(GFile *file, GAsyncResult *result, GTask *data);
static void on_recursive_folder_copy_finish(GFile *file, GAsyncResult *result, GTask *data);
static void copy_folder_queue_async(GTask *task);
static void copy_recursive_closure_free(CopyRecursiveClosure *ptr);
/**
* copy_recursive_async:
* @src: The source folder
* @dest: Destination folder in which to place the copy of @src
* @flags: #GFileCopyFlags to apply to copy operations
* @prio: I/O priority, e.g. #G_PRIORITY_DEFAULT
* @cancel: #GCancellable that will interrupt the operation when triggered
* @done_cb: Function to call when the operation is finished
* @data: Pointer to pass to @done_cb
*
* Copy the folder @src and all of the files and subfolders in it into the
* folder @dest, asynchronously.
*
* The only @flags supported are #G_FILE_COPY_NONE and #G_FILE_COPY_OVERWRITE.
*/
void
copy_recursive_async(GFile *src, GFile *dest, GFileCopyFlags flags, int prio, GCancellable *cancel,
GAsyncReadyCallback done_cb, void *data)
{
g_return_if_fail(G_IS_FILE(src));
g_return_if_fail(G_IS_FILE(dest));
g_return_if_fail(flags == G_FILE_COPY_NONE || flags == G_FILE_COPY_OVERWRITE);
g_return_if_fail(!cancel || G_IS_CANCELLABLE(cancel));
g_autoptr(GTask) task = g_task_new(src, cancel, done_cb, data);
g_task_set_priority(task, prio);
CopyRecursiveClosure *task_data = g_new0(CopyRecursiveClosure, 1);
g_autofree char *basename = g_file_get_basename(src);
task_data->dest_folder = g_file_get_child(dest, basename);
task_data->files_to_copy = g_queue_new();
task_data->folders_to_copy = g_queue_new();
task_data->flags = flags;
g_task_set_task_data(task, task_data, (GDestroyNotify)copy_recursive_closure_free);
g_file_make_directory_async(task_data->dest_folder, prio, cancel,
(GAsyncReadyCallback)on_recursive_make_dir_finish, g_steal_pointer(&task));
}
/**
* copy_recursive_finish:
* @src: The source folder
* @result: The #GAsyncResult passed to the callback
* @error_out: (nullable): Return location for a #GError
*
* Complete the asynchronous copy operation started by copy_recursive_async().
*
* Returns: %TRUE if the operation completed successfully, %FALSE on error.
*/
bool
copy_recursive_finish(GFile *src, GAsyncResult *result, GError **error_out)
{
g_return_val_if_fail(G_IS_FILE(src), false);
g_return_val_if_fail(G_IS_TASK(result), false);
g_return_val_if_fail(g_task_is_valid(result, src), false);
return g_task_propagate_boolean(G_TASK(result), error_out);
}
static void
on_recursive_make_dir_finish(GFile *file, GAsyncResult *result, GTask *task_ptr)
{
g_autoptr(GTask) task = g_steal_pointer(&task_ptr);
g_autoptr(GError) error = NULL;
GCancellable *cancel = g_task_get_cancellable(task);
int prio = g_task_get_priority(task);
if (!g_file_make_directory_finish(G_FILE(file), result, &error)) {
/* With the OVERWRITE flag, don't error out when the folder already
* exists. (Hopefully plopping all the files in the existing folder is
* sufficient. If not, another way to do this would be to delete the
* existing folder recursively, so that extra existing files not in the
* source don't remain in the destination.) */
CopyRecursiveClosure *data = g_task_get_task_data(task);
bool overwrite = !!(data->flags & G_FILE_COPY_OVERWRITE);
if (!overwrite || !g_error_matches(error, G_IO_ERROR, G_IO_ERROR_EXISTS)) {
g_autofree char *path = g_file_get_path(file);
g_task_return_prefixed_error(task, g_steal_pointer(&error),
"Error creating destination folder %s: ", path);
return;
}
}
GFile *src = g_task_get_source_object(task);
g_file_enumerate_children_async(src, "standard::*", G_FILE_QUERY_INFO_NONE, prio, cancel,
(GAsyncReadyCallback)on_recursive_file_enumerate_finish, g_steal_pointer(&task));
}
static void
on_recursive_file_enumerate_finish(GFile *file, GAsyncResult *result, GTask *task_ptr)
{
g_autoptr(GTask) task = g_steal_pointer(&task_ptr);
g_autoptr(GError) error = NULL;
GCancellable *cancel = g_task_get_cancellable(task);
int prio = g_task_get_priority(task);
g_autoptr(GFileEnumerator) children = g_file_enumerate_children_finish(G_FILE(file), result, &error);
if (!children) {
g_autofree char *path = g_file_get_path(file);
g_task_return_prefixed_error(task, g_steal_pointer(&error),
"Error reading folder %s: ", path);
return;
}
g_file_enumerator_next_files_async(children, 10, prio, cancel,
(GAsyncReadyCallback)on_recursive_file_next_files_finish, g_steal_pointer(&task));
}
static void
on_recursive_file_next_files_finish(GFileEnumerator *children, GAsyncResult *result, GTask *task_ptr)
{
g_autoptr(GTask) task = g_steal_pointer(&task_ptr);
g_autoptr(GError) error = NULL;
GCancellable *cancel = g_task_get_cancellable(task);
int prio = g_task_get_priority(task);
g_autolist(GFileInfo) next_files = g_file_enumerator_next_files_finish(children, result, &error);
if (error) {
g_autofree char *path = g_file_get_path(g_file_enumerator_get_container(children));
g_task_return_prefixed_error(task, g_steal_pointer(&error),
"Error reading files from folder %s: ", path);
return;
}
CopyRecursiveClosure *data = g_task_get_task_data(task);
if (next_files) {
for (GList *iter = next_files; iter != NULL; iter = g_list_next(iter)) {
GFileInfo *info = G_FILE_INFO(iter->data);
GFileType type = g_file_info_get_file_type(info);
g_autoptr(GFile) file = g_file_enumerator_get_child(children, info);
switch (type) {
case G_FILE_TYPE_DIRECTORY:
g_queue_push_tail(data->folders_to_copy, g_steal_pointer(&file));
break;
case G_FILE_TYPE_REGULAR:
g_queue_push_tail(data->files_to_copy, g_steal_pointer(&file));
break;
default:
g_warning("Unhandled file type %d in recursive copy: %s", type, g_file_info_get_name(info));
continue;
}
}
g_file_enumerator_next_files_async(children, 10, prio, cancel,
(GAsyncReadyCallback)on_recursive_file_next_files_finish, g_steal_pointer(&task));
return;
}
copy_file_queue_async(g_steal_pointer(&task));
}
static void
copy_file_queue_async(GTask *task_ptr)
{
g_autoptr(GTask) task = task_ptr;
CopyRecursiveClosure *data = g_task_get_task_data(task);
g_autoptr(GFile) file = g_queue_pop_head(data->files_to_copy);
if (file) {
GCancellable *cancel = g_task_get_cancellable(task);
int prio = g_task_get_priority(task);
g_autofree char *basename = g_file_get_basename(file);
g_autoptr(GFile) dest = g_file_get_child(data->dest_folder, basename);
g_file_copy_async(file, dest, data->flags, prio, cancel,
/* progress_callback = */ NULL, NULL,
(GAsyncReadyCallback)on_recursive_file_copy_finish, g_steal_pointer(&task));
return;
}
copy_folder_queue_async(g_steal_pointer(&task));
}
static void
on_recursive_file_copy_finish(GFile *file, GAsyncResult *result, GTask *task_ptr)
{
g_autoptr(GTask) task = task_ptr;
g_autoptr(GError) error = NULL;
if (!g_file_copy_finish(file, result, &error)) {
g_autofree char *path = g_file_get_path(file);
g_task_return_prefixed_error(task, g_steal_pointer(&error),
"Error copying file %s: ", path);
return;
}
copy_file_queue_async(g_steal_pointer(&task));
}
static void
copy_folder_queue_async(GTask *task_ptr)
{
g_autoptr(GTask) task = task_ptr;
CopyRecursiveClosure *data = g_task_get_task_data(task);
g_autoptr(GFile) folder = g_queue_pop_head(data->folders_to_copy);
if (folder) {
GCancellable *cancel = g_task_get_cancellable(task);
int prio = g_task_get_priority(task);
copy_recursive_async(folder, data->dest_folder, data->flags, prio, cancel,
(GAsyncReadyCallback)on_recursive_folder_copy_finish, g_steal_pointer(&task));
return;
}
g_task_return_boolean(task, true);
}
static void
on_recursive_folder_copy_finish(GFile *folder, GAsyncResult *result, GTask *task_ptr)
{
g_autoptr(GTask) task = task_ptr;
g_autoptr(GError) error = NULL;
if (!copy_recursive_finish(folder, result, &error)) {
g_autofree char *path = g_file_get_path(folder);
g_task_return_prefixed_error(task, g_steal_pointer(&error),
"Error copying folder %s: ", path);
return;
}
copy_folder_queue_async(g_steal_pointer(&task));
}
static void
copy_recursive_closure_free(CopyRecursiveClosure *ptr) {
g_object_unref(ptr->dest_folder);
g_queue_free_full(ptr->files_to_copy, g_object_unref);
g_queue_free_full(ptr->folders_to_copy, g_object_unref);
g_free(ptr);
}
You are welcome to take this code and customize it to your needs. I’m putting it into the public domain so hopefully nobody else has to go through this.
Although if you really want to, it could be improved by implementing progress callbacks like g_file_copy_async() has.
Just so you can understand what’s going on at a glance, here’s what it would look like in about 30 lines of JavaScript, with async/await style:
async function copyRecursive(src, dest, flags, prio, cancel) {
const destFolder = dest.get_child(src.get_basename());
const overwrite = !!(flags & Gio.FileCopyFlags.OVERWRITE);
try {
await destFolder.make_directory_async(prio, cancel);
} catch (error) {
if (!overwrite || !error.matches(Gio.IOErrorEnum, Gio.IOErrorEnum.EXISTS))
throw error;
}
const children = await src.enumerate_children_async('standard::*',
Gio.FileQueryInfoFlags.NONE, prio, cancel);
let nextFiles;
const filesToCopy = [];
const foldersToCopy = [];
while ((nextFiles = await children.next_files_async(10, prio, cancel)).length) {
const {
[Gio.FileType.REGULAR]: files,
[Gio.FileType.DIRECTORY]: folders,
} = Object.groupBy(nextFiles, info => info.get_file_type());
foldersToCopy.push(...folders?.map(info => children.get_child(info)) ?? []);
filesToCopy.push(...files?.map(info => children.get_child(info)) ?? []);
}
for (const file of filesToCopy) {
const dest = destFolder.get_child(file.get_basename());
await file.copy_async(dest, flags, prio, cancel, null, null);
}
for (const folder of foldersToCopy)
await copyRecursive(folder, destFolder, flags, prio, cancel);
}
(This excludes the imports and calls to Gio._promisify that you would have to do; hopefully we’ll get native async operations in GNOME 48!)
[1] C++ before C++11 used to be a worse experience than C. However, I don’t have to deal with that because the three major JS engines use C++17. It’s … its own category of special, but better. ↩️
[2] No, ChatGPT couldn’t do it either; it made up GIO APIs that don’t exist. If that programming technique is on the table, then sure, it’d have been a lot easier. ↩️
At the root of the GNOME desktop is an object-oriented technology called GObject. GObjects are reference counted, but not garbage collected. As long as their reference count is nonzero, they are “alive”, and when their reference count drops to zero, they are deleted from memory.
The Mad Scientist Review 