In 2010 I was working with evince, the gnome PDF document viewer, trying to add some accessibility to PDF files. That was really hard, not because GTK+ or ATK technology but because the PDF format itself. The PDF format is really cool for printing because you know that the piece of paper will look the same as the PDF doc, and because it's vector it scales and don't loose quality and files are smaller than image files, but almost all PDF files have not any metadata for sections, headings, tablets or so, this depends on the creation tool, but it's really hard to deal with PDF content text, because you don't know event if the text that you're reading is really in the same order that you read from the PDF.

After my fight against the PDF format hell and poppler, I discovered the epub format that's a really simple format for electronic books. An epub is a zip with some XML files describing the book index and every chapter is a xhtml and xhtml is a good format compared to PDF because you can parse easily with any XML lib and the content is tagged and well structured so you know what's a heading, what's a paragraph, etc.

So I started to write a simple C library to read epub files, thinking about add epub support to evince. That's how libgepub was born. I tried to integrate libgepub in evince, I've something working, rendering with webkit, but nothing really useful, because evince needs pages and it's not easy to split an xhtml file in pages with the same height, because xhtml is continuous text and it adapts to the page width, so I give up and leave this branch.

gnome-books

After some time, I discovered gnome-books, and the idea of epub support comes to me again. Books has an initial epub support, that consists only in showing in the collection, but when you click on the book, an error message was shown because there's no epub preview support.

I've libgepub with GIR support and books was written with gjs so it was easy for me to add an initial support for epub documents using libgepub and rendering with webkit.

So libgepub is used in a gnome app to render epub documents and for that reason gnome depends on libgepub now.

Rustifying everything

In 2017 I discover rust and fell in love with this language, so I wanted to write some code with rust and I started porting this epub library and I ended creating the epub crate, that's basically the same implementation but with rust instead of C, the API is really similar.

After that, I read somewhere that Federico was porting the librsvg code to rust, so I thought that I can do the same with libgepub and replace almost all the C code with this new rust crate.

I copied all the code for autotools to build using cargo and I've copied to the glue code from librsvg and adapt to my own lib. That worked really well and I was able to remove the core libgepub C code and replace with the new rust code.

But rust is really new and it was not full supported in all distributions, so release depending on rust will make the release people's live harder. That's the reason because I decided to leave the C version for now and wait a little to make the rust migration.

And here we're, the libgepub has changed a little, some fixes and small changes, but the main change is that now we're using meson to build instead of autotools, and it's not easy to integrate a rust+cargo lib in the meson desc. There's a PR in meson to support this but it seems that meson devs doesn't like the idea.

So yesterday I was playing around with meson to get back the rust code working in libgepub, and was not easy, but now I've a working build configuration that works. It's a hack with a custom script to build with cargo, but it's working so I'm really happy.

Show me the code

To integrate the rust epub crate in the libgepub C lib I needed to write a simple cargo project with the glue code that converts the rust types to glib/c types and expose that in a C lib, and that's the libgepub_internals, that's full of glue rust+glib code like:

#[no_mangle]
pub extern "C" fn epub_new(path: *const libc::c_char) -> *mut EpubDoc {
    let my_path = unsafe { &String::from_glib_none(path) };
    let doc = EpubDoc::new(my_path);
    let doc = doc.unwrap();
    Box::into_raw(Box::new(doc))
}

#[no_mangle]
pub unsafe extern "C" fn epub_destroy(raw_doc: *mut EpubDoc) {
    assert!(!raw_doc.is_null());
    let _ = Box::from_raw(raw_doc);
}

Then, in the gepub-doc.c, that's the glib object that expose the GIR API, I added calls to this functions, defining each function heading that'll be used from the rust static lib at link time:

// Rust
void      *epub_new(char *path);
void       epub_destroy(void *doc);
void      *epub_get_resource(void *doc, const char *path, int *size);
void      *epub_get_resource_by_id(void *doc, const char *id, int *size);
void      *epub_get_metadata(void *doc, const char *mdata);
void      *epub_get_resource_mime(void *doc, const char *path);
void      *epub_get_resource_mime_by_id(void *doc, const char *id);
void      *epub_get_current_mime(void *doc);
void      *epub_get_current(void *doc, int *size);
void      *epub_get_current_with_epub_uris(void *doc, int *size);
void       epub_set_page(void *doc, guint page);
guint      epub_get_num_pages(void *doc);
guint      epub_get_page(void *doc);
gboolean   epub_next_page(void *doc);
gboolean   epub_prev_page(void *doc);
void      *epub_get_cover(void *doc);
void      *epub_resource_path(void *doc, const char *id);
void      *epub_current_path(void *doc);
void      *epub_current_id(void *doc);
void      *epub_get_resources(void *doc);
guint      epub_resources_get_length(void *er);

gchar     *epub_resources_get_id(void *er, gint i);
gchar     *epub_resources_get_mime(void *er, gint i);
gchar     *epub_resources_get_path(void *er, gint i);

And then calling to this functions:

static gboolean
gepub_doc_initable_init (GInitable     *initable,
                         GCancellable  *cancellable,
                         GError       **error)
{
    GepubDoc *doc = GEPUB_DOC (initable);

    g_assert (doc->path != NULL);
    doc->rust_epub_doc = epub_new (doc->path);
    if (!doc->rust_epub_doc) {
        if (error != NULL) {
            g_set_error (error, gepub_error_quark (), GEPUB_ERROR_INVALID,
                         "Invalid epub file: %s", doc->path);
        }
        return FALSE;
    }

    return TRUE;
}

To make this work, I needed to add the libgepub_internals dependency to the libgepub meson.build like this:

gepub_deps = [
  dependency('gepub_internals', fallback: ['libgepub_internals', 'libgepub_internals_dep']),
  dependency('webkit2gtk-4.0'),
  dependency('libsoup-2.4'),
  dependency('glib-2.0'),
  dependency('gobject-2.0'),
  dependency('gio-2.0'),
  dependency('libxml-2.0'),
  dependency('libarchive')
]

This definition looks for the gepub_internals lib in the libgepub_internals subproject with this meson.build:

project(
  'libgepub_internals', 'rust',
  version: '3.29.6',
  license: 'GPLv3',
)

libgepub_internals_version = meson.project_version()
version_array = libgepub_internals_version.split('.')
libgepub_internals_major_version = version_array[0].to_int()
libgepub_internals_minor_version = version_array[1].to_int()
libgepub_internals_version_micro = version_array[2].to_int()

libgepub_internals_prefix = get_option('prefix')

cargo = find_program('cargo', required: true)
cargo_vendor = find_program('cargo-vendor', required: false)
cargo_script = find_program('scripts/cargo.sh')
grabber = find_program('scripts/grabber.sh')
cargo_release = find_program('scripts/release.sh')

c = run_command(grabber)
sources = c.stdout().strip().split('\n')

cargo_build = custom_target('cargo-build',
                        build_by_default: true,
                        input: sources,
                        output: ['libgepub_internals'],
                        install: false,
                        command: [cargo_script, '@CURRENT_SOURCE_DIR@', '@OUTPUT@'])

libgepub_internals_lib = static_library('gepub_internals', cargo_build)

cc = meson.get_compiler('c')
th = dependency('threads')
libdl = cc.find_library('dl')

libgepub_internals_dep = declare_dependency(
  link_with: libgepub_internals_lib,
  dependencies: [th, libdl],
  sources: cargo_build,
)

I'm using here a custom_target to build the lib using a custom script that simply calls to cargo and then copies the result lib to the correct place:

if [[ $DEBUG = true ]]
then
    echo "DEBUG MODE"
    cargo build --manifest-path $1/Cargo.toml && cp $1/target/debug/libgepub_internals.a $2.a
else
    echo "RELEASE MODE"
    cargo build --manifest-path $1/Cargo.toml --release && cp $1/target/release/libgepub_internals.a $2.a
fi

Then I declared the static_library and the dependency with declare_dependency. I need to add threads and dl because the epub crate depends on it and this works!

I'll need to vendor all dep crates with cargo-vendor for releasing, but I think that this is working and it's the way to go with libgepub.

The future of libgepub + rust

Currently, with the libgepub_internals lib, the gepub-doc.c code is basically to provide a gobject and GIR information to be able to work with epub docs, but the real work is done in Rust. The gnome-class provides a simple way to build this gobject with rust code, but currently it's not completed and there's no way to generate GIR, but in the future, it could be cool to remove the gepub-doc.c code and generate all with gnome-class. I can wait until Federico writes the piece of code that I need for this or maybe I should contribute to gnome-class to be able to do this.

Libgepub also provides a widget that inherits from WebkitWebView to render the book. That widget is written in C to provide GIR data also and we can try to do the same and use gnome-class to write this widget.

But for now, we're really far from this, we need to spend some time improving gnome-class to be able to write all the code in rust and expose the gobject GIR. Meantime we can start to use rust with this glue code, and that's great, because if you've a gobject library and you want to migrate to rust, you don't need to migrate all the code at once, you can do the same that librsvg is doing and migrate function by function and that's really cool.