jellytau/src-tauri/src/player/video_surface.rs

//! The native video surface: mpv drawn *behind* Tauri's webview, without
//! touching the widget tree.
//!
//! # Why there is no overlay here
//!
//! The obvious arrangement — wrap the webview in a `GtkOverlay` with a
//! `GtkGLArea` beneath — attaches cleanly and then aborts the process on the
//! first click. `tauri-runtime-wry` connects a button-press handler to the
//! webview that walks a hard-coded path:
//!
//! ```text
//! webview.parent()          // "This one should be GtkBox"
//!        .parent()          // ...and this one the GtkWindow
//!        .downcast::<gtk::Window>().unwrap()
//! ```
//!
//! An overlay makes that chain `webview → GtkOverlay → GtkBox`, the downcast
//! fails, and because the panic is non-unwinding it takes the app with it.
//! Nothing in configuration avoids it: on Linux the handler is attached
//! *unconditionally* (the Windows path guards it behind `is_decorated()`), and
//! the decoration check that would make it inert runs *after* the unwrap.
//!
//! So the widget tree is left exactly as Tauri built it. GTK draws a container
//! before its children, so rendering into the vbox's own `draw` handler puts the
//! picture underneath the webview for free — the same z-order, no reparenting,
//! one less widget, and nothing a Tauri upgrade can invalidate by assuming its
//! own layout.
//!
//! TRACES: UR-080 | DR-231, DR-232, DR-233, IR-033

use std::cell::RefCell;
use std::ffi::c_void;
use std::rc::Rc;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;

use gtk::prelude::*;
use gtk::{gdk, glib};
use log::{error, info, warn};

use super::mpv_render::MpvRenderContext;

/// GL enum for `gdk_cairo_draw_from_gl`'s `source_type`. GDK takes the GL
/// constant itself rather than an enum of its own.
const GL_TEXTURE: i32 = 0x1702;

/// Everything the draw handler needs, shared with the GTK callbacks.
struct SurfaceState {
    gl: Option<gdk::GLContext>,
    render: Option<MpvRenderContext>,
    mpv: *mut libmpv_sys::mpv_handle,
    /// Set by mpv's update callback (on an mpv thread), cleared by the frame
    /// clock (on the main thread). The whole cross-thread contract.
    frame_ready: Arc<AtomicBool>,
    /// The boxed clone of `frame_ready` handed to mpv, reclaimed on teardown.
    /// Null when no callback is registered.
    callback_ctx: *mut Arc<AtomicBool>,
    // One-shot diagnostic latches; see `draw`.
    logged_first_draw: bool,
    logged_first_frame: bool,
    /// Last size we logged, so a size change re-reports rather than staying silent.
    logged_size: (i32, i32),
    logged_no_gl: bool,
    logged_no_window: bool,
    logged_no_size: bool,
    logged_render_fail: bool,
}

impl SurfaceState {
    /// Tear down in the order DR-232 requires, with the GL context current.
    ///
    /// The update callback is unregistered before the context is freed (inside
    /// `destroy`), and the GL objects go while their context is still bound.
    /// Getting this wrong is DR-184 on Android restated — a surface outliving
    /// its player — and is the likeliest cause of the one unexplained SIGSEGV
    /// the spike recorded.
    fn teardown(&mut self) {
        if let Some(render) = self.render.take() {
            if let Some(gl) = &self.gl {
                gl.make_current();
            }
            // Unregisters the callback before freeing the context.
            unsafe { render.destroy() };
        }
        // Only now is it safe to reclaim what the callback was holding: mpv can
        // no longer reach it. Freeing it first would be the use-after-free this
        // ordering exists to prevent.
        if !self.callback_ctx.is_null() {
            unsafe { drop(Box::from_raw(self.callback_ctx)) };
            self.callback_ctx = std::ptr::null_mut();
        }
        self.gl = None;
    }
}

/// A live video surface. Dropping it tears the render context down.
pub struct VideoSurface {
    state: Rc<RefCell<SurfaceState>>,
    widget: gtk::Box,
    handlers: Vec<glib::SignalHandlerId>,
}

impl Drop for VideoSurface {
    fn drop(&mut self) {
        for id in self.handlers.drain(..) {
            self.widget.disconnect(id);
        }
        self.state.borrow_mut().teardown();
        self.widget.queue_draw();
        info!("[VideoSurface] detached");
    }
}

/// mpv's update callback. Runs on an mpv thread, so it does the least possible:
/// flags the state and asks GTK to redraw on the main loop.
///
/// **Nothing here may block or re-enter the player.** The project's deadlock
/// gotcha applies with full force — this is called from mpv's own threads.
///
/// TRACES: UR-080 | DR-233
unsafe extern "C" fn on_mpv_update(ctx: *mut c_void) {
    if ctx.is_null() {
        return;
    }
    // Runs on an *mpv* thread. It therefore does exactly one thing that is safe
    // to do from there: set an atomic flag.
    //
    // It must not touch GTK, and specifically must not schedule work with
    // `idle_add_local*`, which requires the calling thread to own the default
    // main context — from here that panics with "default main context already
    // acquired by another thread". Nor can it hold the `Rc<RefCell<..>>` state:
    // an `Rc` is not `Send`, and cloning one from two threads races its
    // refcount.
    //
    // The frame clock on the widget picks the flag up on the main thread. See
    // `install_frame_clock`.
    let flag = &*(ctx as *const Arc<AtomicBool>);
    flag.store(true, Ordering::Release);
}

/// Start drawing mpv's video underneath the webview.
///
/// `vbox` is Tauri's `default_vbox()` — the container the webview already lives
/// in. It is not modified; only a `draw` handler is added.
///
/// Must run on the GTK main thread.
///
/// TRACES: UR-080 | DR-231, DR-232, DR-233
pub fn attach(vbox: &gtk::Box, mpv: *mut libmpv_sys::mpv_handle) -> bool {
    if mpv.is_null() {
        warn!("[VideoSurface] no mpv handle; native video unavailable");
        return false;
    }

    let state = Rc::new(RefCell::new(SurfaceState {
        gl: None,
        render: None,
        mpv,
        frame_ready: Arc::new(AtomicBool::new(false)),
        callback_ctx: std::ptr::null_mut(),
        logged_first_draw: false,
        logged_first_frame: false,
        logged_size: (0, 0),
        logged_no_gl: false,
        logged_no_window: false,
        logged_no_size: false,
        logged_render_fail: false,
    }));

    let mut handlers = Vec::new();

    // The GL context can only be created once the widget has a GdkWindow, which
    // is what `realize` announces. Creating it earlier leaves nothing to attach
    // to — the same ordering constraint the render context has.
    let realize_state = state.clone();
    handlers.push(vbox.connect_realize(move |widget| {
        if let Err(e) = init_gl(widget, &realize_state) {
            error!("[VideoSurface] GL init failed: {e}");
        }
    }));

    // A render context outliving its GL context is the defect DR-232 exists to
    // prevent, so teardown is bound to `unrealize` rather than left to Drop.
    let unrealize_state = state.clone();
    handlers.push(vbox.connect_unrealize(move |_| {
        unrealize_state.borrow_mut().teardown();
    }));

    // Drive the render loop from the widget's frame clock, on the main thread,
    // rendering only when mpv actually has a frame.
    //
    // Both nearby mistakes were made and are worth naming, because each has a
    // symptom that points somewhere else:
    //
    //   - Waiting on mpv's update callback before rendering deadlocks. mpv does
    //     not progress until the client renders. The file loads, one frame
    //     appears, and everything stops — no picture, no audio, a spinner that
    //     never clears. It reads as a broken stream.
    //   - Rendering unconditionally every tick and reporting a swap each time
    //     tells mpv a frame reached the screen far more often than one did. It
    //     plays, and judders badly. It reads as a GPU or compositing limit.
    //
    // Polling `has_frame` each tick is neither.
    //
    // The frame clock only ticks while the widget is mapped, so this costs
    // nothing when the window is hidden.
    //
    // TRACES: UR-080 | DR-233
    let tick_state = state.clone();
    vbox.add_tick_callback(move |widget, _clock| {
        // Ask mpv, on the main thread, whether there is anything new. The
        // update callback's flag is only a hint that something *may* have
        // happened; `has_frame` is the authority, and asking it here is what
        // keeps this from either deadlocking or over-presenting.
        let ready = match tick_state.try_borrow() {
            Ok(s) => {
                s.frame_ready.swap(false, Ordering::AcqRel);
                match s.render.as_ref() {
                    Some(render) => unsafe { render.has_frame() },
                    None => false,
                }
            }
            Err(_) => false,
        };
        if ready {
            widget.queue_draw();
        }
        glib::ControlFlow::Continue
    });

    let draw_state = state.clone();
    handlers.push(vbox.connect_draw(move |widget, cr| {
        draw(widget, cr, &draw_state);
        // Propagate: the webview is a child and must still draw over us.
        glib::Propagation::Proceed
    }));

    // The window is already up by the time we are called, so run the init the
    // `realize` signal would have.
    if vbox.is_realized() {
        if let Err(e) = init_gl(vbox, &state) {
            error!("[VideoSurface] GL init failed: {e}");
        }
    }

    info!("[VideoSurface] attached to Tauri's vbox without reparenting");
    // The surface lives as long as the window. Held in a thread-local rather
    // than returned, because it owns `Rc` and GTK types and so is neither `Send`
    // nor `Sync` — it cannot go into Tauri's managed state, and leaking it would
    // give up the ability to tear it down at all.
    //
    // Teardown does not depend on this being dropped: it is driven by the
    // widget's `unrealize`, which is the signal that actually means "your GL
    // context is going away" (DR-232).
    LIVE_SURFACE.with(|cell| {
        *cell.borrow_mut() = Some(VideoSurface {
            state,
            widget: vbox.clone(),
            handlers,
        });
    });
    true
}

thread_local! {
    /// The one live surface, on the GTK main thread.
    static LIVE_SURFACE: RefCell<Option<VideoSurface>> = const { RefCell::new(None) };
}

/// Drop the live surface, if there is one. Idempotent.
///
/// TRACES: UR-080 | DR-232
#[allow(dead_code)]
pub fn detach() {
    LIVE_SURFACE.with(|cell| {
        cell.borrow_mut().take();
    });
}

/// Create the GL context and the mpv render context over it.
fn init_gl(widget: &gtk::Box, state: &Rc<RefCell<SurfaceState>>) -> Result<(), String> {
    if state.borrow().render.is_some() {
        return Ok(());
    }
    let window = widget.window().ok_or("widget has no GdkWindow")?;

    let gl = window
        .create_gl_context()
        .map_err(|e| format!("create_gl_context: {e}"))?;
    gl.realize().map_err(|e| format!("realize: {e}"))?;
    gl.make_current();

    let mpv = state.borrow().mpv;
    let mut render =
        unsafe { MpvRenderContext::new(mpv) }.ok_or("mpv render context creation failed")?;

    // The callback needs an owned handle that outlives this function, so a
    // clone of the flag is boxed and leaked. `Arc<AtomicBool>` rather than the
    // state itself: it is the only thing that may cross to an mpv thread. The
    // pointer is kept so teardown can reclaim it — after the callback is
    // unregistered, never before.
    let flag = state.borrow().frame_ready.clone();
    let ctx_box: *mut Arc<AtomicBool> = Box::into_raw(Box::new(flag));
    unsafe { render.set_update_callback(Some(on_mpv_update), ctx_box as *mut c_void) };

    let mut s = state.borrow_mut();
    s.gl = Some(gl);
    s.render = Some(render);
    s.callback_ctx = ctx_box;
    info!("[VideoSurface] GL and render context ready");
    Ok(())
}

/// Draw the current frame, if there is one.
///
/// Runs *before* the children, which is what puts the picture behind the
/// webview. Deliberately forgiving: no frame, no GL, or a borrowed state all
/// mean "draw nothing this pass" rather than an error — the webview then paints
/// over an untouched background, which is exactly the pre-native appearance.
fn draw(widget: &gtk::Box, cr: &gtk::cairo::Context, state: &Rc<RefCell<SurfaceState>>) {
    // Report each way of doing nothing exactly once. Without this the whole
    // path is invisible: a draw handler that never runs, one that bails on a
    // zero allocation, and one that renders perfectly all look identical from
    // outside — and mpv stalls if frames are never consumed, so "no audio and
    // it hangs" is a plausible symptom of *any* of them.
    fn once(flag: &mut bool, msg: &str) {
        if !*flag {
            *flag = true;
            warn!("[VideoSurface] not drawing: {msg}");
        }
    }

    let Ok(mut s) = state.try_borrow_mut() else {
        return;
    };
    if !s.logged_first_draw {
        s.logged_first_draw = true;
        info!("[VideoSurface] draw handler running");
    }
    let Some(gl) = s.gl.clone() else {
        let f = &mut s.logged_no_gl;
        once(f, "no GL context");
        return;
    };
    let Some(window) = widget.window() else {
        let f = &mut s.logged_no_window;
        once(f, "widget has no GdkWindow");
        return;
    };

    let scale = widget.scale_factor();
    let width = widget.allocated_width() * scale;
    let height = widget.allocated_height() * scale;
    if width <= 0 || height <= 0 {
        let f = &mut s.logged_no_size;
        once(f, "zero allocation");
        return;
    }

    gl.make_current();

    // Render and end the mutable borrow before touching the latches again.
    let rendered = match s.render.as_mut() {
        Some(render) => unsafe { render.render(width, height) },
        None => return,
    };
    let Some(texture) = rendered else {
        let f = &mut s.logged_render_fail;
        once(f, "mpv render produced no texture");
        return;
    };
    // Log the first frame, and again whenever the target size changes. Latching
    // this once per session hid the case that matters: a second file, rendered
    // at a different size, in a window that never moved. "The picture is a small
    // box in the middle" and "the picture fills the widget" are indistinguishable
    // from outside without it.
    if !s.logged_first_frame || s.logged_size != (width, height) {
        s.logged_first_frame = true;
        s.logged_size = (width, height);
        info!("[VideoSurface] rendering {width}x{height} (texture {texture})");
    }

    unsafe {
        cr.draw_from_gl(
            &window,
            texture as i32,
            GL_TEXTURE,
            scale,
            0,
            0,
            width,
            height,
        );
        // Tell mpv the frame reached the screen. Without this it has nothing to
        // pace against — see DR-233.
        if let Some(render) = s.render.as_ref() {
            render.report_swap();
        }
    }
}