Canvas: deleting an image key that is in use

[asajeffrey]

A lovely webrender panic:

stack backtrace:
   0:     0x55f7bd9bd8c4 - backtrace::backtrace::libunwind::trace
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace<closure>
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/mod.rs:42
   1:     0x55f7bd9b5d6f - backtrace::capture::{{impl}}::new
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/capture.rs:64
   2:     0x55f7b7f4a346 - servo::main::{{closure}}
                        at /home/ajeffrey/github/asajeffrey/servo/ports/servo/main.rs:129
   3:     0x55f7bda82a2a - std::panicking::rust_panic_with_hook
                        at /checkout/src/libstd/panicking.rs:550
   4:     0x55f7bda828c4 - std::panicking::begin_panic<collections::string::String>
                        at /checkout/src/libstd/panicking.rs:511
   5:     0x55f7bda827f9 - std::panicking::begin_panic_fmt
                        at /checkout/src/libstd/panicking.rs:495
   6:     0x55f7bc202e8e - webrender::resource_cache::{{impl}}::get_image_properties::{{closure}}
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/resource_cache.rs:540
   7:     0x55f7bc113701 - core::option::{{impl}}::unwrap_or_else<&webrender::resource_cache::ImageResource,closure>
                        at /checkout/src/libcore/option.rs:370
   8:     0x55f7bc202c66 - webrender::resource_cache::{{impl}}::get_image_properties
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/resource_cache.rs:539
   9:     0x55f7bc06be68 - webrender::frame::{{impl}}::flatten_item
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/frame.rs:546
  10:     0x55f7bc06da28 - webrender::frame::{{impl}}::flatten_items
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/frame.rs:788
  11:     0x55f7bc06b33e - webrender::frame::{{impl}}::flatten_stacking_context
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/frame.rs:455
  12:     0x55f7bc06d0f9 - webrender::frame::{{impl}}::flatten_item
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/frame.rs:678
  13:     0x55f7bc06da28 - webrender::frame::{{impl}}::flatten_items
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/frame.rs:788
  14:     0x55f7bc06d71b - webrender::frame::{{impl}}::flatten_root
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/frame.rs:757
  15:     0x55f7bc06a599 - webrender::frame::{{impl}}::create
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/frame.rs:347
  16:     0x55f7bbf6b213 - webrender::render_backend::{{impl}}::build_scene
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/render_backend.rs:500
  17:     0x55f7bc1fcdfd - webrender::render_backend::{{impl}}::run::{{closure}}
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/render_backend.rs:251
  18:     0x55f7bc1f8099 - webrender::profiler::{{impl}}::profile<(),closure>
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/profiler.rs:154
  19:     0x55f7bbf69192 - webrender::render_backend::{{impl}}::run
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/render_backend.rs:244
  20:     0x55f7bc21a033 - webrender::renderer::{{impl}}::new::{{closure}}
                        at /home/ajeffrey/github/asajeffrey/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/a787f8e/webrender/src/renderer.rs:1158
  21:     0x55f7bc1295ca - std::sys_common::backtrace::__rust_begin_short_backtrace<closure,()>
                        at /checkout/src/libstd/sys_common/backtrace.rs:136
  22:     0x55f7bc12be13 - std::thread::{{impl}}::spawn::{{closure}}::{{closure}}<closure,()>
                        at /checkout/src/libstd/thread/mod.rs:364
  23:     0x55f7bc1d005a - std::panic::{{impl}}::call_once<(),closure>
                        at /checkout/src/libstd/panic.rs:296
  24:     0x55f7bc12da49 - std::panicking::try::do_call<std::panic::AssertUnwindSafe<closure>,()>
                        at /checkout/src/libstd/panicking.rs:454
  25:     0x55f7bda89b8a - panic_unwind::__rust_maybe_catch_panic
                        at /checkout/src/libpanic_unwind/lib.rs:98
  26:     0x55f7bc12c1fc - std::panicking::try<(),std::panic::AssertUnwindSafe<closure>>
                        at /checkout/src/libstd/panicking.rs:433
  27:     0x55f7bc12aef5 - std::panic::catch_unwind<std::panic::AssertUnwindSafe<closure>,()>
                        at /checkout/src/libstd/panic.rs:361
  28:     0x55f7bc12bc3b - std::thread::{{impl}}::spawn::{{closure}}<closure,()>
                        at /checkout/src/libstd/thread/mod.rs:363
  29:     0x55f7bc168113 - alloc::boxed::{{impl}}::call_box<(),closure>
                        at /checkout/src/liballoc/boxed.rs:648
  30:     0x55f7bda81685 - alloc::boxed::{{impl}}::call_once<(),()>
                        at /checkout/src/liballoc/boxed.rs:658
                         - std::sys_common::thread::start_thread
                        at /checkout/src/libstd/sys_common/thread.rs:21
                         - std::sys::imp::thread::{{impl}}::new::thread_start
                        at /checkout/src/libstd/sys/unix/thread.rs:84
  31:     0x7fd8c1d706b9 - start_thread
  32:     0x7fd8c18903dc - clone
  33:                0x0 - <unknown>

The cause is

servo/components/canvas/canvas_paint_thread.rs

Line 549 in 2843eb5

self.webrender_api.delete_image(image_key);

:

    fn recreate(&mut self, size: Size2D<i32>) {
        self.drawtarget = CanvasPaintThread::create(size);
        // Webrender doesn't let images change size, so we clear the webrender image key.
        if let Some(image_key) = self.image_key.take() {
            self.webrender_api.delete_image(image_key);
        }
    }

The key is being deleted immediately, but it might still be in use by the display list builder. Really, we should flag the key for being deleted in the next epoch, not the current epoch.

[asajeffrey]

cc @glennw

[asajeffrey]

Looking at the code for html images, it looks like the reason why we're not hitting the problem there is that we're just never removing images from the image cache: #12881.