Going forwards in history to a webgl page panics

[jdm]

STR:

1. load https://threejs.org/examples
2. choose the cloth example
3. watch it load
4. go back in history
5. go forward in history

Expected:
Cloth example continues running.

Actual:

[2019-03-06T21:00:19Z ERROR canvas::webgl_thread] Last GL operation failed: BindBuffer(34962, Some(WebGLBufferId(2)))
[2019-03-06T21:00:19Z WARN  servo] Panic hook called.
assertion failed: `(left == right)`
  left: `1286`,
 right: `0`: Unexpected WebGL error: 0x506 (1286) (thread WebGLThread, at components/canvas/webgl_thread.rs:1437)
stack backtrace:
   0:        0x10fe07c34 - __ZN9backtrace9backtrace5trace17h9f3458ac04360c1aE
                        at /Users/jdm/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/backtrace/mod.rs:42
   1:        0x10fe025dc - __ZN9backtrace7capture9Backtrace14new_unresolved17h21921a7295046aa9E
                        at /Users/jdm/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/capture.rs:88
   2:        0x10fe0253e - __ZN9backtrace7capture9Backtrace3new17h18226acc03ed089aE
                        at /Users/jdm/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/capture.rs:63
   3:        0x109a61065 - __ZN5servo4main28_$u7b$$u7b$closure$u7d$$u7d$17h6511ddc8f2676f98E
                        at ports/servo/non_android_main.rs:110
   4:        0x11121cf90 - __ZN3std9panicking20rust_panic_with_hook17hc96a2ab9717cf47bE
                        at src/libstd/panicking.rs:482
   5:        0x11121c9dc - __ZN3std9panicking18continue_panic_fmt17h829e4b1e6e9a8f39E
                        at src/libstd/panicking.rs:385
   6:        0x11121c930 - __ZN3std9panicking15begin_panic_fmt17h3a4ef0ccd095e4a0E
                        at src/libstd/panicking.rs:340
   7:        0x10e5ccda6 - __ZN6canvas12webgl_thread9WebGLImpl5apply17h917b1f0ddac611b4E
                        at components/canvas/webgl_thread.rs:1437
   8:        0x10e60138f - __ZN6canvas10gl_context16GLContextWrapper13apply_command17hf77262daa18fd2eeE
                        at components/canvas/gl_context.rs:164
   9:        0x10e5bbaa1 - __ZN52_$LT$canvas..webgl_thread..WebGLThread$LT$VR$GT$$GT$20handle_webgl_command17hd3d963302b8b7085E
                        at components/canvas/webgl_thread.rs:207
  10:        0x10e5bada9 - __ZN52_$LT$canvas..webgl_thread..WebGLThread$LT$VR$GT$$GT$10handle_msg17h7f91a54c252d5633E
                        at components/canvas/webgl_thread.rs:169
  11:        0x10e5ba76e - __ZN52_$LT$canvas..webgl_thread..WebGLThread$LT$VR$GT$$GT$5start28_$u7b$$u7b$closure$u7d$$u7d$17h84f50790bf709a1eE
                        at components/canvas/webgl_thread.rs:106
  12:        0x10e5e66e3 - __ZN3std10sys_common9backtrace28__rust_begin_short_backtrace17h56911288a4a75b0fE
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/sys_common/backtrace.rs:136
  13:        0x10e736d50 - __ZN3std6thread7Builder15spawn_unchecked28_$u7b$$u7b$closure$u7d$$u7d$28_$u7b$$u7b$closure$u7d$$u7d$17h1648794bce76646aE
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/thread/mod.rs:469
  14:        0x10e5d4a83 - __ZN101_$LT$std..panic..AssertUnwindSafe$LT$F$GT$$u20$as$u20$core..ops..function..FnOnce$LT$$LP$$RP$$GT$$GT$9call_once17h29285d785e46d502E
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/panic.rs:309
  15:        0x10e5e6c3f - __ZN3std9panicking3try7do_call17h299d8907f865ed34E
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/panicking.rs:297
  16:        0x11122621e - ___rust_maybe_catch_panic
                        at src/libpanic_unwind/lib.rs:87
  17:        0x10e5e6b24 - __ZN3std9panicking3try17hb778a41606c7c91dE
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/panicking.rs:276
  18:        0x10e5e6983 - __ZN3std5panic12catch_unwind17hb221ebc2569bff03E
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/panic.rs:388
  19:        0x10e736923 - __ZN3std6thread7Builder15spawn_unchecked28_$u7b$$u7b$closure$u7d$$u7d$17h0dc9110979604618E
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/libstd/thread/mod.rs:468
  20:        0x10e745447 - __ZN50_$LT$F$u20$as$u20$alloc..boxed..FnBox$LT$A$GT$$GT$8call_box17h5ad430700f108549E
                        at /rustc/0ea22717a1e01fa535534b85a5347a7e49fc79de/src/liballoc/boxed.rs:749
  21:        0x11122564b - __ZN3std3sys4unix6thread6Thread3new12thread_start17hb65474f74178fcb3E
                        at src/libstd/sys_common/thread.rs:14
  22:     0x7fffe4b1793a - __pthread_body
  23:     0x7fffe4b17886 - __pthread_start

[jdm]

In a release build I hit this assertion immediately when going back in history on macOS:

[2019-03-06T21:04:33Z ERROR canvas::webgl_thread] Last GL operation failed: Clear(17664)
assertion failed: `(left == right)`
  left: `1286`,
 right: `0`: Unexpected WebGL error: 0x506 (1286) (thread WebGLThread, at components/canvas/webgl_thread.rs:1437)

[jdm]

The patch I wrote for #22977 makes the assertion I just mentioned go away, which is the same as #20924.

[jdm]

The original assertion is triggered by a GC of an extraneous webgl context that's created at https://github.com/mrdoob/three.js/blob/63eca7a87695703e23ccce326f95ffba259e8e19/examples/js/WebGL.js#L13 and never used again.

[jdm]

Specifically, removing the GL context from self.contexts triggers the assertion. I suspect the Drop implementation for NativeGLContext is at fault on macOS.

[jdm]

Dropping the GLContext drops the DrawBuffer, but if the context is not actually current we end up deleting framebuffers and renderbuffers for an inactive context. That's no good!