Thanks for the report, I'll look into it.

note to self: likely able to reproduce more easily by using workarounds to enable that path on desktop.

Unfortunately, on my Mac, unsetting feature_flags_.native_vertex_array_object causes a bunch of gl_tests errors, even with my patch reverted. I don't know why that's true. One error I tracked down was an INVALID_OPERATION in the driver's glVertexAttribDivisorANGLE here:

https://cs.chromium.org/chromium/src/gpu/command_buffer/service/gles2_cmd_decoder.cc?l=10111&rcl=aefd808a350f919fd976f1187bc1fedec1845241

Not sure how that error is even possible. But I think it was a red herring.

Able to repro by enabling use_client_side_arrays_for_stream_buffers on my Pixel phone.

Working on a fix here:
https://chromium-review.googlesource.com/#/c/chromium/src/+/685984

We may want to merge this back to M-62 to prevent regressions. I don't know what kind of regressions this might cause, but since the bug is in context switching I think it could flakily cause serious issues.

Agreed, we should merge to 62.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/728c5405bd66c8303b5f6f6eba750efe3e9b7847

commit 728c5405bd66c8303b5f6f6eba750efe3e9b7847
Author: Kai Ninomiya <kainino@chromium.org>
Date: Sun Oct 01 06:00:39 2017

Fix bug in VAO emulation, recently introduced by a workaround

The workaround was added here: https://crrev.com/c/636513

Bug:  768324 
Test: GLVirtualContextsTest.VertexArrayObjectRestoreDefault
      (with use_client_side_arrays_for_stream_buffers enabled).

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I1f0b00a4ab9ab4bf5d8fbd33f567af3b0f41de58
Reviewed-on: https://chromium-review.googlesource.com/685984
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505508}
[modify] https://crrev.com/728c5405bd66c8303b5f6f6eba750efe3e9b7847/gpu/command_buffer/service/gles2_cmd_decoder.cc
[modify] https://crrev.com/728c5405bd66c8303b5f6f6eba750efe3e9b7847/gpu/command_buffer/service/vertex_attrib_manager.cc
[modify] https://crrev.com/728c5405bd66c8303b5f6f6eba750efe3e9b7847/gpu/command_buffer/service/vertex_attrib_manager.h

Requesting to merge #12. See #9 and #11.

This bug requires manual review: M62 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), bhthompson@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

what is the risk associated with merging this into M62 at this point? Could this break other stuffs? Also, can we have a test in place to prevent this from happening in the future?

We consider the risk very low. This is only a very slight logical change - and we're confident that it's more correct now. Plus it only affects Imagination and ARM.

I will work on finding a way to test this change more reliably. (Perhaps we can run GLVirtualContextsTest.VertexArrayObjectRestoreDefault with the extra workaround force-enabled.)

Thanks for thinking about finding a way to test it. Please verify your fix in the latest canary and merge it into 3202 if it all looks good.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fead116077da984b849c5653785d6199ec4ca825

commit fead116077da984b849c5653785d6199ec4ca825
Author: Kai Ninomiya <kainino@chromium.org>
Date: Tue Oct 03 22:10:18 2017

Fix bug in VAO emulation, recently introduced by a workaround

The workaround was added here: https://crrev.com/c/636513

Bug:  768324 
Test: GLVirtualContextsTest.VertexArrayObjectRestoreDefault
      (with use_client_side_arrays_for_stream_buffers enabled).

TBR=kainino@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I1f0b00a4ab9ab4bf5d8fbd33f567af3b0f41de58
Reviewed-on: https://chromium-review.googlesource.com/685984
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#505508}(cherry picked from commit 728c5405bd66c8303b5f6f6eba750efe3e9b7847)
Reviewed-on: https://chromium-review.googlesource.com/699049
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/branch-heads/3202@{#564}
Cr-Branched-From: fa6a5d87adff761bc16afc5498c3f5944c1daa68-refs/heads/master@{#499098}
[modify] https://crrev.com/fead116077da984b849c5653785d6199ec4ca825/gpu/command_buffer/service/gles2_cmd_decoder.cc
[modify] https://crrev.com/fead116077da984b849c5653785d6199ec4ca825/gpu/command_buffer/service/vertex_attrib_manager.cc
[modify] https://crrev.com/fead116077da984b849c5653785d6199ec4ca825/gpu/command_buffer/service/vertex_attrib_manager.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3a1ae8cc2edec3007e1b34dfaed43547392709ee

commit 3a1ae8cc2edec3007e1b34dfaed43547392709ee
Author: Kai Ninomiya <kainino@chromium.org>
Date: Thu Oct 05 01:21:15 2017

Add coverage to regression test for  issue 768324 

Regression test for
GLVirtualContextsTest.VertexArrayObjectRestoreDefault
on devices with use_client_side_arrays_for_stream_buffers and
use_virtualized_gl_contexts.

Bug:  768324 
Test: WithWorkarounds/GLVirtualContextsTest.VertexArrayObjectRestoreDefault/1
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I1b1f8fb791a02c614f1a4c9f869016c198902a16
Reviewed-on: https://chromium-review.googlesource.com/699459
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506606}
[modify] https://crrev.com/3a1ae8cc2edec3007e1b34dfaed43547392709ee/gpu/command_buffer/tests/gl_virtual_contexts_unittest.cc