[ozone/x11] Fixed the crash during drag and drop.

The X11Window sometimes did not provide data to the drag and drop client
on drag entering the window, which later (on handling further events)
resulted in DesktopDragDropClientOzone::UpdateTargetAndCreateDropEvent()
dereferencing nullptr and thus creating an invalid reference to data
in the DropTargetEvent, which then caused the crash.

Why exactly the window might not provide data cannot be known for sure.
The essential point in the X11 drag and drop session is XdndPosition
event tht is handled by X11Window::UpdateDrag(), where the drag data is
always created.  The newly created data should then be passed to
DesktopDragDropClientOzone::OnDragEnter(), but because this call must
happen exactly once per drag and drop session, the X11Window has the
notified_enter_ flag for that; the call to OnDragEnter() happens iff
the flag was reset, and the flag is then set and kept till the end of
the session.  So the likely reason of the crash was that notified_enter_
was true before the call to DesktopDragDropClientOzone::OnDragEnter(),
which resulted in the call not happening.

X11Window::StartDrag() resets the flag, so the only case when things
could go wrong is the drag incoming from another window.

The code around had DCHECKs that assert valid state of the data, but
they did not fire neither in tests nor during development, which
suggests that the event is quite rare.

Theoretically, the reason could be incorrect sequence of incoming
events: either the previous drag and drop did not end properly, or the
entering drag did not send some events.

This CL fixes the situation with two changes:
1. The notified_enter_ flag is reset on entering the foreign drag, thus
   ensuring the correct state at the beginning of the incoming drag
   session.
2. The DesktopDragDropClientOzone::UpdateTargetAndCreateDropEvent()
   checks data_to_drop_ before passing it further, so nullptr should
   never be dereferenced anymore.

Bug: 1151836
Change-Id: Id007d5ee463fbaeaf4311166c72a397f48a8e9ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558344
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Maksim Sisov (GMT+2) <msisov@igalia.com>
Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Cr-Commit-Position: refs/heads/master@{#832822}
3 files changed
tree: a8f27a21c7ab3a4b656703aabf9c65fdcf08ad74
  1. .clang-format
  2. .clang-tidy
  3. .eslintrc.js
  4. .git-blame-ignore-revs
  5. .gitattributes
  6. .gitignore
  7. .gn
  8. .vpython
  9. .vpython3
  10. .yapfignore
  11. AUTHORS
  12. BUILD.gn
  13. CODE_OF_CONDUCT.md
  14. DEPS
  15. DIR_METADATA
  16. ENG_REVIEW_OWNERS
  17. LICENSE
  18. LICENSE.chromium_os
  19. OWNERS
  20. PRESUBMIT.py
  21. PRESUBMIT_test.py
  22. PRESUBMIT_test_mocks.py
  23. README.md
  24. WATCHLISTS
  25. android_webview/
  26. apps/
  27. ash/
  28. base/
  29. build/
  30. build_overrides/
  31. buildtools/
  32. cc/
  33. chrome/
  34. chromecast/
  35. chromeos/
  36. cloud_print/
  37. codelabs/
  38. codereview.settings
  39. components/
  40. content/
  41. courgette/
  42. crypto/
  43. dbus/
  44. device/
  45. docs/
  46. extensions/
  47. fuchsia/
  48. gin/
  49. google_apis/
  50. google_update/
  51. gpu/
  52. headless/
  53. infra/
  54. ios/
  55. ipc/
  56. jingle/
  57. media/
  58. mojo/
  59. native_client_sdk/
  60. net/
  61. pdf/
  62. ppapi/
  63. printing/
  64. remoting/
  65. rlz/
  66. sandbox/
  67. services/
  68. skia/
  69. sql/
  70. storage/
  71. styleguide/
  72. testing/
  73. third_party/
  74. tools/
  75. ui/
  76. url/
  77. weblayer/
README.md

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