RFR: 8280468: Crashes in getConfigColormap, getConfigVisualId, XVisualIDFromVisual on Linux

[Sergey Bylokhov]

On Fri, 21 Jan 2022 17:02:38 GMT, Maxim Kartashev <duke at openjdk.java.net> wrote:

> These crashes were not reproducible, so the fix is based on a hypothesis that there are two possible reasons for them:
> 1. `makeDefaultConfig()` returning `NULL`.
> 2. A race condition when the number of screens changes.
> The race scenario: `X11GraphisDevice.makeDefaultConfiguration()` is called on EDT so the call can race with `X11GraphisDevice.invalidate()` that re-sets the screen number of the device; the latter is invoked on the `AWT-XAWT` thread from `X11GraphicsEnvironment.rebuildDevices()`. So by the time `makeDefaultConfiguration()` makes a native call with the device's current screen number, the `x11Screens` array maintained by the native code could have become shorter. And the native methods like `Java_sun_awt_X11GraphicsDevice_getConfigColormap()` assume that the number passed to them is always current and valid. The AWT lock only protects against the changes during the native methods invocation and does not protect against them being called with an outdated screen number. With a larger screen number, those methods read past the end of the `x11Screens` array.
> 
> The fix for (1) is to die gracefully instead of crashing in an attempt to de-reference a `NULL` pointer, which might happen upon returning from `makeDefaultConfig()` in `getAllConfigs()`.
> 
> The fix for (2) is to eliminate the race by protecting `X11GraphisDevice.screen` with the AWT lock such that it doesn't change when the native methods working with it are active.
> 
> We've been shipping JetBrains Runtime with this fix for a few months now and there were no crash reports with those specific patterns against the versions with the fix.

I think we can update the fix by these suggestions above:

- Can we remove the `X11GraphicsDevice.configLock` from the `X11GraphicsDevice` and use awt lock instead? I am not sure can we get a deadlock for the current code where we will have `X11GraphicsDevice.configLock` -> `awt lock` in the `X11GraphicsDevice`, and `awt lock` -> `X11GraphicsDevice.configLock` in the `XCanvasPeer. getAppropriateGraphicsConfiguration` (Or in some other similar code)

- The code in the `XCanvasPeer` most probably can be rewritten w/o using the screenNum and the internal API, by the iteration over the newDev.getConfigurations() and check the visual of each config. In this case, we will reuse the locks added to the 
`X11GraphicsDevice`

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PR: https://git.openjdk.java.net/jdk/pull/7182