RFR: 8264990: Fix segfault when accessing local storage in webview

[Matthias Bläsing]

On Thu, 8 Apr 2021 06:58:14 GMT, Matthias Bläsing <github.com+2179736+matthiasblaesing at openjdk.org> wrote:

> The functions from FileSystemJava are called from different threads the
> root problem manifests because the JNI FindClass function behaves
> differently when called from a context that is the ancestor of a java
> frame compared to when called in isolation.
> 
> A segmentation fault is observed when local storage of a webview is
> accessed. At that time a new native thread is spun up and that sets up
> the local storage, by calling into the JVM via
> WTF::FileSystem::makeAllDirectories. At that point GetFileSystemClass is
> invoked to get a referenc to the java implementation of the FileSystem.
> As this is is called from a new native thread (no java context
> available), JNI uses the system classloader to locate the class. This
> fails if the JavaFX modules are not on the boot module/class path.
> 
> Instead on relying on fetching the class reference everytime it is
> needed, this change fetches it once when the JavaFX library is loaded
> and stores it in the WTF namespace.
> 
> In addition to this it was observed, that there is no attachment to the
> JVM done when calling into the filesystem. No fault was observed, but
> the JNI specs indicate, that the JNIEnv interface is only valid when
> attached.

The issue reference is the ID I got after I requested a bug to be created - I'm waiting to have it being opened up.

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PR: https://git.openjdk.java.net/jfx/pull/458