RFR: 8257189: Handle concurrent updates of MH.form better

[Claes Redestad]

On Thu, 26 Nov 2020 21:23:16 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> Concurrent updates may lead to redundant LambdaForms created and unnecessary class loading when those are compiled. 
> 
> Most notably, it severely affects MethodHandle customization: when a MethodHandle is called from multiple threads, every thread starts customization which takes enough time for other threads to join, but only one of those customizations will be picked.
> 
> Coordination between threads requesting the updates and letting a single thread proceed avoids the aforementioned problem. Moreover, there's no need to wait until the update in-flight is over: all other threads (except the one performing the update) can just proceed with the invocation using the existing MH.form. 
> 
> Testing:
> - manually monitored the behavior on a stress test from [JDK-8252049](https://bugs.openjdk.java.net/browse/JDK-8252049)
> - tier1-4

Looks good to me! 

A couple of trivial nits inline..

src/java.base/share/classes/java/lang/invoke/MethodHandle.java line 459:

> 457:     // asTypeCache is not private so that invokers can easily fetch it
> 458: 
> 459:     /*non-public*/

Remove `/*non-public*/`

src/java.base/share/classes/java/lang/invoke/MethodHandle.java line 1769:

> 1767:      */
> 1768:     /*non-public*/
> 1769:     void updateForm(Function<LambdaForm,LambdaForm> updater) {

`Function<LambdaForm, LambdaForm>`

src/java.base/share/classes/java/lang/invoke/MethodHandle.java line 1754:

> 1752:         updateForm(new Function<>() {
> 1753:                 public LambdaForm apply(LambdaForm oldForm) {
> 1754:                     return oldForm.customize(mh);

I think you can drop line 1751 and write this as `return oldForm.customize(MethodHandle.this);`. Simpler and might avoid an additional, implicit capture.

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Marked as reviewed by redestad (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1472