[crac] RFR: Allow users to pass new properties on restore

[Ashutosh Mehra]

On Thu, 5 May 2022 09:00:17 GMT, Anton Kozlov <akozlov at openjdk.org> wrote:

> I think we need to choose one model (JVMTI and j.l.S.properties are identified for now), follow it, and try to understand how much it satisfies users' needs. Or at least ours :)

>From this perspective I think we can go with not updating `_system_properties` on restore. Right now the requirement, as I understand, is to provide a way to specify new or update existing application specific properties on restore which can be achieved by just updating the properties in `j.l.System`.

> I would like to distinguish setting a value for a property and the way or the point when the value is processed by some unrelated entity to the way the property is set. Handling is a very implementation detail.

I get your point. Let's drop the "modifiable" marker on the properties and just update the `java.lang.System.getProperties` view so that applications can make use of the new/updated properties.

> Having that there is another pass over all of the arguments anyway, is it possible to provide all explicit native arguments to the function doing the restore (os::Linux::restore for now)? Then we won't need to introduce a lot of aux code in the VM and it will be simpler to move out VM eventually.

Did you mean we just pass the properties and application arguments directly to `os::Linux::restore()` instead of getting it through `Arguments` class? It should be possible. I guess as soon as we get access to the args, we can call `os::Linux::restore()` and avoid all the JVM initialization process.
The `args` parameter to `Threads::create_vm()` has all the stuff that we need to pass to `os::Linux::restore()`. We can do a quick pass over `args` to extract the properties and application arguments. 
The java launcher adds following properties to the `args` in addition to the user specified:
1. -Djava.class.path
2. -Dsun.java.command
3. -Dsun.java.launcher

We can ignore 1 and 3, collect the application arguments from 2, filter any option starting with `-D` and pass the set to `os::Linux::restore()`. Is that what you are suggesting?

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PR: https://git.openjdk.java.net/crac/pull/21