[crac] RFR: 8350845: [CRaC] Support C/R engines in form of a library [v3]

[Timofei Pushkin]

> Adds support for C/R engines implemented in form of dynamic libraries.
> 
> In the proposed API the engine controls the set of the configurable options — they are mainly passed by the user via the new `CRaCEngineOptions` VM option, but some may also be passed by the VM. Other VM and/or engine implementations may extend the API via "extensions".
> 
> The JVM is supposed to know what configuration options and API extensions are supported by calling `can_configure` and `get_extension`.
> 
> The user is supposed to know which options they can pass by asking the engine with `-XX:CRaCEngineOptions=help` (support for this is engine-dependent). Example of how this looks for bundled engines:
> 
> $ java -XX:CRaCEngineOptions=help
> crexec - pseudo-CRaC-engine used to relay data from JVM to a "real" engine implemented as an executable (instead of a library). The engine executable is expected to have CRaC-CRIU-like CLI. Support of the configuration options also depends on the engine executable.
> 
> Configuration options:
> * keep_running=<true/false> (default: false) - keep the process running after the checkpoint or kill it.
> * direct_map=<true/false> (default: false) - on restore, map process data directly from saved files. This may speedup the restore but the resulting process will not be the same as before the checkpoint.
> * args=<string> (default: "") - free space-separated arguments passed directly to the engine executable, e.g. "--arg1 --arg2 --arg3".
> 
> 
> Notable related changes included in the patch:
> - The ability to pass arguments to C/R engine was removed from `CRaCEngine` because it can now be implemented through `CRaCEngineOptions`. E.g. `-XX:CRaCEngine=criu,--verbose` is now `-XX:CRaCEngine=criu -XX:CRaCEngineOptions=args=--verbose`.
> - `CRaCEngine` (as well as the new `CRaCEngineOptions`) VM Option is now not updated in the restored VM. This seems more correct because engine path and options will very likely differ on restore and on checkpoint, so it is not obvious how to combine them — just replacing won't always work. Until now the updates were ignored by the CRaC implementation anyway, so this should only be visible by inspecting the options through JMX.
> - `CRaCRestoreFrom` is now considered set (and thus an attempt to restore is made) only if it is set to a non-empty value, i.e. `-XX:CRaCRestoreFrom=""` won't trigger a (failing) restore attempt anymore. This is to be consistent with how `-XX:CRaCCheckpointTo=""` does not trigger a checkpoint setup.
> - Support for restore data (...

Timofei Pushkin has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

-------------

Changes:
  - all: https://git.openjdk.org/crac/pull/207/files
  - new: https://git.openjdk.org/crac/pull/207/files/8c8864e4..c72d5876

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=207&range=02
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=207&range=01-02

  Stats: 118 lines in 4 files changed: 66 ins; 10 del; 42 mod
  Patch: https://git.openjdk.org/crac/pull/207.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/207/head:pull/207

PR: https://git.openjdk.org/crac/pull/207