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

[Timofei Pushkin]

On Thu, 27 Feb 2025 20:45:30 GMT, Radim Vansa <rvansa at openjdk.org> wrote:

>> 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:CRaCEngine=simengine -XX:CRaCEngineOptions=help -XX:CRaCCheckpointTo=cr -version
>> 
>> 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 options below 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".
>> * help — print this message.
>> 
>> openjdk version "24-internal" 2025-03-18
>> OpenJDK Runtime Environment (slowdebug build 24-internal-adhoc.timpushkin.crac)
>> OpenJDK 64-Bit Server VM (slowdebug build 24-internal-adhoc.timpushkin.crac, mixed mode, sharing)
>> 
>> 
>> 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 ...
>
> test/jdk/jdk/crac/CracEngineOptionsTest.java line 135:
> 
>> 133:                 "-version");
>> 134:         if (opts != null) {
>> 135:             pb.command().add(pb.command().size() - 2, "-XX:CRaCEngineOptions=" + opts);
> 
> `CRaCEngineOptions` is `ccstrlist`, so sometimes you parse options separated by commas (when passed like above) but if you repeat the option the value gets concatenated with newlines. Could you test that usecase as well?

I actually missed this and `\n` is not treated as a separator at all. Will fix and add a test case, thanks.

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PR Review Comment: https://git.openjdk.org/crac/pull/207#discussion_r1974912307