RFR: 8340114: Remove outdated SelectVersion() function from the launcher and update the code comments explaining the code flow [v6]

[Jaikiran Pai]

On Mon, 16 Sep 2024 01:06:27 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   comment improvements
>
> src/java.base/macosx/native/libjli/java_md_macosx.m line 83:
> 
>> 81:  *    point of creating a new thread in CreateExecutionEnvironment, the CreateExecutionEnvironment will check for
>> 82:  *    the state flag to see if a new thread has already been spawned and upon noticing that it has, it will skip
>> 83:  *    spawning any more threads and will return back from CreateExecutionEnvironment.
> 
> My understanding was that this recursive invocation was only needed when switching modes/models. I don't see why it would be needed now.

In the context of the 2 platforms - macosx and unix, the recursive invocation still continues to happen.

For macosx, the `CreateExecutionEnvironment` unconditionally, through an internal `MacOSXStartup` function, spawns a new thread (`pthread_create`). That thread is passed a function pointer pointing to the current executable/process' `main(...)` function (and thus made to execute afresh). That then triggers this recursion where `JLI_Launch` is called again and that then calls into `CreateExecutionEnvironment` all the way to `MacOSXStartup` function which has the necessary knowledge not to spawn another thread the second time.

For unix, in the `CreateExecutionEnvironment` function, there's a specific piece of code which determines whether or not `LD_LIBRARY_PATH` environment variable needs to be set or updated before loading the JVM. This decision of whether or not `LD_LIBRARY_PATH` needs to be updated or set is done in an internal function called `RequiresSetenv`. There are several rules which determine whether or not to set/update that environment variable. Rules like - is musl libc in use; or is the runtime environment AIX; or if the `LD_LIBRARY_PATH` is currently set, then whether the value in the environment variable matches some well known JVM library path patterns (like `lib/client`, `lib/server`). If any of these rules determine that the `LD_LIBRARY_PATH` needs to be set/updated, then the `CreateExecutionEnvironment` function will do the necessary updates to the environment variable value and and exec() the current process with that new value. This then triggers the recursion all the way from the `
 JLI_Launch` back into this `CreateExecutionEnvironment` (which has the necessary knowledge not to exec() once more).

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PR Review Comment: https://git.openjdk.org/jdk/pull/20997#discussion_r1761203849