RFR: 8346433: Cannot use DllMain in hotspot for static builds [v4]

[Thomas Stuefe]

On Wed, 15 Jan 2025 16:34:28 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> To be able to properly support static builds on Windows in [JDK-8346377](https://bugs.openjdk.org/browse/JDK-8346377), we cannot use DllMain, for two reasons:
>> 
>> 1) This is not called for statically linked libraries, and
>> 2) There are multiple DllMain definitions throughout the JDK native libraries, causing name collisions.
>> 
>> While it could have been possible to keep the DllMain function for non-static builds and just use an alternative solution for static builds, I think it is preferable to have a single solution that works as well for both static and dynamic builds.
>> 
>> The DllMain in hotspot is doing work both at DLL load time, and at DLL unload time. Let's go through them to see why this patch is safe.
>> 
>> During DLL load time, the library handle is set, and the hi-res timer is initialized. The `pre_initialize` method from `WindowsDbgHelp` and `SymbolEngine` is called. These two are basically identical; both setup a critical section (a Windows mutex). However, this mutex is only used from a stack local guard object, which is allocated only when calls are made into these classes, which are not happening at bootstrapping time, so this shift in initialization time is harmless.
>> 
>> That shift in time is also very small. The `DllMain` method is called by Windows when the DLL is loaded (by libjli), and the very first thing that JLI does after that is to call `JNI_CreateJavaVM`, which ends up calling  `Threads::create_vm`, which calls `os::init` early on.
>
> Magnus Ihse Bursie has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:
> 
>  - Simplify GetModuleHandleEx for static builds
>  - Merge branch 'master' into dll-main-in-hotspot
>  - Update comments
>  - Merge branch 'master' into dll-main-in-hotspot
>  - Add explanation
>  - Update comments and copyright year
>  - Restore DllMain
>  - 8346433: Cannot use DllMain in hotspot for static builds

Still good!

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Marked as reviewed by stuefe (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/22793#pullrequestreview-2556814398