RFR: 8346433: Cannot use DllMain in hotspot for static builds

[David Holmes]

On Tue, 17 Dec 2024 13:48:59 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.

In terms of this PR I would like to see explicit code for the static build (it could even call DllMain!) with zero perturbations to the initialization sequence of the dynamic build (no matter how safe they may appear) - please.

As a general comment on the static build I have concerns about the global ordering of things currently handled by DllMain and static initialization, but that is for them to check and ensure things work as expected. Fortunately most libs should be independent of each other and so the order things happen for different libs may not matter.

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PR Comment: https://git.openjdk.org/jdk/pull/22793#issuecomment-2552973472