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.

As I wrote in JBS it is okay to refactor the code so that DllMain calls the same code snippets that the static build will. It is the timing of that code execution that needs to be unperturbed. The same potentially with tear-down. I don't know when the DllMain detach code runs in relation to when an at_exit function may run! ??

The sorry tale of ForceTimeHighResolution is told in [JDK-6435126](https://bugs.openjdk.org/browse/JDK-6435126).

But the other pre-initialize functions are also an issue as they need to be called as soon as possible in the VM's lifecycle (`os::init` could be way too late!) in case of crashes.

For a static build how are you figuring out and controlling the order of all of the normal "load time" initialization: static constructors, DllMain, ... ??

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PR Comment: https://git.openjdk.org/jdk/pull/22793#issuecomment-2550359640