On Tue, 17 Dec 2024 11:51:51 GMT, Magnus Ihse Bursie <i...@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. > > In this case, the `DllMain` function did two things: > > 1) At startup, it called SetModuleHandle. This has been moved to the > `JNI_OnLoad` function, which is called by the JVM right after loading the > DLL, or in the case of a static build, when the `awt` native library has been > requested. > > 2) At DLL unload -- for debug builds only -- it "disabled" the mutexes for > the DTrace and DMem debug systems. In this case, "disable" means writing NULL > to the mutexes, causing any further calls to the continue without locking, > since the enter/exit calls only do the locking if the mutex is not NULL. > (This is pre-existing code so I am not discussing the soundness of this > approach.) > > But why did we need to do that? After the DLL unloading, which is done by > Windows when the process is exiting, no code should be executing in AWT, > right? No, wrong. There are three static objects, one instance of > AwtDebugSupport and two instances of GDIHashtable, and the destructors of > these objects are called by Windows at the time of process shutdown. I have > not been able to confirm that the `DllMain` code is guaranteed to be called > before the destructors are called, so I guess the fact that this ever worked > has just been a lucky coincidence. > > I have solved this by disabling the mutexes in the destructors themselves, > thereby guaranteeing that they are disabled before the last few calls to the > DTrace/DMem calls are made. There is no guarantee in which order these > destructors are made, so I do the same on both locations. (There is no bad > effect from calling these twice in a row, it's just setting a mutex to NULL). > > Finally, I have wrapped the `DEF_JNI_OnLoad` function in `extern "C" { ... > }`. This is needed since DEF_JNI_OnLoad is a macro that actually creates two > functions, where one is a wrapper calling the other which has a generated > name, and without this, the C++ name mangling messed things up. (To be clear: > this means tha... This pull request has now been integrated. Changeset: 893d00ac Author: Magnus Ihse Bursie <i...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/893d00ac7d118620b56ce147d447c6fca80bbe7b Stats: 46 lines in 1 file changed: 19 ins; 26 del; 1 mod 8346388: Cannot use DllMain in libawt for static builds Reviewed-by: aivanov, prr ------------- PR: https://git.openjdk.org/jdk/pull/22790
