Extension dylibs aren't being unloaded on Hot Reload

[tishin]

This was originally mentioned in #273
I made a simplistic entry point using GDExtension target as the only dependency:

import GDExtension

@_cdecl("swift_entry_point") public func enterExtension (interface: OpaquePointer?, library: OpaquePointer?, extension: OpaquePointer?) -> UInt8 {
    guard let library, let interface, let `extension` else { return 0 }
    initializeSwiftModule (interface, library, `extension`)
    return 1
}

public func initializeSwiftModule (_ godotGetProcAddrPtr: OpaquePointer, _ libraryPtr: OpaquePointer, _ extensionPtr: OpaquePointer) {
    let initialization = UnsafeMutablePointer<GDExtensionInitialization> (extensionPtr)
    initialization.pointee.deinitialize = extension_deinitialize
    initialization.pointee.initialize = extension_initialize
    initialization.pointee.minimum_initialization_level = GDEXTENSION_INITIALIZATION_SCENE
}

func extension_initialize (userData: UnsafeMutableRawPointer?, l: GDExtensionInitializationLevel) {}
func extension_deinitialize (userData: UnsafeMutableRawPointer?, l: GDExtensionInitializationLevel) {}

final class SomeClass {}

And configured gdextension as reloadable:

[configuration]
entry_symbol = "swift_entry_point"
compatibility_minimum = 4.2
reloadable = true

This was enough to reproduce the issue:

1. Build the extension
2. Run godot
3. Build the extension again replacing the dylib
   This will result in

objc[79981]: Class _TtC15GodotPlayground9SomeClass is implemented in both /gd/GodotPlayground/.build/arm64-apple-macosx/debug/libGodotPlayground.dylib (0x1113b80d8) and /gd/GodotPlayground/.build/arm64-apple-macosx/debug/libGodotPlayground.dylib (0x11fc440d8). One of the two will be used. Which one is undefined.

[tishin]

In fact the extension can be as simple as

@_cdecl("swift_entry_point") public func enterExtension (interface: OpaquePointer?, library: OpaquePointer?, extension: OpaquePointer?) -> UInt8 {
    return 1
}

class SomeClass {}

with no dependencies at all and the issue will still be reproducible. It just needs a little tweak in Godot's source code (godotengine/godot#87938), since Godot does not properly handle null in initialization.deinitialize

Looking into Godot's extension reload, dlclose(p_library_handle) on the old library does not fail, but the dlopen after it reports class implementation ambiguity. That being said, dlclose success does not mean the library is supposed to be unloaded immediately:

The function dlclose() decrements the reference count on the dynamic library handle handle. If the reference count drops to zero and no other loaded libraries use symbols in it, then the dynamic library is unloaded.

[migueldeicaza]

I do not think this is a Godot issue, but an interoperability issue between Swift and loading dynamic modules.

I created a very simple sample (https://tirania.org/tmp/demo-swift-dlopen.tar.gz) that exhibits the same issue, here is the sample for the main program:

import Foundation

func run () {
        let h = dlopen ("/tmp/libj.dylib", RTLD_LAZY)
        let methodSymbol = dlsym (h, "swift_entry_point")
        typealias MethodFunction = @convention(c) () -> UInt8
        let method = unsafeBitCast(methodSymbol, to: MethodFunction.self)
        let result = method()
        print("Result of method call: \(result)")

        // Close the dynamic library
        print ("Closing: \(dlclose(h))")
}

run ()
print ("Swap the library")
getchar()
run ();

And then I replace libj.dylib with libk.dylib in the pause, and I get the same error.

[migueldeicaza]

And I can no longer reproduce the error. I am not sure what I did, but now I am able to load both libraries, and the warning is gone, and the libraries do show the right output.