Initial version

.editorconfig

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+root = true
+
+[*]
+indent_style = tab
+indent_size = 4
+end_of_line = crlf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = false
+
+[*.yml]
+indent_style = space
+indent_size = 2

.github/check.yml

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+name: check
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Check
+        run: cargo check --verbose

.github/release.yml

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+name: Release
+
+permissions:
+  contents: write
+
+on:
+  push:
+    tags:
+      - v[0-9]+.*
+
+jobs:
+  create-release:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: taiki-e/create-gh-release-action@v1
+        with:
+          changelog: CHANGELOG.md
+          token: ${{ secrets.GITHUB_TOKEN }}
+
+  upload-assets:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: taiki-e/upload-rust-binary-action@v1
+        with:
+          bin: multiblox
+          token: ${{ secrets.GITHUB_TOKEN }}

.gitignore

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+/target

.vscode/settings.json

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+{
+	"editor.formatOnSave": true
+}

CHANGELOG.md

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+# 0.1.0
+
+Initial version.
+
+Creates a mutex with CreateMutexA with name `ROBLOX_singletonMutex`, stays in system tray.
+
+Windows only.

Cargo.toml

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+[package]
+name = "multiblox"
+version = "0.1.0"
+edition = "2021"
+
+[profile.release]
+panic = "abort"
+strip = true
+opt-level = "z"
+lto = true
+codegen-units = 1
+
+[dependencies]
+trayicon = "0.1.3"
+winapi = { version = "0.3.9", features = ["winuser"] }

README.md

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+# Multiblox
+
+Simple program to allow running multiple Roblox client instances at once.
+
+## Downloading
+
+Download a build from the [Releases](https://github.com/DvvCz/Multiblox/releases/latest)
+
+## Usage
+
+Open Multiblox **before** opening Roblox, and it should move to your system tray / "hidden icons".
+
+You should be able to open multiple clients while it is there.
+
+## ⚠️ Note
+
+This is not associated with the Roblox corporation in any way.
+
+I doubt you would get banned for using this, but keep in mind there could always be a chance.

rustfmt.toml

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+hard_tabs = true

src/main.rs

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+mod singleton;
+
+#[derive(PartialEq, Clone)]
+enum Events {
+	Close,
+}
+
+fn main() {
+	// If you're overwhelmed by the amount of code here, don't worry
+	// This single variable declaration is all that matters with regards to patching Roblox.
+	// The rest of it just makes this use a system tray icon.
+	let singleton = singleton::Singleton::new().expect("Failed to acquire mutex");
+
+	let (stop_send, stop_recv) = std::sync::mpsc::channel::<bool>();
+	let (event_send, event_recv) = std::sync::mpsc::channel::<Events>();
+
+	let icon = trayicon::TrayIconBuilder::new()
+		.sender(event_send)
+		.icon_from_buffer(include_bytes!("../assets/icon.ico"))
+		.tooltip("Multiblox")
+		.menu(trayicon::MenuBuilder::new().item("Close", Events::Close))
+		.build()
+		.expect("Failed to create trayicon");
+
+	let join = std::thread::spawn(move || {
+		event_recv.iter().next();
+		let _ = stop_send.send(true); // Couldn't have hung up
+	});
+
+	loop {
+		use winapi::um::winuser;
+
+		let _ = icon;
+		let _ = singleton;
+
+		if let Ok(true) = stop_recv.try_recv() {
+			let _ = join.join(); // Can't possibly panic
+			break;
+		}
+
+		let mut msg = core::mem::MaybeUninit::uninit();
+		let bret = unsafe { winuser::GetMessageA(msg.as_mut_ptr(), 0 as _, 0, 0) };
+		if bret > 0 {
+			unsafe {
+				winuser::TranslateMessage(msg.as_ptr());
+				winuser::DispatchMessageA(msg.as_ptr());
+			}
+		} else {
+			break;
+		}
+	}
+}

src/singleton.rs

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+extern "system" {
+	fn CreateMutexA(
+		opts: *const core::ffi::c_void,
+		takeover: u32,
+		name: *const i8,
+	) -> *const core::ffi::c_void;
+	fn ReleaseMutex(m: *const core::ffi::c_void);
+	fn CloseHandle(m: *const core::ffi::c_void);
+}
+
+pub struct Singleton {
+	mutex: *const core::ffi::c_void,
+}
+
+impl Singleton {
+	#[inline]
+	pub fn new() -> core::option::Option<Self> {
+		let mutex = unsafe {
+			CreateMutexA(
+				core::ptr::null(),
+				1,
+				"ROBLOX_singletonMutex\0".as_ptr() as _,
+			)
+		};
+		if mutex.is_null() {
+			None
+		} else {
+			Some(Self { mutex })
+		}
+	}
+}
+
+impl Drop for Singleton {
+	#[inline]
+	fn drop(&mut self) {
+		unsafe {
+			ReleaseMutex(self.mutex);
+			CloseHandle(self.mutex);
+		}
+	}
+}