feat: add threading to Scheduler, fix desktop background selection freeze issue

programs/src/threadtest/main.cpp

@@ -0,0 +1,101 @@
+/*
+    * main.cpp
+    * Threading smoke test for MontaukOS.
+    * Copyright (c) 2026 Daniel Hammer
+    *
+    * Spawns two worker threads that increment a shared counter under a
+    * userspace mutex, joins them, and reports the result.
+*/
+
+#include <montauk/syscall.h>
+#include <montauk/thread.h>
+
+using montauk::Mutex;
+
+static constexpr int ITERS_PER_THREAD = 100000;
+
+struct WorkerState {
+    Mutex* lock;
+    int* counter;
+    int delta;
+};
+
+static int worker(void* raw) {
+    auto* s = (WorkerState*)raw;
+    for (int i = 0; i < ITERS_PER_THREAD; i++) {
+        s->lock->lock();
+        *(s->counter) += s->delta;
+        s->lock->unlock();
+    }
+    return s->delta;
+}
+
+static void print_int(int v) {
+    char buf[16];
+    int n = 0;
+    if (v == 0) {
+        buf[n++] = '0';
+    } else {
+        bool neg = v < 0;
+        unsigned int u = neg ? (unsigned int)(-(long long)v) : (unsigned int)v;
+        char tmp[16]; int t = 0;
+        while (u) { tmp[t++] = (char)('0' + (u % 10)); u /= 10; }
+        if (neg) buf[n++] = '-';
+        while (t) buf[n++] = tmp[--t];
+    }
+    buf[n] = '\0';
+    montauk::print(buf);
+}
+
+extern "C" void _start() {
+    Mutex lock;
+    int counter = 0;
+
+    WorkerState a{&lock, &counter, +1};
+    WorkerState b{&lock, &counter, +2};
+
+    int tidA = montauk::thread_spawn(worker, &a);
+    int tidB = montauk::thread_spawn(worker, &b);
+    if (tidA < 0 || tidB < 0) {
+        montauk::print("threadtest: spawn failed\n");
+        montauk::exit(1);
+    }
+
+    montauk::print("threadtest: spawned tids ");
+    print_int(tidA);
+    montauk::print(" and ");
+    print_int(tidB);
+    montauk::print("\n");
+
+    int codeA = -1, codeB = -1;
+    if (montauk::thread_join(tidA, &codeA) != 0) {
+        montauk::print("threadtest: join A failed\n");
+        montauk::exit(2);
+    }
+    if (montauk::thread_join(tidB, &codeB) != 0) {
+        montauk::print("threadtest: join B failed\n");
+        montauk::exit(3);
+    }
+
+    int expected = ITERS_PER_THREAD * (1 + 2);
+    montauk::print("threadtest: counter=");
+    print_int(counter);
+    montauk::print(" expected=");
+    print_int(expected);
+    montauk::print(" codes=");
+    print_int(codeA);
+    montauk::print("/");
+    print_int(codeB);
+    montauk::print("\n");
+
+    if (counter != expected) {
+        montauk::print("threadtest: FAIL (counter mismatch)\n");
+        montauk::exit(4);
+    }
+    if (codeA != 1 || codeB != 2) {
+        montauk::print("threadtest: FAIL (exit codes)\n");
+        montauk::exit(5);
+    }
+    montauk::print("threadtest: PASS\n");
+    montauk::exit(0);
+}