Linux debug: hot restart no longer crashes with live mpv players

Hot restart frees the old isolate's NativeCallables while mpv handles stay
alive; media_kit then quits them on the next start and mpv calls the dead
wakeup callback (SIGSEGV on the mpv 'core' thread). A debug-only runner
hook clears every live handle's wakeup callback on the engine's
on-pre-engine-restart signal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/linux/runner/hot_restart_guard.cc

@@ -0,0 +1,69 @@
+#include "hot_restart_guard.h"
+
+#ifdef NDEBUG
+
+void hot_restart_guard_attach(FlView* view) {}
+
+#else
+
+#include <dlfcn.h>
+#include <stdint.h>
+#include <unistd.h>
+
+namespace {
+
+// Mirrors media_kit's NativeReferenceHolder (debug only): a calloc'd buffer
+// of kReferenceBufferSize intptr slots holding live mpv_handle addresses,
+// its address written in decimal to <tmp>/<kFilePrefix><pid>.
+constexpr int kReferenceBufferSize = 512;
+constexpr char kFilePrefix[] = "com.alexmercerind.media_kit.NativeReferenceHolder.";
+
+using SetWakeupCallback = void (*)(void* ctx, void (*cb)(void*), void* d);
+
+SetWakeupCallback find_set_wakeup_callback() {
+  static const char* const kNames[] = {"libmpv.so.2", "libmpv.so", "libmpv.so.1"};
+  for (const char* name : kNames) {
+    void* lib = dlopen(name, RTLD_NOW | RTLD_NOLOAD);
+    if (lib == nullptr) continue;
+    auto fn = reinterpret_cast<SetWakeupCallback>(
+        dlsym(lib, "mpv_set_wakeup_callback"));
+    dlclose(lib);  // Drops only the reference NOLOAD added.
+    if (fn != nullptr) return fn;
+  }
+  return nullptr;
+}
+
+// Runs on the engine's UI thread before the old isolate shuts down, so the
+// buffer is not being written concurrently. Clearing the callback takes the
+// same lock mpv calls it under: once this returns, mpv never calls it again.
+// The handles stay in the buffer: media_kit's startup in the new isolate
+// still sends them "quit".
+void on_pre_engine_restart(FlEngine* engine, gpointer user_data) {
+  g_autofree gchar* name = g_strdup_printf("%s%d", kFilePrefix, getpid());
+  g_autofree gchar* path = g_build_filename(g_get_tmp_dir(), name, nullptr);
+  g_autofree gchar* contents = nullptr;
+  if (!g_file_get_contents(path, &contents, nullptr, nullptr)) return;
+  const guint64 address = g_ascii_strtoull(contents, nullptr, 10);
+  if (address == 0) return;
+  const SetWakeupCallback set_wakeup = find_set_wakeup_callback();
+  if (set_wakeup == nullptr) return;
+  const auto* slots = reinterpret_cast<const intptr_t*>(address);
+  int cleared = 0;
+  for (int i = 0; i < kReferenceBufferSize; i++) {
+    if (slots[i] == 0) continue;
+    set_wakeup(reinterpret_cast<void*>(slots[i]), nullptr, nullptr);
+    cleared++;
+  }
+  if (cleared > 0) {
+    g_message("hot restart: unhooked %d mpv wakeup callback(s)", cleared);
+  }
+}
+
+}  // namespace
+
+void hot_restart_guard_attach(FlView* view) {
+  g_signal_connect(fl_view_get_engine(view), "on-pre-engine-restart",
+                   G_CALLBACK(on_pre_engine_restart), nullptr);
+}
+
+#endif