diff --git a/src/global_safety.h b/src/global_safety.h
new file mode 100644
index 000000000..2156e2c55
--- /dev/null
+++ b/src/global_safety.h
@@ -0,0 +1,73 @@
+// Support for enforcing correct access to globals.
+#ifndef FISH_GLOBAL_SAFETY_H
+#define FISH_GLOBAL_SAFETY_H
+
+#include "config.h"  // IWYU pragma: keep
+
+#include "common.h"
+
+#include <cassert>
+
+// fish is multithreaded. Global (which includes function and file-level statics) when used naively
+// may therefore lead to data races. Use the following types to characterize and enforce correct
+// access patterns.
+
+namespace detail {
+// An empty value type that cannot be copied or moved.
+// Include this as an instance variable to prevent globals from being copied or moved.
+struct fixed_t {
+    fixed_t(const fixed_t &) = delete;
+    fixed_t(fixed_t &&) = delete;
+    fixed_t &operator=(fixed_t &&) = delete;
+    fixed_t &operator=(const fixed_t &) = delete;
+    fixed_t() = default;
+};
+}  // namespace detail
+
+/// A mainthread_t variable may only be accessed on the main thread.
+template <typename T>
+class mainthread_t : detail::fixed_t {
+    T value_{};
+
+   public:
+    mainthread_t(T value) : value_(std::move(value)) {}
+    mainthread_t() = default;
+
+    operator T &() {
+        ASSERT_IS_MAIN_THREAD();
+        return value_;
+    }
+
+    operator const T &() const {
+        ASSERT_IS_MAIN_THREAD();
+        return value_;
+    }
+
+    void operator=(T value) {
+        ASSERT_IS_MAIN_THREAD();
+        value_ = std::move(value);
+    }
+};
+
+/// A latch variable may only be set once, on the main thread.
+/// The value is a immortal.
+template <typename T>
+class latch_t : detail::fixed_t {
+    T *value_{};
+
+   public:
+    operator T *() { return value_; }
+    operator const T *() const { return value_; }
+
+    T *operator->() { return value_; }
+    const T *operator->() const { return value_; }
+
+    template <typename... Args>
+    void emplace(Args &&... args) {
+        ASSERT_IS_MAIN_THREAD();
+        assert(value_ == nullptr && "Latch variable initialized multiple times");
+        value_ = new T(std::forward<Args>(args)...);
+    }
+};
+
+#endif