2 files changed, 123 insertions, 0 deletions
diff --git a/src/libutil/async-semaphore.hh b/src/libutil/async-semaphore.hh
new file mode 100644
index 000000000..f8db31a68
--- /dev/null
+++ b/src/libutil/async-semaphore.hh
@@ -0,0 +1,122 @@
+#pragma once
+/// @file
+/// @brief A semaphore implementation usable from within a KJ event loop.
+
+#include <cassert>
+#include <kj/async.h>
+#include <kj/common.h>
+#include <kj/exception.h>
+#include <kj/list.h>
+#include <kj/source-location.h>
+#include <memory>
+#include <optional>
+
+namespace nix {
+
+class AsyncSemaphore
+{
+public:
+    class [[nodiscard("destroying a semaphore guard releases the semaphore immediately")]] Token
+    {
+        struct Release
+        {
+            void operator()(AsyncSemaphore * sem) const
+            {
+                sem->unsafeRelease();
+            }
+        };
+
+        std::unique_ptr<AsyncSemaphore, Release> parent;
+
+    public:
+        Token() = default;
+        Token(AsyncSemaphore & parent, kj::Badge<AsyncSemaphore>) : parent(&parent) {}
+
+        bool valid() const
+        {
+            return parent != nullptr;
+        }
+    };
+
+private:
+    struct Waiter
+    {
+        kj::PromiseFulfiller<Token> & fulfiller;
+        kj::ListLink<Waiter> link;
+        kj::List<Waiter, &Waiter::link> & list;
+
+        Waiter(kj::PromiseFulfiller<Token> & fulfiller, kj::List<Waiter, &Waiter::link> & list)
+            : fulfiller(fulfiller)
+            , list(list)
+        {
+            list.add(*this);
+        }
+
+        ~Waiter()
+        {
+            if (link.isLinked()) {
+                list.remove(*this);
+            }
+        }
+    };
+
+    const unsigned capacity_;
+    unsigned used_ = 0;
+    kj::List<Waiter, &Waiter::link> waiters;
+
+    void unsafeRelease()
+    {
+        used_ -= 1;
+        while (used_ < capacity_ && !waiters.empty()) {
+            used_ += 1;
+            auto & w = waiters.front();
+            w.fulfiller.fulfill(Token{*this, {}});
+            waiters.remove(w);
+        }
+    }
+
+public:
+    explicit AsyncSemaphore(unsigned capacity) : capacity_(capacity) {}
+
+    KJ_DISALLOW_COPY_AND_MOVE(AsyncSemaphore);
+
+    ~AsyncSemaphore()
+    {
+        assert(waiters.empty() && "destroyed a semaphore with active waiters");
+    }
+
+    std::optional<Token> tryAcquire()
+    {
+        if (used_ < capacity_) {
+            used_ += 1;
+            return Token{*this, {}};
+        } else {
+            return {};
+        }
+    }
+
+    kj::Promise<Token> acquire()
+    {
+        if (auto t = tryAcquire()) {
+            return std::move(*t);
+        } else {
+            return kj::newAdaptedPromise<Token, Waiter>(waiters);
+        }
+    }
+
+    unsigned capacity() const
+    {
+        return capacity_;
+    }
+
+    unsigned used() const
+    {
+        return used_;
+    }
+
+    unsigned available() const
+    {
+        return capacity_ - used_;
+    }
+};
+}
diff --git a/src/libutil/meson.build b/src/libutil/meson.build
index a3f21de59..89eeed133 100644
--- a/src/libutil/meson.build
+++ b/src/libutil/meson.build
@@ -53,6 +53,7 @@ libutil_headers = files(
   'archive.hh',
   'args/root.hh',
   'args.hh',
+  'async-semaphore.hh',
   'backed-string-view.hh',
   'box_ptr.hh',
   'canon-path.hh',