libutil: add an async semaphore implementation

like a normal semaphore, but with awaitable acquire actions. this is
primarily intended as an intermediate concurrency limiting device in
the Worker code, but it may find other uses over time. we do not use
std::counting_semaphore as a base because the counter of that is not
inspectable as will be needed for Worker. we also do not need atomic
operations for cross-thread consistency since we don't have multiple
threads (thanks to kj event loops being confined to a single thread)

Change-Id: Ie2bcb107f3a2c0185138330f7cbba4cec6cbdd95

2 files changed, 76 insertions, 0 deletions

diff --git a/tests/unit/libutil/async-semaphore.cc b/tests/unit/libutil/async-semaphore.cc
new file mode 100644
index 000000000..12b52885d
--- /dev/null
+++ b/tests/unit/libutil/async-semaphore.cc
@@ -0,0 +1,74 @@
+#include "async-semaphore.hh"
+
+#include <gtest/gtest.h>
+#include <kj/async.h>
+
+namespace nix {
+
+TEST(AsyncSemaphore, counting)
+{
+    kj::EventLoop loop;
+    kj::WaitScope waitScope(loop);
+
+    AsyncSemaphore sem(2);
+
+    ASSERT_EQ(sem.available(), 2);
+    ASSERT_EQ(sem.used(), 0);
+
+    auto a = kj::evalNow([&] { return sem.acquire(); });
+    ASSERT_EQ(sem.available(), 1);
+    ASSERT_EQ(sem.used(), 1);
+    auto b = kj::evalNow([&] { return sem.acquire(); });
+    ASSERT_EQ(sem.available(), 0);
+    ASSERT_EQ(sem.used(), 2);
+
+    auto c = kj::evalNow([&] { return sem.acquire(); });
+    auto d = kj::evalNow([&] { return sem.acquire(); });
+
+    ASSERT_TRUE(a.poll(waitScope));
+    ASSERT_TRUE(b.poll(waitScope));
+    ASSERT_FALSE(c.poll(waitScope));
+    ASSERT_FALSE(d.poll(waitScope));
+
+    a = nullptr;
+    ASSERT_TRUE(c.poll(waitScope));
+    ASSERT_FALSE(d.poll(waitScope));
+
+    {
+        auto lock = b.wait(waitScope);
+        ASSERT_FALSE(d.poll(waitScope));
+    }
+
+    ASSERT_TRUE(d.poll(waitScope));
+
+    ASSERT_EQ(sem.available(), 0);
+    ASSERT_EQ(sem.used(), 2);
+    c = nullptr;
+    ASSERT_EQ(sem.available(), 1);
+    ASSERT_EQ(sem.used(), 1);
+    d = nullptr;
+    ASSERT_EQ(sem.available(), 2);
+    ASSERT_EQ(sem.used(), 0);
+}
+
+TEST(AsyncSemaphore, cancelledWaiter)
+{
+    kj::EventLoop loop;
+    kj::WaitScope waitScope(loop);
+
+    AsyncSemaphore sem(1);
+
+    auto a = kj::evalNow([&] { return sem.acquire(); });
+    auto b = kj::evalNow([&] { return sem.acquire(); });
+    auto c = kj::evalNow([&] { return sem.acquire(); });
+
+    ASSERT_TRUE(a.poll(waitScope));
+    ASSERT_FALSE(b.poll(waitScope));
+
+    b = nullptr;
+    a = nullptr;
+
+    ASSERT_TRUE(c.poll(waitScope));
+}
+
+}
diff --git a/tests/unit/meson.build b/tests/unit/meson.build
index 8ff0b5ec5..5baf10de2 100644
--- a/tests/unit/meson.build
+++ b/tests/unit/meson.build
@@ -39,6 +39,7 @@ liblixutil_test_support = declare_dependency(
 )
 
 libutil_tests_sources = files(
+  'libutil/async-semaphore.cc',
   'libutil/canon-path.cc',
   'libutil/checked-arithmetic.cc',
   'libutil/chunked-vector.cc',
@@ -76,6 +77,7 @@ libutil_tester = executable(
     liblixexpr_mstatic,
     liblixutil_test_support,
     nlohmann_json,
+    kj,
   ],
   cpp_pch : cpp_pch,
 )