semaphore: add support for multiple units in wait() and post()

Use Nadav's idea of iterating over the list and selecting wait records
that fit the available units.

core/semaphore.cc

@@ -6,34 +6,35 @@ semaphore::semaphore(unsigned val)
 {
 }
 
-void semaphore::post()
+void semaphore::post(unsigned units)
 {
-    auto wr = with_lock(*_mtx, [this] () -> wait_record* {
-        if (_waiters.empty()) {
-            ++_val;
-            return nullptr;
+    with_lock(*_mtx, [=] {
+        _val += units;
+        auto i = _waiters.begin();
+        while (_val > 0 && i != _waiters.end()) {
+            auto p = i++;
+            auto& wr = *p;
+            if (wr->units <= _val) {
+                _val -= wr->units;
+                wr->owner->wake();
+                wr->owner = nullptr;
+                _waiters.erase(p);
+            }
         }
-        auto wr = _waiters.front();
-        _waiters.pop_front();
-        return wr;
     });
-    if (wr) {
-        auto t = wr->owner;
-        wr->owner = nullptr;
-        t->wake();
-    }
 }
 
-void semaphore::wait()
+void semaphore::wait(unsigned units)
 {
     bool wait = false;
     wait_record wr;
     wr.owner = nullptr;
     with_lock(*_mtx, [&] {
-        if (_val > 0) {
-            --_val;
+        if (_val >= units) {
+            _val -= units;
         } else {
             wr.owner = sched::thread::current();
+            wr.units = units;
             _waiters.push_back(&wr);
             wait = true;
         }
@@ -43,12 +44,12 @@ void semaphore::wait()
         sched::thread::wait_until([&] { return !wr.owner; });
 }
 
-bool semaphore::trywait()
+bool semaphore::trywait(unsigned units)
 {
     bool ok = false;
     with_lock(*_mtx, [&] {
-        if (_val > 0) {
-            --_val;
+        if (_val > units) {
+            _val -= units;
             ok = true;
         }
     });

include/osv/semaphore.hh

@@ -9,14 +9,15 @@
 class semaphore {
 public:
     explicit semaphore(unsigned val);
-    void post();
-    void wait();
-    bool trywait();
+    void post(unsigned units = 1);
+    void wait(unsigned units = 1);
+    bool trywait(unsigned units = 1);
 private:
     unsigned _val;
     std::unique_ptr<mutex> _mtx;
     struct wait_record {
         sched::thread* owner;
+        unsigned units;
     };
     std::list<wait_record*> _waiters;
 };