Avoid getting stuck forever on monitor.Signal (#1976)

lib/monitor.toit

@@ -128,6 +128,7 @@ A signal synchronization primitive.
 This class must not be extended.
 */
 monitor Signal:
+  waiters_ /int := 0
   current_ /int := 0
   awaited_ /int := 0
 
@@ -169,17 +170,20 @@ monitor Signal:
 
   // Helper method for condition waiting.
   wait_ [condition] -> none:
-    while true:
-      awaited := awaited_
-      if current_ == awaited:
+    waiters_++
+    try:
+      while true:
+        awaited := awaited_
+        awaited_ = ++awaited
+        await: current_ >= awaited
+        if condition.call: return
+    finally:
+      if waiters_-- == 1:
         // No other task is waiting for this signal to be raised,
         // so it is safe to reset the counters. This helps avoid
         // the ever increasing counter issue that may lead to poor
         // performance in (very) extreme cases.
-        current_ = awaited = 0
-      awaited_ = ++awaited
-      await: current_ >= awaited
-      if condition.call: return
+        current_ = awaited_ = 0
 
 /**
 A synchronization gate.

src/primitive_flash_registry.cc

@@ -208,7 +208,10 @@ PRIMITIVE(grant_access) {
   if (!grant) FAIL(MALLOC_FAILED);
   Locker locker(OS::global_mutex());
   for (auto it : grants) {
-    if (it->offset() == offset && it->size() == size) FAIL(ALREADY_IN_USE);
+    if (it->offset() == offset && it->size() == size) {
+      delete grant;
+      FAIL(ALREADY_IN_USE);
+    }
   }
   grants.prepend(grant);
   return process->null_object();

tests/monitor-test.toit

@@ -31,6 +31,7 @@ run:
   test-process-messages-on-leave
   test-gate
   test-latch
+  test-signal
 
 monitor A:
   foo-ready := false
@@ -487,6 +488,40 @@ test-latch:
         l3.set --exception exception
   expect-throw 99: l3.get
 
+test-signal:
+  done := Semaphore
+  order := []
+  signal := Signal
+  t0 := false
+  t1 := false
+  t2 := false
+  task::
+    signal.wait: t0
+    order.add 0
+    done.up
+  task::
+    signal.wait: t1
+    order.add 1
+    done.up
+  // Make sure we can raise the signal here without making
+  // progress and without getting any of the tasks already
+  // waiting stuck.
+  signal.raise
+  task::
+    signal.wait: t2
+    order.add 2
+    done.up
+  t0 = true
+  signal.raise
+  done.down
+  t1 = true
+  signal.raise
+  done.down
+  t2 = true
+  signal.raise
+  done.down
+  expect-list-equals [0, 1, 2] order
+
 test-semaphore:
   semaphore := Semaphore
   expect-equals 0 semaphore.count