Add FiberSequencer::reset

include/silk/fibers/sequencer.h

@@ -126,6 +126,12 @@ class FiberSequencer
         return true;
     }
 
+    /**
+     * Rebase the counter to @p value, up or down. The caller guarantees quiescence: no registered waiter
+     * and no concurrent increment / advance / wait.
+     */
+    void reset(uint64_t value) noexcept;
+
     /**
      * Transition into the stopped state and wake every unreached waiter with
      * ECANCELED. After this, every unreached wait completes with ECANCELED

src/fibers/sequencer.cpp

@@ -7,6 +7,12 @@
 namespace silk
 {
 
+void FiberSequencer::reset(uint64_t value) noexcept
+{
+    SILK_ASSERT(waiters.empty());
+    counter.store(value, std::memory_order_release);
+}
+
 void FiberSequencer::registerWaiter(uint64_t token, Future * future) noexcept
 {
     // Slow path: register future in the request queue for the next combiner to process.

src/fibers/tests/sequencer-test.cpp

@@ -47,6 +47,31 @@ TEST(FiberSequencer, waitAlreadySatisfied)
     EXPECT_EQ(sequencer.wait(1), 0);
 }
 
+TEST(FiberSequencer, resetRebasesBelowCounter)
+{
+    FiberSequencer sequencer;
+    bool advanced = sequencer.advance(10);
+    ASSERT_TRUE(advanced);
+
+    sequencer.reset(3);
+    ASSERT_EQ(sequencer.get(), 3u);
+
+    // A wait at or below the rebased counter completes immediately.
+    int r = sequencer.wait(3);
+    ASSERT_EQ(r, 0);
+
+    // A wait above it parks until advance reaches the token again.
+    FiberSequencer::Future future;
+    sequencer.wait(4, &future);
+    int err;
+    ASSERT_FALSE(future.isSet(&err));
+
+    advanced = sequencer.advance(4);
+    ASSERT_TRUE(advanced);
+    ASSERT_TRUE(future.isSet(&err));
+    ASSERT_EQ(err, 0);
+}
+
 TEST(FiberSequencer, stopCancelsUnreachedWaiters)
 {
     FiberSequencer sequencer;