From 74575f2e67938e8f41022b897e9da1f43e400812 Mon Sep 17 00:00:00 2001
From: Vadim Skipin <vadim.skipin@clickhouse.com>
Date: Fri, 24 Jul 2026 18:43:29 +0000
Subject: [PATCH] Add FiberSequencer::reset

Rebase the counter to an arbitrary value, up or down, under a caller-
guaranteed quiescence contract: no registered waiter (asserted against
the waiter tree) and no concurrent operation. advance stays monotone;
reset serves callers that re-derive a baseline, like a storage
recovery attempt whose durable prefix legitimately regresses.
---
 include/silk/fibers/sequencer.h     |  6 ++++++
 src/fibers/sequencer.cpp            |  6 ++++++
 src/fibers/tests/sequencer-test.cpp | 25 +++++++++++++++++++++++++
 3 files changed, 37 insertions(+)

diff --git a/include/silk/fibers/sequencer.h b/include/silk/fibers/sequencer.h
index 39f8278..bc77f2c 100644
--- a/include/silk/fibers/sequencer.h
+++ b/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
diff --git a/src/fibers/sequencer.cpp b/src/fibers/sequencer.cpp
index 0e11e3c..5f2a92d 100644
--- a/src/fibers/sequencer.cpp
+++ b/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.
diff --git a/src/fibers/tests/sequencer-test.cpp b/src/fibers/tests/sequencer-test.cpp
index 80ca756..29116df 100644
--- a/src/fibers/tests/sequencer-test.cpp
+++ b/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;