From ce00a07d88f3bc529ceca44d203b809c075e8c15 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Zdyba=C5=82?= <tomek@zdybal.lap.pl>
Date: Wed, 5 Jul 2023 00:05:28 +0200
Subject: [PATCH] refactor(sync): get rid of RWMutex and TryLock

---
 sync/sync.go             |  2 +-
 sync/sync_getter.go      | 24 +++++++++++++++++-------
 sync/sync_getter_test.go |  1 +
 3 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/sync/sync.go b/sync/sync.go
index ce66b6a9..733baac9 100644
--- a/sync/sync.go
+++ b/sync/sync.go
@@ -72,7 +72,7 @@ func NewSyncer[H header.Header](
 	return &Syncer[H]{
 		sub:         sub,
 		store:       syncStore[H]{Store: store},
-		getter:      syncGetter[H]{Getter: getter},
+		getter:      *newSyncGetter(getter),
 		triggerSync: make(chan struct{}, 1), // should be buffered
 		Params:      &params,
 	}, nil
diff --git a/sync/sync_getter.go b/sync/sync_getter.go
index 7b81ee01..88f6df96 100644
--- a/sync/sync_getter.go
+++ b/sync/sync_getter.go
@@ -10,32 +10,36 @@ import (
 
 // syncGetter is a Getter wrapper that ensure only one Head call happens at the time
 type syncGetter[H header.Header] struct {
-	getterLk   sync.RWMutex
+	getterLk   sync.Mutex
+	getterCond *sync.Cond
 	isGetterLk atomic.Bool
 	header.Getter[H]
 }
 
+func newSyncGetter[H header.Header](g header.Getter[H]) *syncGetter[H] {
+	getter := syncGetter[H]{Getter: g}
+	getter.getterCond = sync.NewCond(&getter.getterLk)
+	return &getter
+}
+
 // Lock locks the getter for single user.
 // Reports 'true' if the lock was held by the current routine.
 // Does not require unlocking on 'false'.
 func (se *syncGetter[H]) Lock() bool {
 	// the lock construction here ensures only one routine is freed at a time
 	// while others wait via Rlock
-	locked := se.getterLk.TryLock()
+	locked := se.isGetterLk.CompareAndSwap(false, true)
 	if !locked {
-		se.getterLk.RLock()
-		defer se.getterLk.RUnlock()
-		return false
+		se.await()
 	}
-	se.isGetterLk.Store(locked)
 	return locked
 }
 
 // Unlock unlocks the getter.
 func (se *syncGetter[H]) Unlock() {
 	se.checkLock("Unlock without preceding Lock on syncGetter")
-	se.getterLk.Unlock()
 	se.isGetterLk.Store(false)
+	se.getterCond.Broadcast()
 }
 
 // Head must be called with held Lock.
@@ -50,3 +54,9 @@ func (se *syncGetter[H]) checkLock(msg string) {
 		panic(msg)
 	}
 }
+
+func (se *syncGetter[H]) await() {
+	se.getterLk.Lock()
+	se.getterCond.Wait()
+	se.getterLk.Unlock()
+}
diff --git a/sync/sync_getter_test.go b/sync/sync_getter_test.go
index 06fcd7ad..5989f812 100644
--- a/sync/sync_getter_test.go
+++ b/sync/sync_getter_test.go
@@ -20,6 +20,7 @@ func TestSyncGetterHead(t *testing.T) {
 
 	fex := &fakeGetter[*headertest.DummyHeader]{}
 	sex := &syncGetter[*headertest.DummyHeader]{Getter: fex}
+	sex.getterCond = sync.NewCond(&sex.getterLk)
 
 	var wg sync.WaitGroup
 	for i := 0; i < 100; i++ {