From aad612d097bb2377369aa20e55ccc3250599a929 Mon Sep 17 00:00:00 2001
From: "Sascha L. Teichmann" <sascha.teichmann@intevation.de>
Date: Mon, 20 Mar 2017 12:41:24 +0100
Subject: [PATCH] Experimental increased priorities for typical mt-server ops.

---
 cmd/mtredisalize/leveldb.go | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/cmd/mtredisalize/leveldb.go b/cmd/mtredisalize/leveldb.go
index f218ba4..1e9f1a5 100644
--- a/cmd/mtredisalize/leveldb.go
+++ b/cmd/mtredisalize/leveldb.go
@@ -23,6 +23,9 @@ type LevelDBBackend struct {
 
 	changeTracker *changeTracker
 	mutex         sync.RWMutex
+
+	priority      *sync.Cond
+	priorityCount int
 }
 
 type LevelDBSession struct {
@@ -72,7 +75,8 @@ func NewLeveDBBackend(
 		interleaved:   interleaved,
 		encoder:       encoder,
 		decoder:       decoder,
-		changeTracker: changeTracker}
+		changeTracker: changeTracker,
+		priority:      sync.NewCond(new(sync.Mutex))}
 
 	if !interleaved {
 		if err = ldb.buildCoverage(); err != nil {
@@ -84,6 +88,28 @@ func NewLeveDBBackend(
 	return
 }
 
+func (ldb *LevelDBBackend) suspend() {
+	ldb.priority.L.Lock()
+	for ldb.priorityCount > 0 {
+		ldb.priority.Wait()
+	}
+	ldb.priority.L.Unlock()
+}
+
+func (ldb *LevelDBBackend) grab() {
+	ldb.priority.L.Lock()
+	ldb.priorityCount++
+	ldb.priority.L.Unlock()
+}
+
+func (ldb *LevelDBBackend) ungrab() {
+	ldb.priority.L.Lock()
+	if ldb.priorityCount--; ldb.priorityCount <= 0 {
+		ldb.priority.Broadcast()
+	}
+	ldb.priority.L.Unlock()
+}
+
 func (ldb *LevelDBBackend) buildCoverage() error {
 	log.Println("INFO: Start building coverage index (this may take some time)...")
 
@@ -169,6 +195,9 @@ func (ldbs *LevelDBSession) Fetch(hash, key []byte) (value []byte, err error) {
 		return
 	}
 	ldbs.backend.doRead(func(db *leveldb.DB) {
+		ldbs.backend.grab()
+		defer ldbs.backend.ungrab()
+
 		ro := leveldb.NewReadOptions()
 		value, err = ldbs.backend.db.Get(ro, key)
 		//if err != nil {
@@ -202,6 +231,8 @@ func (ldbs *LevelDBSession) Store(hash, key, value []byte) (exists bool, err err
 		return
 	}
 	ldbs.backend.doWrite(func(db *leveldb.DB) {
+		ldbs.backend.grab()
+		defer ldbs.backend.ungrab()
 		if exists, err = keyExists(db, key); err != nil {
 			return
 		}
@@ -424,6 +455,7 @@ func (ldbs *LevelDBSession) interleavedSpatialQuery(
 		//log.Printf("seeking to: %d\n", zmin)
 		it.Seek(common.ToBigEndian(zmin))
 		for it.Valid() {
+			ldbs.backend.suspend()
 			zcode := common.FromBigEndian(it.Key())
 
 			if zcode > zmax {