Fix compacting disable/enable

Enabling and disabling compaction no longer blocks are potentially
causes panics.

db.go

@@ -100,7 +100,6 @@ type DB struct {
 	opts    *Options
 
 	// Mutex for that must be held when modifying the general block layout.
-	// cmtx must be held before acquiring it.
 	mtx    sync.RWMutex
 	blocks []Block
 
@@ -118,7 +117,7 @@ type DB struct {
 
 	// cmtx is used to control compactions and deletions.
 	cmtx               sync.Mutex
-	compacting bool
+	compactionsEnabled bool
 }
 
 type dbMetrics struct {
@@ -203,7 +202,7 @@ func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db
 		compactc:           make(chan struct{}, 1),
 		donec:              make(chan struct{}),
 		stopc:              make(chan struct{}),
-		compacting: true,
+		compactionsEnabled: true,
 	}
 	db.metrics = newDBMetrics(db, r)
 
@@ -371,6 +370,10 @@ func (db *DB) compact() (changes bool, err error) {
 	db.cmtx.Lock()
 	defer db.cmtx.Unlock()
 
+	if !db.compactionsEnabled {
+		return false, nil
+	}
+
 	// Check whether we have pending head blocks that are ready to be persisted.
 	// They have the highest priority.
 	for _, h := range db.completedHeads() {
@@ -579,21 +582,21 @@ func (db *DB) Close() error {
 
 // DisableCompactions disables compactions.
 func (db *DB) DisableCompactions() {
-	if db.compacting {
 	db.cmtx.Lock()
-		db.compacting = false
+	defer db.cmtx.Unlock()
+
+	db.compactionsEnabled = false
 	db.logger.Log("msg", "compactions disabled")
 }
-}
 
 // EnableCompactions enables compactions.
 func (db *DB) EnableCompactions() {
-	if !db.compacting {
-		db.cmtx.Unlock()
-		db.compacting = true
+	db.cmtx.Lock()
+	defer db.cmtx.Unlock()
+
+	db.compactionsEnabled = true
 	db.logger.Log("msg", "compactions enabled")
 }
-}
 
 // Snapshot writes the current data to the directory.
 func (db *DB) Snapshot(dir string) error {