pkg/sorted/sqlkv: don't release gate until iterator is closed

Change-Id: I4c3f5406b89294c09fb1d64aaeffc3c581664acd

pkg/sorted/sqlkv/sqlkv.go

@@ -204,12 +204,10 @@ func (kv *KeyValue) Wipe() error {
 func (kv *KeyValue) Close() error { return kv.DB.Close() }
 
 func (kv *KeyValue) Find(start, end string) sorted.Iterator {
+	var releaseGate func() // nil if unused
 	if kv.Gate != nil {
 		kv.Gate.Start()
-		// TODO(mpl): looks like sqlite considers the db locked until we've closed
-		// the iterator, so we can't do anything else until then. We should probably
-		// move that Unlock to the closing of the iterator. Investigating.
-		defer kv.Gate.Done()
+		releaseGate = kv.Gate.Done
 	}
 	var rows *sql.Rows
 	var err error
@@ -227,6 +225,7 @@ func (kv *KeyValue) Find(start, end string) sorted.Iterator {
 		kv:          kv,
 		rows:        rows,
 		closeCheck:  leak.NewChecker(),
+		releaseGate: releaseGate,
 	}
 	return it
 }
@@ -240,6 +239,7 @@ type iter struct {
 	err error  // accumulated error, returned at Close
 
 	closeCheck  *leak.Checker
+	releaseGate func() // if non-nil, called on Close
 
 	rows *sql.Rows // if non-nil, the rows we're reading from
 
@@ -276,6 +276,9 @@ func (t *iter) Close() error {
 		t.rows.Close()
 		t.rows = nil
 	}
+	if t.releaseGate != nil {
+		t.releaseGate()
+	}
 	err := t.err
 	t.err = errClosed
 	return err