From a346064eaf5de6d9cf6b279faa3bd476ee54e399 Mon Sep 17 00:00:00 2001
From: Dominic Wong <domwongemail@googlemail.com>
Date: Mon, 8 Jun 2020 16:19:22 +0100
Subject: [PATCH] Fix race when opening DB for first time (#1691)

---
 store/file/file.go | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/store/file/file.go b/store/file/file.go
index 71bd0f83..e6856289 100644
--- a/store/file/file.go
+++ b/store/file/file.go
@@ -105,7 +105,6 @@ func (f *fileStore) getDB(database, table string) (*fileHandle, error) {
 	}
 
 	k := key(database, table)
-
 	f.RLock()
 	fd, ok := f.handles[k]
 	f.RUnlock()
@@ -115,6 +114,13 @@ func (f *fileStore) getDB(database, table string) (*fileHandle, error) {
 		return fd, nil
 	}
 
+	// double check locking
+	f.Lock()
+	defer f.Unlock()
+	if fd, ok := f.handles[k]; ok {
+		return fd, nil
+	}
+
 	// create a directory /tmp/micro
 	dir := filepath.Join(DefaultDir, database)
 	// create the database handle
@@ -125,18 +131,16 @@ func (f *fileStore) getDB(database, table string) (*fileHandle, error) {
 	dbPath := filepath.Join(dir, fname)
 
 	// create new db handle
+	// Bolt DB only allows one process to open the file R/W so make sure we're doing this under a lock
 	db, err := bolt.Open(dbPath, 0700, &bolt.Options{Timeout: 5 * time.Second})
 	if err != nil {
 		return nil, err
 	}
-
-	f.Lock()
 	fd = &fileHandle{
 		key: k,
 		db:  db,
 	}
 	f.handles[k] = fd
-	f.Unlock()
 
 	return fd, nil
 }