Cleaned up some code (#706)

Made some changes to improve the quality of the code and comments:

* Only generate the defer code to release the lock on the value directory if it is different from the main directory
* Removed a period from ErrValueThreshold to keep it consistent with the other errors
* Removed an unused image
* Make explicit that if the directory or the value directory don't exist, they will try to be created
* Move a switch case to the appropriate place
* Replace ts += 1 for t++

db.go

@@ -223,12 +223,12 @@ func Open(opt Options) (db *DB, err error) {
 		if err != nil {
 			return nil, err
 		}
+		defer func() {
+			if valueDirLockGuard != nil {
+				_ = valueDirLockGuard.release()
+			}
+		}()
 	}
-	defer func() {
-		if valueDirLockGuard != nil {
-			_ = valueDirLockGuard.release()
-		}
-	}()
 	if !(opt.ValueLogFileSize <= 2<<30 && opt.ValueLogFileSize >= 1<<20) {
 		return nil, ErrValueLogSize
 	}

errors.go

@@ -27,7 +27,7 @@ var (
 
 	// ErrValueThreshold is returned when ValueThreshold is set to a value close to or greater than
 	// uint16.
-	ErrValueThreshold = errors.New("Invalid ValueThreshold, must be lower than uint16.")
+	ErrValueThreshold = errors.New("Invalid ValueThreshold, must be lower than uint16")
 
 	// ErrKeyNotFound is returned when key isn't found on a txn.Get.
 	ErrKeyNotFound = errors.New("Key not found")

options.go

@@ -31,10 +31,11 @@ import (
 type Options struct {
 	// 1. Mandatory flags
 	// -------------------
-	// Directory to store the data in. Should exist and be writable.
+	// Directory to store the data in. If it doesn't exist, Badger will
+	// try to create it for you.
 	Dir string
-	// Directory to store the value log in. Can be the same as Dir. Should
-	// exist and be writable.
+	// Directory to store the value log in. Can be the same as Dir. If it
+	// doesn't exist, Badger will try to create it for you.
 	ValueDir string
 
 	// 2. Frequently modified flags

txn.go

@@ -616,13 +616,13 @@ func runTxnCallback(cb *txnCb) {
 	switch {
 	case cb == nil:
 		panic("txn callback is nil")
+	case cb.user == nil:
+		panic("Must have caught a nil callback for txn.CommitWith")
 	case cb.err != nil:
 		cb.user(cb.err)
 	case cb.commit != nil:
 		err := cb.commit()
 		cb.user(err)
-	case cb.user == nil:
-		panic("Must have caught a nil callback for txn.CommitWith")
 	default:
 		cb.user(nil)
 	}

value_test.go

@@ -102,7 +102,7 @@ func TestValueGCManaged(t *testing.T) {
 
 	var ts uint64
 	newTs := func() uint64 {
-		ts += 1
+		ts++
 		return ts
 	}