Close only waits for write transactions to finish

[vansante]

Hi,

This is a followup on: #80

I have been testing with closing BoltDB and ran into panic's in view transactions after calling Close(). After looking at the Close() code I think I can conclude that it waits for write transactions to finish by locking on db.rwlock but it does not wait for read transactions.

The only read lock a view transaction keeps open is the db.mmaplock, but the Close() function also only requires a read lock on that. So it does not wait for them. Am I correct in this conclusion?

If true, I think the documentation should be amended to reflect that it does not wait for read transactions, or the Close() function should acquire a full lock on db.mmaplock so that it also waits for read transactions. That said, I am not familiar enough with BoltDB's code to know what other consequences that might have.

I'm willing to submit a PR if you tell me which option you prefer :)

[vansante]

To illustrate the problem, I created a small test script:

package main

import (
	"log"
	"time"
	bolt "github.com/coreos/bbolt"
)

func main() {
	log.Println("Opening DB (update)")
	db, err := bolt.Open("my.db", 0600, nil)
	if err != nil {
		log.Fatal(err)
	}

	start := make(chan bool)
	go func() {
		db.Update(func(tx *bolt.Tx) error {
			close(start)
			time.Sleep(time.Second * 4)

			bk, err := tx.CreateBucket([]byte("NewBucket"))
			if err != nil {
				return err
			}

			return bk.Put([]byte("newKey"), []byte("newVal"))
		})
	}()

	// Wait until the transaction has begun
	<-start
	log.Println("Closing DB (update)")
	// This will block on the update transaction
	db.Close()
	log.Println("Closed DB (update)")

	log.Println("Opening DB (view)")
	db, err = bolt.Open("my.db", 0600, nil)
	if err != nil {
		log.Fatal(err)
	}

	start = make(chan bool)
	go func() {
		db.View(func(tx *bolt.Tx) error {
			close(start)
			time.Sleep(time.Second * 4)

			bk := tx.Bucket([]byte("NewBucket"))
			bk.Get([]byte("newKey"))

			bk = tx.Bucket([]byte("AnotherNotExistingBucket"))
			return nil
		})
	}()

	// Wait until the transaction has begun
	<-start
	log.Println("Closing DB (view)")
	// This will not block on the view transaction, and the view transaction could panic
	db.Close()
	log.Println("Closed DB (view)")
}

This gives the following output:

2018/03/29 14:17:19 Opening DB (update)
2018/03/29 14:17:19 Closing DB (update)
2018/03/29 14:17:23 Closed DB (update)
2018/03/29 14:17:23 Opening DB (view)
2018/03/29 14:17:23 Closing DB (view)
2018/03/29 14:17:23 Closed DB (view)

As you can clearly see there is a 4 second wait period in the closing of the update transaction, but there is not for the view transaction.

[xiang90]

probably it should wait for all transactions, not just writes.

[vansante]

@xiang90 Do you need anything more for this issue?

[vtolstov]

ping...

[vansante]

The PR is merged, so Im closing this :)