prevent closing concurrently with other operations.

Add a `sync.RWMutex` protecting the Close operation.
Follows the same pattern as in go-ds-badger.
Address #29 / ipfs/kubo#6880

Author: willscott

datastore.go

@@ -3,6 +3,7 @@ package leveldb
 import (
 	"os"
 	"path/filepath"
+	"sync"
 
 	ds "github.com/ipfs/go-datastore"
 	dsq "github.com/ipfs/go-datastore/query"
@@ -16,8 +17,9 @@ import (
 
 type Datastore struct {
 	*accessor
-	DB   *leveldb.DB
-	path string
+	DB      *leveldb.DB
+	path    string
+	closeLk sync.RWMutex
 }
 
 var _ ds.Datastore = (*Datastore)(nil)
@@ -52,11 +54,13 @@ func NewDatastore(path string, opts *Options) (*Datastore, error) {
 		return nil, err
 	}
 
-	return &Datastore{
+	ds := Datastore{
 		accessor: &accessor{ldb: db, syncWrites: true},
 		DB:       db,
 		path:     path,
-	}, nil
+	}
+	ds.accessor.closeLk = &ds.closeLk
+	return &ds, nil
 }
 
 // An extraction of the common interface between LevelDB Transactions and the DB itself.
@@ -74,9 +78,12 @@ type levelDbOps interface {
 type accessor struct {
 	ldb        levelDbOps
 	syncWrites bool
+	closeLk    *sync.RWMutex
 }
 
 func (a *accessor) Put(key ds.Key, value []byte) (err error) {
+	a.closeLk.RLock()
+	defer a.closeLk.RUnlock()
 	return a.ldb.Put(key.Bytes(), value, &opt.WriteOptions{Sync: a.syncWrites})
 }
 
@@ -85,6 +92,8 @@ func (a *accessor) Sync(prefix ds.Key) error {
 }
 
 func (a *accessor) Get(key ds.Key) (value []byte, err error) {
+	a.closeLk.RLock()
+	defer a.closeLk.RUnlock()
 	val, err := a.ldb.Get(key.Bytes(), nil)
 	if err != nil {
 		if err == leveldb.ErrNotFound {
@@ -96,18 +105,26 @@ func (a *accessor) Get(key ds.Key) (value []byte, err error) {
 }
 
 func (a *accessor) Has(key ds.Key) (exists bool, err error) {
+	a.closeLk.RLock()
+	defer a.closeLk.RUnlock()
 	return a.ldb.Has(key.Bytes(), nil)
 }
 
-func (d *accessor) GetSize(key ds.Key) (size int, err error) {
-	return ds.GetBackedSize(d, key)
+func (a *accessor) GetSize(key ds.Key) (size int, err error) {
+	a.closeLk.RLock()
+	defer a.closeLk.RUnlock()
+	return ds.GetBackedSize(a, key)
 }
 
 func (a *accessor) Delete(key ds.Key) (err error) {
+	a.closeLk.RLock()
+	defer a.closeLk.RUnlock()
 	return a.ldb.Delete(key.Bytes(), &opt.WriteOptions{Sync: a.syncWrites})
 }
 
 func (a *accessor) Query(q dsq.Query) (dsq.Results, error) {
+	a.closeLk.RLock()
+	defer a.closeLk.RUnlock()
 	var rnge *util.Range
 
 	// make a copy of the query for the fallback naive query implementation.
@@ -159,6 +176,8 @@ func (a *accessor) Query(q dsq.Query) (dsq.Results, error) {
 // DiskUsage returns the current disk size used by this levelDB.
 // For in-mem datastores, it will return 0.
 func (d *Datastore) DiskUsage() (uint64, error) {
+	d.closeLk.RLock()
+	defer d.closeLk.RUnlock()
 	if d.path == "" { // in-mem
 		return 0, nil
 	}
@@ -182,19 +201,23 @@ func (d *Datastore) DiskUsage() (uint64, error) {
 
 // LevelDB needs to be closed.
 func (d *Datastore) Close() (err error) {
+	d.closeLk.Lock()
+	defer d.closeLk.Unlock()
 	return d.DB.Close()
 }
 
 type leveldbBatch struct {
 	b          *leveldb.Batch
 	db         *leveldb.DB
+	closeLk    *sync.RWMutex
 	syncWrites bool
 }
 
 func (d *Datastore) Batch() (ds.Batch, error) {
 	return &leveldbBatch{
 		b:          new(leveldb.Batch),
 		db:         d.DB,
+		closeLk:    &d.closeLk,
 		syncWrites: d.syncWrites,
 	}, nil
 }
@@ -205,6 +228,8 @@ func (b *leveldbBatch) Put(key ds.Key, value []byte) error {
 }
 
 func (b *leveldbBatch) Commit() error {
+	b.closeLk.RLock()
+	defer b.closeLk.RUnlock()
 	return b.db.Write(b.b, &opt.WriteOptions{Sync: b.syncWrites})
 }
 
@@ -220,18 +245,24 @@ type transaction struct {
 }
 
 func (t *transaction) Commit() error {
+	t.closeLk.RLock()
+	defer t.closeLk.RUnlock()
 	return t.tx.Commit()
 }
 
 func (t *transaction) Discard() {
+	t.closeLk.RLock()
+	defer t.closeLk.RUnlock()
 	t.tx.Discard()
 }
 
 func (d *Datastore) NewTransaction(readOnly bool) (ds.Txn, error) {
+	d.closeLk.RLock()
+	defer d.closeLk.RUnlock()
 	tx, err := d.DB.OpenTransaction()
 	if err != nil {
 		return nil, err
 	}
-	accessor := &accessor{ldb: tx, syncWrites: false}
+	accessor := &accessor{ldb: tx, syncWrites: false, closeLk: &d.closeLk}
 	return &transaction{accessor, tx}, nil
 }