fix(fsrepo/datastore) allow goroutines to share the datastore.

doh! I forgot to make sure leveldb is only opened once. thanks for catching this @mappum

* You may be wondering why we don't just share pointers to FSRepos. We
  want to manage the lifecycle of the FSRepo by tracking its `state`.
  Thus each FSRepo/goroutine requires private instance variables. For
  this reason, each `fsrepo.At(p)` caller must get its own goroutine.

* There's a test in `fsrepo` because callers desire the ability to Open
  from multiple goroutines. There's a test in `component` because this
  is where the actual work needs to go in order to provide the desired
  contract. If the `component` package moves, the assurances need to
  move along with it.

cc @whyrusleeping @jbenet

side note: there are a couple packages in FSRepo that it might be
worthwhile to extract once the dust settles on this feature-set.

repo/fsrepo/component/datastore.go

 package component
 
 import (
+	"errors"
+	"sync"
+
 	datastore "github.com/jbenet/go-ipfs/Godeps/_workspace/src/github.com/jbenet/go-datastore"
 	levelds "github.com/jbenet/go-ipfs/Godeps/_workspace/src/github.com/jbenet/go-datastore/leveldb"
 	ldbopts "github.com/jbenet/go-ipfs/Godeps/_workspace/src/github.com/syndtr/goleveldb/leveldb/opt"
 	config "github.com/jbenet/go-ipfs/repo/config"
+	counter "github.com/jbenet/go-ipfs/repo/fsrepo/counter"
 	dir "github.com/jbenet/go-ipfs/repo/fsrepo/dir"
 	util "github.com/jbenet/go-ipfs/util"
 	ds2 "github.com/jbenet/go-ipfs/util/datastore2"
 	debugerror "github.com/jbenet/go-ipfs/util/debugerror"
 )
 
-var _ Component = &DatastoreComponent{}
-var _ Initializer = InitDatastoreComponent
-var _ InitializationChecker = DatastoreComponentIsInitialized
+var (
+	_ Component             = &DatastoreComponent{}
+	_ Initializer           = InitDatastoreComponent
+	_ InitializationChecker = DatastoreComponentIsInitialized
+
+	dsLock         sync.Mutex // protects openersCounter and datastores
+	openersCounter *counter.Openers
+	datastores     map[string]ds2.ThreadSafeDatastoreCloser
+)
+
+func init() {
+	openersCounter = counter.NewOpenersCounter()
+	datastores = make(map[string]ds2.ThreadSafeDatastoreCloser)
+}
 
 func InitDatastoreComponent(path string, conf *config.Config) error {
 	// The actual datastore contents are initialized lazily when Opened.
@@ -41,24 +56,57 @@ func DatastoreComponentIsInitialized(path string) bool {
 }
 
 // DatastoreComponent abstracts the datastore component of the FSRepo.
-// NB: create with makeDatastoreComponent function.
 type DatastoreComponent struct {
-	path string
-	ds   ds2.ThreadSafeDatastoreCloser
+	path string                        // required
+	ds   ds2.ThreadSafeDatastoreCloser // assigned when repo is opened
 }
 
+func (dsc *DatastoreComponent) SetPath(p string)                         { dsc.path = p }
+func (dsc *DatastoreComponent) Datastore() datastore.ThreadSafeDatastore { return dsc.ds }
+
 // Open returns an error if the config file is not present.
 func (dsc *DatastoreComponent) Open() error {
+
+	dsLock.Lock()
+	defer dsLock.Unlock()
+
+	// if no other goroutines have the datastore Open, initialize it and assign
+	// it to the package-scoped map for the goroutines that follow.
+	if openersCounter.NumOpeners(dsc.path) == 0 {
 		ds, err := levelds.NewDatastore(dsc.path, &levelds.Options{
 			Compression: ldbopts.NoCompression,
 		})
 		if err != nil {
-		return err
+			return errors.New("unable to open leveldb datastore")
+		}
+		datastores[dsc.path] = ds
+	}
+
+	// get the datastore from the package-scoped map and record self as an
+	// opener.
+	ds, dsIsPresent := datastores[dsc.path]
+	if !dsIsPresent {
+		// This indicates a programmer error has occurred.
+		return errors.New("datastore should be available, but it isn't")
 	}
 	dsc.ds = ds
+	openersCounter.AddOpener(dsc.path) // only after success
 	return nil
 }
 
-func (dsc *DatastoreComponent) Close() error                             { return dsc.ds.Close() }
-func (dsc *DatastoreComponent) SetPath(p string)                         { dsc.path = p }
-func (dsc *DatastoreComponent) Datastore() datastore.ThreadSafeDatastore { return dsc.ds }
+func (dsc *DatastoreComponent) Close() error {
+
+	dsLock.Lock()
+	defer dsLock.Unlock()
+
+	// decrement the Opener count. if this goroutine is the last, also close
+	// the underlying datastore (and remove its reference from the map)
+
+	openersCounter.RemoveOpener(dsc.path)
+
+	if openersCounter.NumOpeners(dsc.path) == 0 {
+		delete(datastores, dsc.path) // remove the reference
+		return dsc.ds.Close()
+	}
+	return nil
+}

repo/fsrepo/component/datastore_test.go

+package component
+
+import (
+	"io/ioutil"
+	"path/filepath"
+	"testing"
+
+	"github.com/jbenet/go-ipfs/repo/fsrepo/assert"
+)
+
+// swap arg order
+func testRepoPath(t *testing.T, path ...string) string {
+	name, err := ioutil.TempDir("", filepath.Join(path...))
+	if err != nil {
+		t.Fatal(err)
+	}
+	return name
+}
+
+func TestOpenMoreThanOnceInSameProcess(t *testing.T) {
+	t.Parallel()
+	path := testRepoPath(t)
+	dsc1 := DatastoreComponent{path: path}
+	dsc2 := DatastoreComponent{path: path}
+	assert.Nil(dsc1.Open(), t, "first repo should open successfully")
+	assert.Nil(dsc2.Open(), t, "second repo should open successfully")
+
+	assert.Nil(dsc1.Close(), t)
+	assert.Nil(dsc2.Close(), t)
+}

repo/fsrepo/fsrepo_test.go

@@ -125,3 +125,17 @@ func TestDatastorePersistsFromRepoToRepo(t *testing.T) {
 	assert.Nil(r2.Close(), t)
 	assert.True(bytes.Compare(expected, actual) == 0, t, "data should match")
 }
+
+func TestOpenMoreThanOnceInSameProcess(t *testing.T) {
+	t.Parallel()
+	path := testRepoPath("", t)
+	assert.Nil(Init(path, &config.Config{}), t)
+
+	r1 := At(path)
+	r2 := At(path)
+	assert.Nil(r1.Open(), t, "first repo should open successfully")
+	assert.Nil(r2.Open(), t, "second repo should open successfully")
+
+	assert.Nil(r1.Close(), t)
+	assert.Nil(r2.Close(), t)
+}