Add lock around write operations in graph

The graph uses a persistent database connection so
a lock is required because our current sqlite3 driver
does not implement retry logic when the ErrBusy is received.

gograph/gograph.go

@@ -4,6 +4,7 @@ import (
 	"database/sql"
 	"fmt"
 	"path"
+	"sync"
 )
 
 const (
@@ -47,6 +48,7 @@ type WalkFunc func(fullPath string, entity *Entity) error
 // Graph database for storing entities and their relationships
 type Database struct {
 	conn *sql.DB
+	mux  sync.Mutex
 }
 
 // Create a new graph database initialized with a root entity
@@ -54,7 +56,7 @@ func NewDatabase(conn *sql.DB, init bool) (*Database, error) {
 	if conn == nil {
 		return nil, fmt.Errorf("Database connection cannot be nil")
 	}
-	db := &Database{conn}
+	db := &Database{conn: conn}
 
 	if init {
 		if _, err := conn.Exec(createEntityTable); err != nil {
@@ -99,7 +101,9 @@ func (db *Database) Close() error {
 
 // Set the entity id for a given path
 func (db *Database) Set(fullPath, id string) (*Entity, error) {
-	// FIXME: is rollback implicit when closing the connection?
+	db.mux.Lock()
+	defer db.mux.Unlock()
+
 	rollback := func() {
 		db.conn.Exec("ROLLBACK")
 	}
@@ -256,6 +260,9 @@ func (db *Database) RefPaths(id string) Edges {
 
 // Delete the reference to an entity at a given path
 func (db *Database) Delete(name string) error {
+	db.mux.Lock()
+	defer db.mux.Unlock()
+
 	if name == "/" {
 		return fmt.Errorf("Cannot delete root entity")
 	}
@@ -276,6 +283,9 @@ func (db *Database) Delete(name string) error {
 // Walk the graph to make sure all references to the entity
 // are removed and return the number of references removed
 func (db *Database) Purge(id string) (int, error) {
+	db.mux.Lock()
+	defer db.mux.Unlock()
+
 	rollback := func() {
 		db.conn.Exec("ROLLBACK")
 	}
@@ -310,6 +320,9 @@ func (db *Database) Purge(id string) (int, error) {
 
 // Rename an edge for a given path
 func (db *Database) Rename(currentName, newName string) error {
+	db.mux.Lock()
+	defer db.mux.Unlock()
+
 	parentPath, name := splitPath(currentName)
 	newParentPath, newEdgeName := splitPath(newName)
 

gograph/gograph_test.go

@@ -3,6 +3,7 @@ package gograph
 import (
 	_ "code.google.com/p/gosqlite/sqlite3"
 	"database/sql"
+	"fmt"
 	"os"
 	"path"
 	"strconv"
@@ -501,3 +502,39 @@ func TestGetNameWithTrailingSlash(t *testing.T) {
 		t.Fatalf("Entity should not be nil")
 	}
 }
+
+func TestConcurrentWrites(t *testing.T) {
+	db, dbpath := newTestDb(t)
+	defer destroyTestDb(dbpath)
+
+	errs := make(chan error, 2)
+
+	save := func(name string, id string) {
+		if _, err := db.Set(fmt.Sprintf("/%s", name), id); err != nil {
+			errs <- err
+		}
+		errs <- nil
+	}
+	purge := func(id string) {
+		if _, err := db.Purge(id); err != nil {
+			errs <- err
+		}
+		errs <- nil
+	}
+
+	save("/1", "1")
+
+	go purge("1")
+	go save("/2", "2")
+
+	any := false
+	for i := 0; i < 2; i++ {
+		if err := <-errs; err != nil {
+			any = true
+			t.Log(err)
+		}
+	}
+	if any {
+		t.Fatal()
+	}
+}