db: only allow one open connection for in memory databases

sqlite3 in memory databases do not support concurrent writes. Limit
number of open connections to prevent race conditions.

db/conn.go

@@ -67,7 +67,14 @@ func NewConnection(cfg Config) (*gorp.DbMap, error) {
 		if err != nil {
 			return nil, err
 		}
-		// NOTE(ericchiang): sqlite does NOT work with SetMaxIdleConns.
+		if u.Host == ":memory:" {
+			// NOTE(ericchiang): sqlite3 coordinates concurrent clients through file locks.
+			// In memory databases do not support concurrent calls. Limit the number of
+			// open connections to 1.
+			//
+			// See: https://www.sqlite.org/faq.html#q5
+			db.SetMaxOpenConns(1)
+		}
 		dialect = gorp.SqliteDialect{}
 	default:
 		return nil, errors.New("unrecognized database driver")

db/conn_test.go

@@ -0,0 +1,39 @@
+package db
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/coreos/dex/connector"
+)
+
+// TestConcurrentSqliteConns tests concurrent writes to a single in memory database.
+func TestConcurrentSqliteConns(t *testing.T) {
+	dbMap := NewMemDB()
+	repo := NewConnectorConfigRepo(dbMap)
+
+	var (
+		once sync.Once
+		wg   sync.WaitGroup
+	)
+
+	n := 1000
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		go func() {
+			configs := []connector.ConnectorConfig{
+				&connector.LocalConnectorConfig{ID: "local"},
+			}
+			// Setting connector configs both deletes and writes to a single table
+			// within a transaction.
+			if err := repo.Set(configs); err != nil {
+				// Don't print 1000 errors, only the first.
+				once.Do(func() {
+					t.Errorf("concurrent connections to sqlite3: %v", err)
+				})
+			}
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+}