Support MongoDB session-wide write concern (#3646)

* Initial work on write concern support, set for the lifetime of the session

* Add base64 encoded value support, include docs and tests

* Handle error from json.Unmarshal, fix test and docs

* Remove writeConcern struct, move JSON unmarshal to Initialize

* Return error on empty mapping of write_concern into mgo.Safe struct

plugins/database/mongodb/mongodb_test.go

@@ -16,6 +16,8 @@ import (
 
 const testMongoDBRole = `{ "db": "admin", "roles": [ { "role": "readWrite" } ] }`
 
+const testMongoDBWriteConcern = `{ "wmode": "majority", "wtimeout": 5000 }`
+
 func prepareMongoDBTestContainer(t *testing.T) (cleanup func(), retURL string) {
 	if os.Getenv("MONGODB_URL") != "" {
 		return func() {}, os.Getenv("MONGODB_URL")
@@ -129,6 +131,44 @@ func TestMongoDB_CreateUser(t *testing.T) {
 	}
 }
 
+func TestMongoDB_CreateUser_writeConcern(t *testing.T) {
+	cleanup, connURL := prepareMongoDBTestContainer(t)
+	defer cleanup()
+
+	connectionDetails := map[string]interface{}{
+		"connection_url": connURL,
+		"write_concern":  testMongoDBWriteConcern,
+	}
+
+	dbRaw, err := New()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	db := dbRaw.(*MongoDB)
+	err = db.Initialize(connectionDetails, true)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	statements := dbplugin.Statements{
+		CreationStatements: testMongoDBRole,
+	}
+
+	usernameConfig := dbplugin.UsernameConfig{
+		DisplayName: "test",
+		RoleName:    "test",
+	}
+
+	username, password, err := db.CreateUser(statements, usernameConfig, time.Now().Add(time.Minute))
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if err := testCredsExist(t, connURL, username, password); err != nil {
+		t.Fatalf("Could not connect with new credentials: %s", err)
+	}
+}
+
 func TestMongoDB_RevokeUser(t *testing.T) {
 	cleanup, connURL := prepareMongoDBTestContainer(t)
 	defer cleanup()