Set MaxIdleConns to reduce connection churn (postgresql physical) (#6967)

* Set MaxIdleConns to reduce connection churn (postgresql physical)

* Make new  "max_idle_connection" config option for physical postgresql

* Add docs for "max_idle_connections" for postgresql storage

* Add minimum version to docs for max_idle_connections

physical/postgresql/postgresql.go

@@ -114,6 +114,18 @@ func NewPostgreSQLBackend(conf map[string]string, logger log.Logger) (physical.B
 		maxParInt = physical.DefaultParallelOperations
 	}
 
+	maxIdleConnsStr, maxIdleConnsIsSet := conf["max_idle_connections"]
+	var maxIdleConns int
+	if maxIdleConnsIsSet {
+		maxIdleConns, err = strconv.Atoi(maxIdleConnsStr)
+		if err != nil {
+			return nil, errwrap.Wrapf("failed parsing max_idle_connections parameter: {{err}}", err)
+		}
+		if logger.IsDebug() {
+			logger.Debug("max_idle_connections set", "max_idle_connections", maxIdleConnsStr)
+		}
+	}
+
 	// Create PostgreSQL handle for the database.
 	db, err := sql.Open("postgres", connURL)
 	if err != nil {
@@ -121,6 +133,10 @@ func NewPostgreSQLBackend(conf map[string]string, logger log.Logger) (physical.B
 	}
 	db.SetMaxOpenConns(maxParInt)
 
+	if maxIdleConnsIsSet {
+		db.SetMaxIdleConns(maxIdleConns)
+	}
+
 	// Determine if we should use a function to work around lack of upsert (versions < 9.5)
 	var upsertAvailable bool
 	upsertAvailableQuery := "SELECT current_setting('server_version_num')::int >= 90500"

physical/postgresql/postgresql_test.go

@@ -100,6 +100,20 @@ func TestPostgreSQLBackend(t *testing.T) {
 	}
 }
 
+func TestPostgreSQLBackendMaxIdleConnectionsParameter(t *testing.T) {
+	_, err := NewPostgreSQLBackend(map[string]string{
+		"connection_url":       "some connection url",
+		"max_idle_connections": "bad param",
+	}, logging.NewVaultLogger(log.Debug))
+	if err == nil {
+		t.Error("Expected invalid max_idle_connections param to return error")
+	}
+	expectedErrStr := "failed parsing max_idle_connections parameter: strconv.Atoi: parsing \"bad param\": invalid syntax"
+	if err.Error() != expectedErrStr {
+		t.Errorf("Expected: \"%s\" but found \"%s\"", expectedErrStr, err.Error())
+	}
+}
+
 // Similar to testHABackend, but using internal implementation details to
 // trigger the lock failure scenario by setting the lock renew period for one
 // of the locks to a higher value than the lock TTL.