Check unique constraint errors when manually inserting migrations (#2712)

This should avoid unnecessary logging on startup if the migration (were
we need `InsertMigration`) was already executed.
This now checks for "unique constraint errors" for SQLite and Postgres
and fails the startup process if the migration couldn't be manually
inserted for some other reason.

internal/sqlutil/unique_constraint.go

@@ -0,0 +1,38 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !wasm
+// +build !wasm
+
+package sqlutil
+
+import (
+	"github.com/lib/pq"
+	"github.com/mattn/go-sqlite3"
+)
+
+// IsUniqueConstraintViolationErr returns true if the error is an unique_violation error
+func IsUniqueConstraintViolationErr(err error) bool {
+	switch e := err.(type) {
+	case *pq.Error:
+		return e.Code == "23505"
+	case pq.Error:
+		return e.Code == "23505"
+	case *sqlite3.Error:
+		return e.Code == sqlite3.ErrConstraint
+	case sqlite3.Error:
+		return e.Code == sqlite3.ErrConstraint
+	}
+	return false
+}