Disallows private-scoped migrations with default names. (#328)

* Add critical & fatal error to migration process if a private context is discovered. Add an error but allow continuing if a different unexpected name is discovered.

* code doc for new method.

* fix private fluent benchmark migration.

Sources/FluentBenchmark/Tests/MigratorTests.swift

@@ -164,7 +164,7 @@ extension FluentBenchmarker {
     }
 }
 
-private struct ErrorMigration: Migration {
+internal struct ErrorMigration: Migration {
     init() { }
 
     struct Error: Swift.Error { }

Sources/FluentKit/Migration/Migration.swift

@@ -6,6 +6,10 @@ public protocol Migration {
 
 extension Migration {
     public var name: String {
+        return defaultName
+    }
+
+    internal var defaultName: String {
         return String(reflecting: Self.self)
     }
 }

Sources/FluentKit/Migration/Migrator.swift

@@ -134,9 +134,29 @@ private final class DatabaseMigrator {
     // MARK: Setup
 
     func setupIfNeeded() -> EventLoopFuture<Void> {
-        return MigrationLog.migration.prepare(on: self.database).flatMap {
-            self.fixPrereleaseMigrationNames()
+        return MigrationLog.migration.prepare(on: self.database)
+            .flatMap(self.preventUnstableNames)
+            .flatMap(self.fixPrereleaseMigrationNames)
+    }
+
+    /// An unstable name is a name that is not the same every time migrations
+    /// are run.
+    ///
+    /// For example, the default name for `Migrations` in private contexts
+    /// will include an identifier that can change from one execution to the next.
+    private func preventUnstableNames() -> EventLoopFuture<Void> {
+        for migration in self.migrations {
+            let migrationName = migration.name
+            guard migration.name == migration.defaultName else { continue }
+            guard migrationName.contains("$") else { continue }
+
+            if migrationName.contains("unknown context at") {
+                self.database.logger.critical("The migration at \(migrationName) is in a private context. Either explicitly give it a name by adding the `var name: String` property or make the migration `internal` or `public` instead of `private`.")
+                fatalError("Private migrations not allowed")
+            }
+            self.database.logger.error("The migration at \(migrationName) has an unexpected default name. Consider giving it an explicit name by adding a `var name: String` property before applying these migrations.")
         }
+        return self.database.eventLoop.makeSucceededFuture(())
     }
 
     // This migration just exists to smooth the gap between
@@ -153,14 +173,12 @@ private final class DatabaseMigrator {
         var nameOverrides = Set<String>()
 
         for migration in self.migrations {
-            let releaseCandidateDefaultName = "\(type(of: migration))"
-            let v4DefaultName = String(reflecting:type(of: migration))
-
-            // if the migration does not override the default name.
-            // NOTE we compare to the _current_ style of default, not
-            // the release candidate style of default name.
-            if migration.name == v4DefaultName {
-                migrationNameMap[releaseCandidateDefaultName] = v4DefaultName
+            // if the migration does not override the default name
+            // then it is a candidate for a name change.
+            if migration.name == migration.defaultName {
+                let releaseCandidateDefaultName = "\(type(of: migration))"
+
+                migrationNameMap[releaseCandidateDefaultName] = migration.defaultName
             } else {
                 nameOverrides.insert(migration.name)
             }