CheckSchema: Verify intermediate schema upgrades

doc/04-Upgrading.md

@@ -4,7 +4,8 @@ Some Icinga DB upgrades require manual intervention, others do not. If you need
 point you to the specific upgrade section on this page.
 
 Please note that version upgrades are incremental. If you are upgrading across multiple versions, make sure to follow
-the steps for each of them.
+the steps for each of them. For example, when upgrading from version 1.1.0 to 1.2.0, follow all instructions for
+upgrading to 1.1.1, then all for 1.2.0, including schema upgrades.
 
 ## Database Schema Upgrades
 

pkg/icingadb/schema.go

@@ -18,7 +18,7 @@ const (
 	expectedPostgresSchemaVersion = 5
 )
 
-// ErrSchemaNotExists implies that no Icinga DB schema has been imported.
+// ErrSchemaNotExists implies that no Icinga DB schema has been imported. May result in a later [ImportSchema] call.
 var ErrSchemaNotExists = stderrors.New("no database schema exists")
 
 // ErrSchemaMismatch implies an unexpected schema version, most likely after Icinga DB was updated but the database
@@ -48,12 +48,13 @@ func CheckSchema(ctx context.Context, db *database.DB) error {
 		return ErrSchemaNotExists
 	}
 
-	var version uint16
+	var versions []uint16
+
 	err := retry.WithBackoff(
 		ctx,
 		func(ctx context.Context) error {
-			query := "SELECT version FROM icingadb_schema ORDER BY id DESC LIMIT 1"
-			if err := db.QueryRowxContext(ctx, query).Scan(&version); err != nil {
+			query := "SELECT version FROM icingadb_schema ORDER BY version ASC"
+			if err := db.SelectContext(ctx, &versions, query); err != nil {
 				return database.CantPerformQuery(err, query)
 			}
 			return nil
@@ -65,12 +66,28 @@ func CheckSchema(ctx context.Context, db *database.DB) error {
 		return errors.Wrap(err, "can't check database schema version")
 	}
 
-	if version != expectedDbSchemaVersion {
+	if len(versions) == 0 {
+		return fmt.Errorf("%w: no database schema version is stored in the database", ErrSchemaMismatch)
+	}
+
+	// Check if each schema update between the initial import and the latest version was applied or, in other words,
+	// that no schema update was left out. The loop goes over the ascending sorted array of schema versions, verifying
+	// that each element's successor is the increment of this version, ensuring no gaps in between.
+	for i := 0; i < len(versions)-1; i++ {
+		if versions[i] != versions[i+1]-1 {
+			return fmt.Errorf(
+				"%w: incomplete database schema upgrade: intermediate version v%d is missing,"+
+					" please make sure you have applied all database migrations after upgrading Icinga DB",
+				ErrSchemaMismatch, versions[i]+1)
+		}
+	}
+
+	if latestVersion := versions[len(versions)-1]; latestVersion != expectedDbSchemaVersion {
 		// Since these error messages are trivial and mostly caused by users, we don't need
 		// to print a stack trace here. However, since errors.Errorf() does this automatically,
 		// we need to use fmt instead.
 		return fmt.Errorf("%w: v%d (expected v%d), please make sure you have applied all database"+
-			" migrations after upgrading Icinga DB", ErrSchemaMismatch, version, expectedDbSchemaVersion,
+			" migrations after upgrading Icinga DB", ErrSchemaMismatch, latestVersion, expectedDbSchemaVersion,
 		)
 	}