support for --discard-foreign-keys

This flag makes migration silently and happily discard any existing foreign keys on migrated table. This is useful for intentional dropping of foreign keys, as gh-ost does not otherwise have support for foreign key migration.
At some time in the future gh-ost may support foreign key migration, at which time this flag will be removed

Author: Shlomi Noach

go/base/context.go

@@ -70,6 +70,7 @@ type MigrationContext struct {
 	ApproveRenamedColumns    bool
 	SkipRenamedColumns       bool
 	IsTungsten               bool
+	DiscardForeignKeys       bool
 
 	config      ContextConfig
 	configMutex *sync.Mutex

go/cmd/gh-ost/main.go

@@ -61,6 +61,7 @@ func main() {
 	flag.BoolVar(&migrationContext.ApproveRenamedColumns, "approve-renamed-columns", false, "in case your `ALTER` statement renames columns, gh-ost will note that and offer its interpretation of the rename. By default gh-ost does not proceed to execute. This flag approves that gh-ost's interpretation si correct")
 	flag.BoolVar(&migrationContext.SkipRenamedColumns, "skip-renamed-columns", false, "in case your `ALTER` statement renames columns, gh-ost will note that and offer its interpretation of the rename. By default gh-ost does not proceed to execute. This flag tells gh-ost to skip the renamed columns, i.e. to treat what gh-ost thinks are renamed columns as unrelated columns. NOTE: you may lose column data")
 	flag.BoolVar(&migrationContext.IsTungsten, "tungsten", false, "explicitly let gh-ost know that you are running on a tungsten-replication based topology (you are likely to also provide --assume-master-host)")
+	flag.BoolVar(&migrationContext.DiscardForeignKeys, "discard-foreign-keys", false, "DANGER! This flag will migrate a table that has foreign keys and will NOT create foreign keys on the ghost table, thus your altered table will have NO foreign keys. This is useful for intentional dropping of foreign keys")
 
 	executeFlag := flag.Bool("execute", false, "actually execute the alter & migrate the table. Default is noop: do some tests and exit")
 	flag.BoolVar(&migrationContext.TestOnReplica, "test-on-replica", false, "Have the migration run on a replica, not on the master. At the end of migration replication is stopped, and tables are swapped and immediately swap-revert. Replication remains stopped and you can compare the two tables for building trust")

go/logic/inspect.go

@@ -375,11 +375,15 @@ func (this *Inspector) validateTableForeignKeys() error {
 	if err != nil {
 		return err
 	}
-	if numForeignKeys > 0 {
-		return log.Errorf("Found %d foreign keys related to %s.%s. Foreign keys are not supported. Bailing out", numForeignKeys, sql.EscapeName(this.migrationContext.DatabaseName), sql.EscapeName(this.migrationContext.OriginalTableName))
+	if numForeignKeys == 0 {
+		log.Debugf("Validated no foreign keys exist on table")
+		return nil
 	}
-	log.Debugf("Validated no foreign keys exist on table")
-	return nil
+	if this.migrationContext.DiscardForeignKeys {
+		log.Debugf("Foreign keys found and will be dropped, as per given --discard-foreign-keys flag")
+		return nil
+	}
+	return log.Errorf("Found %d foreign keys related to %s.%s. Foreign keys are not supported. Bailing out", numForeignKeys, sql.EscapeName(this.migrationContext.DatabaseName), sql.EscapeName(this.migrationContext.OriginalTableName))
 }
 
 // validateTableTriggers makes sure no triggers exist on the migrated table