Drop() does not work after https://github.com/golang-migrate/migrate/pull/150

[herpiko]

Describe the Bug

I'm using migrate as lib. Drop() after initializing migrate instance does not work anymore.

Steps to Reproduce
Steps to reproduce the behavior:

	m, err := migrate.New(
		migrationPath,
		url)
	if err != nil {
		log.Fatal(err)
	}
	err = m.Drop()
	if err != nil && err.Error() != "no change" {
		log.Fatal(err)
	}
	err = m.Up()
	if err != nil && err.Error() != "no change" {
		log.Fatal(err)
	}

Expected Behavior

Before 4.2.3, Drop() was always work.

Migrate Version
e.g. v4.2.3

Obtained by running my unit test program that using migrate as lib.

Loaded Source Drivers
N/A

Loaded Database Drivers
postgres

Go Version
go version go1.11.1 linux/amd64

Stacktrace

MIGRATION_PATH=`pwd`/migrations/test go test
db_test.go:69: Migrating postgres://db:db@0.0.0.0:65432/testdb?sslmode=disable from file:///home/path/to/src/migrations/test
db_test.go:78: can't acquire lock
exit status 1
FAIL	xxx/yyy/zzz	0.169s
Makefile:15: recipe for target 'test' failed
make: *** [test] Error 1

[dhui]

Can you provide the full test code?

Are you running migrate in a goroutine? Based on the error message, It looks like you're hitting the case where local lock is being hit. e.g. database.ErrLocked is returned if multiple goroutines are locking migrate. It doesn't seem like an issue where grabbing the Postgres advisory lock is failing.

[herpiko]

Are you running migrate in a goroutine?

No.

@dhui Okay I'll provide the reproducible code ASAP.

[lukaspj]

I'm experiencing this issue, seeing:

tearing down existing database because messages.database.fresh is true
can't acquire lock
failed to tear down database

I'm running on a clean Postgresql instance running inside Kubernetes via Minikube, deployed from Helm.
The code I wrote, it's not a Minimal Working Example, but I wanted to keep everything:
https://gist.github.com/lukaspj/eee69764cca2b58bb58fc368e4658a00

[lukaspj]

It seems to happen in postgres.go line 328

if err := p.ensureVersionTable(); err != nil {

Maybe some lock is not released previously or it might happen inside ensureVersionTable entirely.

Edit: Well it's not ensureVersionTable itself, as it's called earlier in WithInstance, so it must happen in Drop()

Edit2: Happens because p.isLocked is true

[lukaspj]

My guess would be that the Lock is acquired in migrate.go:

migrate/migrate.go

Lines 304 to 312 in f6d624c

	func (m *Migrate) Drop() error {
	if err := m.lock(); err != nil {
	return err
	}
	if err := m.databaseDrv.Drop(); err != nil {
	return m.unlockErr(err)
	}
	return m.unlock()
	}

In the subsequent call to Drop, the database is still locked so when we call ensureVersionTable and it tries to lock the database it dies:

migrate/database/postgres/postgres.go

Lines 328 to 330 in f6d624c

	if err := p.ensureVersionTable(); err != nil {
	return err
	}

Edit: better links