cloud/bmaas: test and document upgrades across migrations

Change-Id: I1c405b0f2ecc10331b79d02deb8a63f3b148f502
Reviewed-on: https://review.monogon.dev/c/monogon/+/1566
Reviewed-by: Tim Windelschmidt <tim@monogon.tech>
Tested-by: Jenkins CI
diff --git a/cloud/bmaas/bmdb/migrations_test.go b/cloud/bmaas/bmdb/migrations_test.go
index 052d5e1..9ee4ae2 100644
--- a/cloud/bmaas/bmdb/migrations_test.go
+++ b/cloud/bmaas/bmdb/migrations_test.go
@@ -1,7 +1,10 @@
 package bmdb
 
 import (
+	"context"
 	"testing"
+
+	"source.monogon.dev/cloud/bmaas/bmdb/model"
 )
 
 // TestMigrateUpDown performs a full-up and full-down migration test on an
@@ -57,3 +60,85 @@
 		t.Fatalf("Initial up migration failed: %v", err)
 	}
 }
+
+func TestMigration1681826233(t *testing.T) {
+	// This migration adds a new nullable field to backoffs.
+
+	// This guarantees that versions [prev, ver] can run concurrently in a cluster.
+	min := uint(1672749980)
+	max := uint(1681826233)
+
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer t.Cleanup(ctxC)
+
+	b := dut()
+	conn, err := b.Open(false)
+	if err != nil {
+		t.Fatalf("Starting empty database failed: %v", err)
+	}
+
+	// First, make sure the change can actually progress if we have some backoffs
+	// already.
+	if err := b.Database.MigrateUpToIncluding(min); err != nil {
+		t.Fatalf("Migration to minimum version failed: %v", err)
+	}
+
+	// Create machine and old-style backoff.
+	q := model.New(conn.db)
+	machine, err := q.NewMachine(ctx)
+	if err != nil {
+		t.Fatalf("Could not create machine: %v", err)
+	}
+	_, err = conn.db.Exec(`
+		INSERT INTO work_backoff
+		    (machine_id, process, until, cause)
+		VALUES
+		    ($1, 'UnitTest1', now(), 'test');
+	`, machine.MachineID)
+	if err != nil {
+		t.Fatalf("Could not create old-style backoff on old version: %v", err)
+	}
+
+	// Migrate to newer version.
+	if err := b.Database.MigrateUpToIncluding(max); err != nil {
+		t.Fatalf("Migration to maximum version failed: %v", err)
+	}
+
+	// The migration should be read succesfully.
+	boffs, err := q.WorkBackoffOf(ctx, model.WorkBackoffOfParams{
+		MachineID: machine.MachineID,
+		Process:   "UnitTest1",
+	})
+	if err != nil {
+		t.Fatalf("Reading backoff failed: %v", err)
+	}
+	if len(boffs) != 1 {
+		t.Errorf("No backoff found")
+	} else {
+		boff := boffs[0]
+		if boff.LastIntervalSeconds.Valid {
+			t.Errorf("Expected interval to be NULL")
+		}
+	}
+
+	// Simultaneously, any concurrently running bmdb user on an older version should
+	// still be able to insert and read backoffs old style.
+	_, err = conn.db.Exec(`
+		INSERT INTO work_backoff
+		    (machine_id, process, until, cause)
+		VALUES
+		    ($1, 'UnitTest2', now(), 'test');
+	`, machine.MachineID)
+	if err != nil {
+		t.Fatalf("Could not create old-style backoff on new version: %v", err)
+	}
+	rows, err := conn.db.Query(`
+		SELECT machine_id, process, until, cause FROM work_backoff
+	`)
+	for rows.Next() {
+		var mid, process, until, cause string
+		if err := rows.Scan(&mid, &process, &until, &cause); err != nil {
+			t.Errorf("Scan failed: %v", err)
+		}
+	}
+}