snapshot: Lock instances before making snapshot

internal/component/instances/wisski_lock.go

@@ -0,0 +1,47 @@
+package instances
+
+import (
+	"errors"
+
+	"github.com/FAU-CDI/wisski-distillery/internal/models"
+)
+
+var ErrLocked = errors.New("instance is locked")
+
+// TryLock attemps to lock this WissKI
+// If this is not possible, returns ErrLocked
+func (wisski WissKI) TryLock() error {
+	table, err := wisski.instances.SQL.QueryTable(true, models.LockTable)
+	if err != nil {
+		return ErrLocked
+	}
+
+	result := table.FirstOrCreate(&models.Lock{}, models.Lock{Slug: wisski.Slug})
+	locked := result.Error == nil && result.RowsAffected == 1
+
+	if !locked {
+		return ErrLocked
+	}
+	return nil
+}
+
+func (wisski WissKI) IsLocked() (locked bool) {
+	table, err := wisski.instances.SQL.QueryTable(true, models.LockTable)
+	if err != nil {
+		return false
+	}
+
+	// check if this instance is locked
+	table.Select("count(*) > 0").Where("slug = ?", wisski.Slug).Find(&locked)
+	return
+}
+
+// Unlock unlocks this WissKI instance and returns if it succeeded
+func (wisski WissKI) Unlock() bool {
+	table, err := wisski.instances.SQL.QueryTable(true, models.LockTable)
+	if err != nil {
+		return false
+	}
+	result := table.Where("slug = ?", wisski.Slug).Delete(&models.Lock{})
+	return result.Error == nil && result.RowsAffected == 1
+}

internal/component/instances/wisski_stack.go

@@ -70,6 +70,11 @@ func (wisski *WissKI) setLastRebuild() error {
 //
 // It also logs the current time into the metadata belonging to this instance.
 func (wisski *WissKI) Build(stream stream.IOStream, start bool) error {
+	if err := wisski.TryLock(); err != nil {
+		return err
+	}
+	defer wisski.Unlock()
+
 	barrel := wisski.Barrel()
 
 	var context component.InstallationContext

internal/component/instances/wisski_status.go

@@ -16,6 +16,8 @@ type WissKIInfo struct {
 	Slug string // slug
 	URL  string // complete URL, including http(s)
 
+	Locked bool // Is this instance currently locked?
+
 	// Information about the running instance
 	Running     bool
 	LastRebuild time.Time
@@ -48,6 +50,12 @@ func (wisski *WissKI) Info(quick bool) (info WissKIInfo, err error) {
 		return
 	})
 
+	// quick check if this instance is locked
+	group.Go(func() (err error) {
+		info.Locked = wisski.IsLocked()
+		return nil
+	})
+
 	// slower checks for extra properties.
 	// these might execute php code or require additional database queries.
 	if !quick {