fix: don't unnecessarily restart scheduler

internal/scheduler/queue.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 
 	"github.com/go-logr/logr"
+	"github.com/leg100/otf/internal"
 	otfrun "github.com/leg100/otf/internal/run"
 	"github.com/leg100/otf/internal/workspace"
 )
@@ -82,7 +83,18 @@ func (q *queue) handleRun(ctx context.Context, run *otfrun.Run) error {
 				// no current run & queue is empty; unlock workspace
 				q.current = nil
 				ws, err := q.Unlock(ctx, q.ws.ID, &run.ID, false)
-				if err != nil {
+				if errors.Is(err, internal.ErrResourceNotFound) {
+					// Workspace not found occurs when the workspace has been
+					// deleted and the scheduler hasn't yet processed the
+					// corresponding "workspace deleted" event. In which there
+					// is nothing to unlock and the scheduler can continue as
+					// normal.
+					return nil
+				} else if err != nil {
+					// Any other error is treated as a transient or unexpected
+					// error, so propagate the error which'll notify the user
+					// via the logs and trigger the scheduler to be restarted
+					// with a backoff-and-retry.
 					return err
 				}
 				q.ws = ws

internal/scheduler/scheduler.go

@@ -177,8 +177,12 @@ func (s *scheduler) handleRunEvent(ctx context.Context, event pubsub.Event[*run.
 	}
 	q, ok := s.queues[event.Payload.WorkspaceID]
 	if !ok {
-		// should never happen
-		s.Error(nil, "workspace queue does not exist for run event", "workspace", event.Payload.WorkspaceID, "run", event.Payload.ID, "event", event.Type)
+		// No queue exists for the workspace because the workspace has
+		// since been deleted, which can occur when run events arrive *after*
+		// the "workspace deleted" event, which is entirely possible with the
+		// way the scheduler processes events.
+		//
+		// In this case no action need be taken on the run.
 		return nil
 	}
 	if err := q.handleRun(ctx, event.Payload); err != nil {

internal/workspace/lock_db.go

@@ -38,9 +38,9 @@ func (db *pgdb) toggleLock(ctx context.Context, workspaceID string, togglefn fun
 			}
 		}
 		if err := q.UpdateWorkspaceLockByID(ctx, params); err != nil {
-			return sql.Error(err)
+			return err
 		}
 		return nil
 	})
-	return ws, err
+	return ws, sql.Error(err)
 }