Hold a merge the controller cannot record now, and hand it over again, instead of taking it (issue 921)

cmd/mesh-controller/related_test.go

@@ -2,6 +2,8 @@ package main
 
 import (
 	"encoding/json"
+	"errors"
+	"fmt"
 	"strings"
 	"testing"
 	"time"
@@ -220,3 +222,14 @@ func TestARetryWaitsOnlyForAnOpenWalkItIsRelatedTo(t *testing.T) {
 		t.Error("an own-path retry went beside an open walk")
 	}
 }
+
+// Plans another controller is working are "not now", not a failure: the merge is held and handed over again
+// (issue 921).
+func TestPlansHeldByAnotherControllerAreNotNow(t *testing.T) {
+	if err := notNow(fmt.Errorf("x: %w", inventory.ErrPlansBusy)); !errors.Is(err, link.ErrTryAgain) {
+		t.Errorf("busy plans read as %v; want ErrTryAgain", err)
+	}
+	if err := notNow(errors.New("no such module")); errors.Is(err, link.ErrTryAgain) {
+		t.Errorf("another failure read as ErrTryAgain: %v", err)
+	}
+}

cmd/mesh-controller/upgrades.go

@@ -294,10 +294,10 @@ func (f following) Announceable(ctx context.Context) ([]link.Announcement, error
 	return out, nil
 }
 
-// notNow marks a store that could not be read right now, so the announcement is held rather than
-// lost; anything else is returned as it was.
+// notNow marks a store that could not be read right now, or plans another controller is working (issue 921), so
+// the announcement is held rather than lost; anything else is returned as it was.
 func notNow(err error) error {
-	if inventory.Unreachable(err) {
+	if inventory.Unreachable(err) || errors.Is(err, inventory.ErrPlansBusy) {
 		return fmt.Errorf("%w: %w", link.ErrTryAgain, err)
 	}
 	return err