don't commit cache mounts on error

Cache mount instances are shared between multiple vertextes/builds
so if one of the cloned instance gets committed reference count
will get corrupted as other parts of the code still see reference as
mountable.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

frontend/gateway/container.go

@@ -127,6 +127,7 @@ type MountRef struct {
 type MountMutableRef struct {
 	Ref        cache.MutableRef
 	MountIndex int
+	NoCommit   bool
 }
 
 type MakeMutable func(m *opspb.Mount, ref cache.ImmutableRef) (cache.MutableRef, error)
@@ -196,6 +197,7 @@ func PrepareMounts(ctx context.Context, mm *mounts.MountManager, cm cache.Manage
 			p.Actives = append(p.Actives, MountMutableRef{
 				MountIndex: i,
 				Ref:        active,
+				NoCommit:   true,
 			})
 			if m.Output != opspb.SkipOutput && ref != nil {
 				p.OutputRefs = append(p.OutputRefs, MountRef{

solver/llbsolver/ops/exec.go

@@ -243,12 +243,16 @@ func (e *execOp) Exec(ctx context.Context, g session.Group, inputs []solver.Resu
 				execMounts[p.OutputRefs[i].MountIndex] = res
 			}
 			for _, active := range p.Actives {
-				ref, cerr := active.Ref.Commit(ctx)
-				if cerr != nil {
-					err = errors.Wrapf(err, "error committing %s: %s", active.Ref.ID(), cerr)
-					continue
+				if active.NoCommit {
+					active.Ref.Release(context.TODO())
+				} else {
+					ref, cerr := active.Ref.Commit(ctx)
+					if cerr != nil {
+						err = errors.Wrapf(err, "error committing %s: %s", active.Ref.ID(), cerr)
+						continue
+					}
+					execMounts[active.MountIndex] = worker.NewWorkerRefResult(ref, e.w)
 				}
-				execMounts[active.MountIndex] = worker.NewWorkerRefResult(ref, e.w)
 			}
 			err = errdefs.WithExecError(err, execInputs, execMounts)
 		} else {