llbsolver: Fix performance of recomputeDigests

Before this, in the case where nothing was mutated the visited memo
would never be updated, thus causing exponential complexity.

Now the memo is updated even when nothing is mutated, just setting old
and new to be the same digest.

Signed-off-by: Erik Sipsma <[email protected]>
(cherry picked from commit ea69a59)

client/client_test.go

@@ -195,6 +195,7 @@ func TestIntegration(t *testing.T) {
 		testMountStubsDirectory,
 		testMountStubsTimestamp,
 		testSourcePolicy,
+		testLLBMountPerformance,
 	)
 }
 
@@ -8945,3 +8946,31 @@ func testSourcePolicy(t *testing.T, sb integration.Sandbox) {
 		require.ErrorContains(t, err, sourcepolicy.ErrSourceDenied.Error())
 	})
 }
+
+func testLLBMountPerformance(t *testing.T, sb integration.Sandbox) {
+	c, err := New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	mntInput := llb.Image("busybox:latest")
+	st := llb.Image("busybox:latest")
+	var mnts []llb.State
+	for i := 0; i < 20; i++ {
+		execSt := st.Run(
+			llb.Args([]string{"true"}),
+		)
+		mnts = append(mnts, mntInput)
+		for j := range mnts {
+			mnts[j] = execSt.AddMount(fmt.Sprintf("/tmp/bin%d", j), mnts[j], llb.SourcePath("/bin"))
+		}
+		st = execSt.Root()
+	}
+
+	def, err := st.Marshal(sb.Context())
+	require.NoError(t, err)
+
+	timeoutCtx, cancel := context.WithTimeout(sb.Context(), time.Minute)
+	defer cancel()
+	_, err = c.Solve(timeoutCtx, def, SolveOpt{}, nil)
+	require.NoError(t, err)
+}

solver/llbsolver/vertex.go

@@ -210,6 +210,7 @@ func recomputeDigests(ctx context.Context, all map[digest.Digest]*pb.Op, visited
 	}
 
 	if !mutated {
+		visited[dgst] = dgst
 		return dgst, nil
 	}
 
@@ -274,7 +275,7 @@ func loadLLB(ctx context.Context, def *pb.Definition, polEngine SourcePolicyEval
 
 	for {
 		newDgst, ok := mutatedDigests[lastDgst]
-		if !ok {
+		if !ok || newDgst == lastDgst {
 			break
 		}
 		lastDgst = newDgst