fix caching issues with Local

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

dagger/pipeline.go

@@ -253,14 +253,33 @@ func (p *Pipeline) Local(ctx context.Context, op *compiler.Value) error {
 			return err
 		}
 	}
-
-	p.fs = p.fs.Set(
+	// FIXME: Remove the `Copy` and use `Local` directly.
+	//
+	// Copy'ing is a costly operation which should be unnecessary.
+	// However, for a mysterious reason, `llb.Local` is returning a different
+	// digest at every run, therefore invalidating the cache.
+	//
+	// By wrapping `llb.Local` inside `llb.Copy`, we get the same digest for
+	// the same content.
+	p.fs = p.fs.Change(func(st llb.State) llb.State {
+		return st.File(
+			llb.Copy(
 				llb.Local(
 					dir,
 					llb.FollowPaths(include),
 					llb.WithCustomName(p.vertexNamef("Local %s", dir)),
+
+					// Without hint, multiple `llb.Local` operations on the
+					// same path get a different digest.
+					llb.SessionID(p.s.SessionID()),
+					llb.SharedKeyHint(dir),
+				),
+				"/",
+				"/",
 			),
 		)
+	})
+
 	return nil
 }
 

dagger/solver.go

@@ -35,6 +35,10 @@ func (s Solver) Scratch() FS {
 	return s.FS(llb.Scratch())
 }
 
+func (s Solver) SessionID() string {
+	return s.c.BuildOpts().SessionID
+}
+
 // Solve will block until the state is solved and returns a Reference.
 func (s Solver) SolveRequest(ctx context.Context, req bkgw.SolveRequest) (bkgw.Reference, error) {
 	// call solve