local: avoid writing to content root on readonly store

A contentstore can be created on top of readonly path and
should not fail unless there is an attempt to write into it.

Currently this fails because new ingest directory is created
always, meaning for example that you can't create a store to
read blobs from OCI layout without it contaminating the OCI
layout files.

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

plugins/content/local/store_test.go

@@ -108,10 +108,6 @@ func TestContentWriter(t *testing.T) {
 	defer cleanup()
 	defer testutil.DumpDirOnFailure(t, tmpdir)
 
-	if _, err := os.Stat(filepath.Join(tmpdir, "ingest")); os.IsNotExist(err) {
-		t.Fatal("ingest dir should be created", err)
-	}
-
 	cw, err := cs.Writer(ctx, content.WithRef("myref"))
 	if err != nil {
 		t.Fatal(err)
@@ -120,6 +116,10 @@ func TestContentWriter(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if _, err := os.Stat(filepath.Join(tmpdir, "ingest")); os.IsNotExist(err) {
+		t.Fatal("ingest dir should be created", err)
+	}
+
 	// reopen, so we can test things
 	cw, err = cs.Writer(ctx, content.WithRef("myref"))
 	if err != nil {