storage: add a unit test to test RocksDB's error message is expected

Add a unit test to test `OpenFile`, `ReadFile`, and `DeleteFile` in RocksDB
return `os.ErrNotExist` error when the file or directory is not found.

Related to: cockroachdb#26259.
Release note: None

pkg/storage/engine/rocksdb_test.go

@@ -1052,3 +1052,64 @@ func TestRocksDBOptions(t *testing.T) {
 		}
 	}
 }
+
+func TestRocksDBFileNotFoundError(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+
+	dir, dirCleanup := testutils.TempDir(t)
+	defer dirCleanup()
+
+	db, err := NewRocksDB(
+		RocksDBConfig{
+			Settings: cluster.MakeTestingClusterSettings(),
+			Dir:      dir,
+		},
+		RocksDBCache{},
+	)
+	if err != nil {
+		t.Fatalf("could not create new rocksdb db instance at %s: %v", dir, err)
+	}
+	defer db.Close()
+
+	// Verify DeleteFile returns os.ErrNotExist if file does not exist.
+	if err := db.DeleteFile("/non/existent/file"); !os.IsNotExist(err) {
+		t.Fatalf("expected IsNotExist, but got %v (%T)", err, err)
+	}
+
+	fname := filepath.Join(dir, "random.file")
+	data := "random data"
+	if f, err := db.OpenFile(fname); err != nil {
+		t.Fatalf("unable to open file with filename %s, got err %v", fname, err)
+	} else {
+		// Write data to file so we can read it later.
+		if err := f.Append([]byte(data)); err != nil {
+			t.Fatalf("error writing data: '%s' to file %s, got err %v", data, fname, err)
+		}
+		if err := f.Sync(); err != nil {
+			t.Fatalf("error syncing data, got err %v", err)
+		}
+		if err := f.Close(); err != nil {
+			t.Fatalf("error closing file %s, got err %v", fname, err)
+		}
+	}
+
+	if b, err := db.ReadFile(fname); err != nil {
+		t.Errorf("unable to read file with filename %s, got err %v", fname, err)
+	} else if string(b) != data {
+		t.Errorf("expected content in %s is '%s', got '%s'", fname, data, string(b))
+	}
+
+	if err := db.DeleteFile(fname); err != nil {
+		t.Errorf("unable to delete file with filename %s, got err %v", fname, err)
+	}
+
+	// Verify ReadFile returns os.ErrNotExist if reading an already deleted file.
+	if _, err := db.ReadFile(fname); !os.IsNotExist(err) {
+		t.Fatalf("expected IsNotExist, but got %v (%T)", err, err)
+	}
+
+	// Verify DeleteFile returns os.ErrNotExist if deleting an already deleted file.
+	if err := db.DeleteFile(fname); !os.IsNotExist(err) {
+		t.Fatalf("expected IsNotExist, but got %v (%T)", err, err)
+	}
+}