Today I Learned

15 posts by michałstaśkiewicz

Setup NAT on mac

Networking on mac can be tricky, the same applies to NAT. If we want our mac to act as NAT we need:

1) Enable forwarding

sudo sysctl -w net.inet.ip.forwarding=1

2) Create a file with our NAT rules for eg nat_rules with our configuration

nat on en0 from en1 to any -> (en0)

3) Start PF via pfctl

sudo pfctl -d                     # stop pfctl
sudo pfctl -F all -f ./nat_rules  # flush all rules and load new ones
sudo pfctl -e                     # start pfctl

Debugging git push(ssh) hanging

Sometimes when we try to push some changes to the repository git push can hang and we cannot get any meaningful information. We can check the GitHub status page but when everything is green there what can we do to get more information(applies only for ssh)

1) Create script like ~/

ssh -vvv "$@"

2) Add permissions chmod u+x ~/

3) Push to the repo GIT_SSH=~/ git push <rest of your command>

We should see what exactly what ssh is doing.

Revealing struct information

In Elixir when we are printing struct we can hide struct’s details implementing Inspect protocol. For example the DateTime struct is doing that so when we are inspecting it it shows us pretty version instead of the detailed one.

iex(1)> now = DateTime.utc_now()
~U[2020-10-06 10:59:58.388743Z]
iex(2)> IO.inspect(now)
~U[2020-10-06 10:59:58.388743Z]

If we want to reveal all the information that the struct contains we can use structs: false option.

iex(5)> IO.inspect(now, structs: false)
  __struct__: DateTime,
  calendar: Calendar.ISO,
  day: 6,
  hour: 10,
  microsecond: {388743, 6},
  minute: 59,
  month: 10,
  second: 58,
  std_offset: 0,
  time_zone: "Etc/UTC",
  utc_offset: 0,
  year: 2020,
  zone_abbr: "UTC"

Using $ sign inside of Makefile

I’ve just learned that if you want to use a dollar sign inside Makefile you need to escape $ with an extra $.

For example:

LATEST_COMMIT := $(shell git log --oneline | head -1 | awk '{print $1}')

It won’t work because make thinks that we are accessing make variable. If we want to use this awk command inside Makefile we need to escape the $

LATEST_COMMIT := $(shell git log --oneline | head -1 | awk '{print $$1}')

Mocking behaviour in child processes

Imagine you have a process which is responsible for consuming some sort of queue and spawning a separate process for each given job.

Each of the job processors later called workers communicates with some infrastructure layers for eg. external API, database, etc. We are injecting infrastructure to workers via configuration:

defp current_impl do
  Application.get_env(:scanner, :ensure_file, __MODULE__.AwsImpl)

Each injected implementation must implement complementary behaviour for eg.

defmodule Scanner.Scans.EnsureFile.Impl do
  @callback call(String.t, String.t) :: {:ok, any()} | {:error, any()}

In this setup, we could mock the whole behaviour using Mox :

# test_suport.ex or other file with your test configuration
Mox.defmock(Scanner.EnsureFileMock, for: Scanner.Scans.EnsureFile.Impl)

# actual test
expect(Scanner.EnsureFileMock, :call, fn ^bucket, ^path -> 
  {:ok, :response} 

When we run this test with the given context we would gen an error:

(Mox.UnexpectedCallError) no expectation defined for

The reason is that Mox by default works in private mode which is applying our mocked behaviour only for the current process - in our scenario our tested module spawns workers which are the real consumers of our mock and the implementation is only mocked for our test process. To allow our child process to consume the mock we need to set Mox in our test the global mode:

setup :set_mox_global

Or we could use:

setup :set_mox_from_context

Which will set the private mode only when a test has async: true otherwise it would use global mode.

Remove sensitive data from git repository.

When you forgot to use secrets from the very beginning - and some secrets landed in your repository eg. login/password/secret_key you can remove them in a simple way using filter-branch for example to remove password you need to use:

git filter-branch --tree-filter "find . -type f -exec sed -i -e 's/password/**REMOVED**/g' {} \;"

It will replace password with **REMOVED** in the whole repo and commits.