fix: partial config update

[Ziinc]

Allows partial updating of a backend config. To accomodate PATCH on nested config values.

Part of O11Y-1578

[jonatanklosko]

Looks good to me! I dropped one suggestion inline :)

[jonatanklosko]

For completeness, you could avoid the stringification and merging here. Currently you have:

logflare/lib/logflare/backends/adaptor/webhook_adaptor.ex

Lines 46 to 52 in 4b84eb5

	@impl Logflare.Backends.Adaptor
	def cast_config(params) do
	{%{}, %{url: :string, headers: :map, http: :string, gzip: :boolean}}
	|> Ecto.Changeset.cast(params, [:url, :headers, :http, :gzip])
	|> Logflare.Utils.default_field_value(:http, "http2")
	|> Logflare.Utils.default_field_value(:gzip, true)
	end

You could change those to accept the existing value and use that instead of the hardcoded %{}:

@impl Logflare.Backends.Adaptor
def cast_config(existing_config, params) do
  {existing_config, %{url: :string, headers: :map, http: :string, gzip: :boolean}}
  |> Ecto.Changeset.cast(params, [:url, :headers, :http, :gzip])
  |> Logflare.Utils.default_field_value(:http, "http2")
  |> Logflare.Utils.default_field_value(:gzip, true)
end

Then here you would pass both config and params and it should just work. You can always pass %{} as existing params, when creating from scratch. This is similar to usual schema changeset function that accepts the struct and params.

[amokan]

Looks good if making the change that Jonatan suggested 👍

[amokan]

Minor nit. Rebinding changeset here could make things flaky if there was downstream code that inspected Backend-specific fields (doesn't happen today) and expected this to represent a Backend changeset.

Could avoid that by not capturing the failed changeset doing something like:

%{valid?: false, errors: errors} ->
  Enum.reduce(errors, changeset, fn {key, {msg, opts}}, cs ->
    Changeset.add_error(cs, :"config.#{key}", msg, opts)
end)