Add ErrorTracker.Filter behavior for modifying error context before saving

lib/error_tracker.ex

@@ -118,7 +118,7 @@ defmodule ErrorTracker do
     {kind, reason} = normalize_exception(exception, stacktrace)
     {:ok, stacktrace} = ErrorTracker.Stacktrace.new(stacktrace)
     {:ok, error} = Error.new(kind, reason, stacktrace)
-    context = Map.merge(get_context(), given_context)
+    context = get_context() |> Map.merge(given_context) |> filter_context_data()
 
     if enabled?() && !ignored?(error, context) do
       {_error, occurrence} = upsert_error!(error, stacktrace, context, reason)
@@ -209,6 +209,16 @@ defmodule ErrorTracker do
     ignorer && ignorer.ignore?(error, context)
   end
 
+  defp filter_context_data(context) do
+    filter_mod = Application.get_env(:error_tracker, :filter)
+
+    if filter_mod do
+      filter_mod.sanitize(context)
+    else
+      context
+    end
+  end
+
   defp normalize_exception(%struct{} = ex, _stacktrace) when is_exception(ex) do
     {to_string(struct), Exception.message(ex)}
   end

lib/error_tracker/filter.ex

@@ -0,0 +1,32 @@
+defmodule ErrorTracker.Filter do
+  @moduledoc """
+  Behaviour for sanitizing & modifying the error context before it's saved.
+
+      defmodule MyApp.ErrorFilter do
+        @behaviour ErrorTracker.Filter
+
+        @impl true
+        def sanitize(context) do
+          context # Modify the context object (add or remove fields as much as you need.)
+        end
+      end
+
+  Once implemented, include it in the ErrorTracker configuration:
+
+    config :error_tracker, filter: MyApp.Filter
+
+  With this configuration in place, the ErrorTracker will call `MyApp.Filter.sanitize/1` to get a context before
+  saving error occurrence.
+
+  > #### A note on performance {: .warning}
+  >
+  > Keep in mind that the `sanitize/1` will be called in the context of the ErrorTracker itself.
+  > Slow code will have a significant impact in the ErrorTracker performance. Buggy code can bring
+  > the ErrorTracker process down.
+  """
+
+  @doc """
+  This function will be given an error context to inspect/modify before it's saved.
+  """
+  @callback sanitize(context :: map()) :: map()
+end

test/error_tracker/filter_test.exs

@@ -0,0 +1,63 @@
+defmodule ErrorTracker.FilterTest do
+  use ErrorTracker.Test.Case
+
+  setup context do
+    if filter = context[:filter] do
+      previous_setting = Application.get_env(:error_tracker, :filter)
+      Application.put_env(:error_tracker, :filter, filter)
+      # Ensure that the application env is restored after each test
+      on_exit(fn -> Application.put_env(:error_tracker, :filter, previous_setting) end)
+    end
+
+    []
+  end
+
+  @sensitive_ctx %{
+    "request" => %{
+      "headers" => %{
+        "accept" => "application/json, text/plain, */*",
+        "authorization" => "Bearer 12341234"
+      }
+    }
+  }
+
+  test "without an filter, context objects are saved as they are." do
+    assert %ErrorTracker.Occurrence{context: ctx} =
+             report_error(fn -> raise "BOOM" end, @sensitive_ctx)
+
+    assert ctx == @sensitive_ctx
+  end
+
+  @tag filter: ErrorTracker.FilterTest.AuthHeaderHider
+  test "user defined filter should be used to sanitize the context before it's saved." do
+    assert %ErrorTracker.Occurrence{context: ctx} =
+             report_error(fn -> raise "BOOM" end, @sensitive_ctx)
+
+    assert ctx != @sensitive_ctx
+
+    cleaned_header_value =
+      ctx |> Map.get("request") |> Map.get("headers") |> Map.get("authorization")
+
+    assert cleaned_header_value == "REMOVED"
+  end
+end
+
+defmodule ErrorTracker.FilterTest.AuthHeaderHider do
+  @behaviour ErrorTracker.Filter
+
+  def sanitize(context) do
+    context
+    |> Enum.map(fn
+      {"authorization", _} ->
+        {"authorization", "REMOVED"}
+
+      o ->
+        o
+    end)
+    |> Enum.map(fn
+      {key, val} when is_map(val) -> {key, sanitize(val)}
+      o -> o
+    end)
+    |> Map.new()
+  end
+end