Attempt to scrub all Plug.Conns in Sentry.PlugCapture (#619)

Closes #477.

lib/sentry/plug_capture.ex

@@ -1,12 +1,15 @@
 defmodule Sentry.PlugCapture do
   @moduledoc """
-  Provides basic functionality to handle and send errors occurring within
+  Provides basic functionality to capture and send errors occurring within
   Plug applications, including Phoenix.
 
-  It is intended for usage with `Sentry.PlugContext`.
+  It is intended for usage with `Sentry.PlugContext`, which adds relevant request
+  metadata to the Sentry context before errors are captured.
 
   ## Usage
 
+  ### With Phoenix
+
   In a Phoenix application, it is important to use this module **before**
   the Phoenix endpoint itself. It should be added to your `endpoint.ex` file:
 
@@ -17,7 +20,9 @@ defmodule Sentry.PlugCapture do
         # ...
       end
 
-  In a Plug application, it can be added *below* your router:
+  ### With Plug
+
+  In a Plug application, you can add this module *below* your router:
 
       defmodule MyApp.PlugRouter do
         use Plug.Router
@@ -32,30 +37,76 @@ defmodule Sentry.PlugCapture do
   > and adds capturing errors and reporting to Sentry. You can still re-override
   > that callback after `use Sentry.PlugCapture` if you need to.
 
+  ## Scrubbing Sensitive Data
+
+  > #### Since v9.1.0 {: .neutral}
+  >
+  > Scrubbing sensitive data in `Sentry.PlugCapture` is available since v9.1.0
+  > of this library.
+
+  Like `Sentry.PlugContext`, this module also supports scrubbing sensitive data
+  out of errors. However, this module has to do some *guessing* to figure
+  out if there are `Plug.Conn` structs to scrub. Right now, the strategy we
+  use follows these steps:
+
+    1. if the error is `Phoenix.ActionClauseError`, we scrub the `Plug.Conn` structs
+      from the `args` field of that exception
+
+  Otherwise, we don't perform any scrubbing. To configure scrubbing, you can use the
+  `:scrubbing` option (see below).
+
+  ## Options
+
+    * `:scrubber` (since v9.1.0) - a term of type `{module, function, args}` that
+      will be invoked to scrub sensitive data from `Plug.Conn` structs. The
+      `Plug.Conn` struct is prepended to `args` before invoking the function,
+      so that the final function will be called as `apply(module, function, [conn | args])`.
+      The function must return a `Plug.Conn` struct. By default, the built-in
+      scrubber does this:
+
+      * scrubs *all* cookies
+      * scrubs sensitive headers just like `Sentry.PlugContext.default_header_scrubber/1`
+      * scrubs sensitive body params just like `Sentry.PlugContext.default_body_scrubber/1`
+
   """
 
-  defmacro __using__(_opts) do
+  defmacro __using__(opts) do
     quote do
+      opts = unquote(Macro.escape(opts))
+      default_scrubber = {unquote(__MODULE__), :default_scrubber, []}
+
+      scrubber =
+        case Keyword.get(opts, :scrubber, default_scrubber) do
+          {mod, fun, args} = scrubber when is_atom(mod) and is_atom(fun) and is_list(args) ->
+            scrubber
+
+          other ->
+            raise ArgumentError,
+                  "expected :scrubber to be a {module, function, args} tuple, got: #{inspect(other)}"
+        end
+
+      @__sentry_scrubber scrubber
+
       @before_compile Sentry.PlugCapture
     end
   end
 
-  defmacro __before_compile__(_) do
+  defmacro __before_compile__(_env) do
     quote do
       defoverridable call: 2
 
       def call(conn, opts) do
         try do
           super(conn, opts)
         rescue
-          e in Plug.Conn.WrapperError ->
-            exception = Exception.normalize(:error, e.reason, e.stack)
-            _ = Sentry.capture_exception(exception, stacktrace: e.stack, event_source: :plug)
-            Plug.Conn.WrapperError.reraise(e)
-
-          e ->
-            _ = Sentry.capture_exception(e, stacktrace: __STACKTRACE__, event_source: :plug)
-            :erlang.raise(:error, e, __STACKTRACE__)
+          err in Plug.Conn.WrapperError ->
+            exception = Exception.normalize(:error, err.reason, err.stack)
+            Sentry.PlugCapture.__capture_exception__(exception, err.stack, @__sentry_scrubber)
+            Plug.Conn.WrapperError.reraise(err)
+
+          exc ->
+            Sentry.PlugCapture.__capture_exception__(exc, __STACKTRACE__, @__sentry_scrubber)
+            :erlang.raise(:error, exc, __STACKTRACE__)
         catch
           kind, reason ->
             message = "Uncaught #{kind} - #{inspect(reason)}"
@@ -66,4 +117,38 @@ defmodule Sentry.PlugCapture do
       end
     end
   end
+
+  @doc false
+  def __capture_exception__(exception, stacktrace, scrubber) do
+    # We can't pattern match here, because we're not guaranteed to have
+    # Phoenix available.
+    exception =
+      if is_struct(exception, Phoenix.ActionClauseError) do
+        update_in(exception, [Access.key!(:args), Access.all()], fn
+          conn when is_struct(conn, Plug.Conn) -> apply_scrubber(conn, scrubber)
+          other -> other
+        end)
+      else
+        exception
+      end
+
+    Sentry.capture_exception(exception, stacktrace: stacktrace, event_source: :plug)
+  end
+
+  @doc false
+  def default_scrubber(conn) do
+    %{
+      conn
+      | cookies: %{},
+        req_headers: Sentry.PlugContext.default_header_scrubber(conn),
+        params: Sentry.PlugContext.default_body_scrubber(conn)
+    }
+  end
+
+  defp apply_scrubber(conn, {mod, fun, args} = _scrubber) do
+    case apply(mod, fun, [conn | args]) do
+      conn when is_struct(conn, Plug.Conn) -> conn
+      other -> raise ":scrubber function must return a Plug.Conn struct, got: #{inspect(other)}"
+    end
+  end
 end