Add structured metadata to code evaluation logs

This will be easier to process from an external log aggregator. Useful when auditing code
evalution in an app server connected to a prod environment.

README.md

@@ -265,9 +265,13 @@ The following environment variables can be used to configure Livebook on boot:
     logging, either of: error, warning, notice, info, debug. Defaults to warning.
 
   * `LIVEBOOK_LOG_METADATA` - a comma-separated list of metadata keys that should
-    be included in the log messages. Currently the only Livebook-spcecific key is
-    users (attached to evaluation and request logs). By default includes only
-    request_id.
+    be included in the log messages. Livebook-specific keys include:
+    - `users` (attached to evaluation and request logs)
+    - `session_mode` (attached to evaluation logs, either "default" or "app")
+    - `code` (attached to evaluation logs, the code being evaluated)
+    - `event` (attached to evaluation logs, currently always "code.evaluate")
+
+    By default includes only `request_id`.
 
   * `LIVEBOOK_LOG_FORMAT` - sets the log output format, either "text" (default)
     for human-readable logs or "json" for structured JSON.

lib/livebook.ex

@@ -104,6 +104,8 @@ defmodule Livebook do
     log_metadata = Livebook.Config.log_metadata!("LIVEBOOK_LOG_METADATA")
     log_format = Livebook.Config.log_format!("LIVEBOOK_LOG_FORMAT") || :text
 
+    config :livebook, :log_format, log_format
+
     case {log_format, log_metadata} do
       {:json, log_metadata} ->
         config :logger, :default_handler,

lib/livebook/session.ex

@@ -2685,8 +2685,20 @@ defmodule Livebook.Session do
   end
 
   def log_code_evaluation(cell, state) do
+    inspected_code = inspect(cell.source, printable_limit: :infinity)
+
+    # For metadata code, we try to use use the raw source, so it's easier to process
+    # the logged code in a external log aggregator.
+    metadata_code =
+      case cell.source do
+        source when is_binary(source) -> source
+        _ -> inspected_code
+      end
+
+    session_mode = state.data.mode
+
     evaluation_users =
-      case state.data.mode do
+      case session_mode do
         :default -> Map.values(state.data.users_map)
         :app -> if(state.deployed_by, do: [state.deployed_by], else: [])
       end
@@ -2695,12 +2707,17 @@ defmodule Livebook.Session do
       [
         """
         Evaluating code
-          Session mode: #{state.data.mode}
+          Session mode: #{session_mode}
           Code: \
         """,
-        inspect(cell.source, printable_limit: :infinity)
+        inspected_code
       ],
-      Livebook.Utils.logger_users_metadata(evaluation_users)
+      Keyword.merge(
+        Livebook.Utils.logger_users_metadata(evaluation_users),
+        session_mode: session_mode,
+        code: metadata_code,
+        event: "code.evaluate"
+      )
     )
   end
 

lib/livebook/utils.ex

@@ -854,6 +854,9 @@ defmodule Livebook.Utils do
             into: %{}
       end
 
-    [users: inspect(list)]
+    case Application.get_env(:livebook, :log_format) do
+      :text -> [users: inspect(list)]
+      :json -> [users: list]
+    end
   end
 end