wip - add LiveView context propagation for distributed tracing

Author: solnic

lib/sentry/opentelemetry/live_view_propagator.ex

@@ -0,0 +1,209 @@
+if Sentry.OpenTelemetry.VersionChecker.tracing_compatible?() and
+     Code.ensure_loaded?(Phoenix.LiveView) do
+  defmodule Sentry.OpenTelemetry.LiveViewPropagator do
+    @moduledoc """
+    Telemetry handler that propagates OpenTelemetry context to LiveView processes.
+
+    This module attaches telemetry handlers for LiveView lifecycle events
+    (mount, handle_params, handle_event) that run BEFORE `opentelemetry_phoenix`
+    creates spans, ensuring the correct parent trace context is attached.
+
+    ## Why This Is Needed
+
+    When a browser makes an HTTP request with distributed tracing headers, the trace
+    context is correctly extracted for the initial request. However, Phoenix LiveView
+    spawns new BEAM processes for lifecycle callbacks.
+
+    `opentelemetry_phoenix` uses telemetry handlers to create spans for these events.
+    If we don't inject the parent context BEFORE those handlers run, each LiveView
+    span becomes a new root trace.
+
+    This module solves this by:
+    1. Attaching telemetry handlers with a lower priority (registered first)
+    2. Storing the trace context in process dictionary via `Sentry.Plug.LiveViewContext`
+    3. Extracting and attaching the context before `opentelemetry_phoenix` creates spans
+
+    ## Usage
+
+    Call `setup/0` in your application's start function, **BEFORE** calling
+    `OpentelemetryPhoenix.setup/1`:
+
+        def start(_type, _args) do
+          # Set up Sentry's LiveView context propagation FIRST
+          Sentry.OpenTelemetry.LiveViewPropagator.setup()
+
+          # Then set up OpentelemetryPhoenix
+          OpentelemetryPhoenix.setup(adapter: :bandit)
+
+          children = [
+            # ...
+          ]
+
+          Supervisor.start_link(children, strategy: :one_for_one)
+        end
+
+    Also add `Sentry.Plug.LiveViewContext` to your router pipeline:
+
+        pipeline :browser do
+          plug :accepts, ["html"]
+          plug :fetch_session
+          # ... other plugs
+          plug Sentry.Plug.LiveViewContext
+        end
+
+    *Available since v10.x.0.*
+    """
+
+    @moduledoc since: "10.8.0"
+
+    require Logger
+
+    @handler_id {__MODULE__, :live_view_context}
+
+    @doc """
+    Attaches telemetry handlers for LiveView context propagation.
+
+    Must be called BEFORE `OpentelemetryPhoenix.setup/1` to ensure handlers
+    run in the correct order.
+    """
+    @spec setup() :: :ok
+    def setup do
+      events = [
+        [:phoenix, :live_view, :mount, :start],
+        [:phoenix, :live_view, :handle_params, :start],
+        [:phoenix, :live_view, :handle_event, :start],
+        [:phoenix, :live_component, :handle_event, :start]
+      ]
+
+      _ =
+        :telemetry.attach_many(
+          @handler_id,
+          events,
+          &__MODULE__.handle_event/4,
+          %{}
+        )
+
+      :ok
+    end
+
+    @doc """
+    Detaches the telemetry handlers. Mainly useful for testing.
+    """
+    @spec teardown() :: :ok | {:error, :not_found}
+    def teardown do
+      :telemetry.detach(@handler_id)
+    end
+
+    @doc false
+    def handle_event(_event, _measurements, %{socket: socket} = _meta, _config) do
+      # During static render (HTTP request phase), we mark spans to be filtered
+      # since the HTTP span already covers this phase. Real LiveView interactions
+      # happen over WebSocket.
+      if static_render?(socket) do
+        # Store in process dict that this is a static render - the span processor
+        # can check this to filter out redundant LiveView spans
+        Process.put(:sentry_lv_static_render, true)
+        :ok
+      else
+        # WebSocket connection - clear the static render flag and propagate context
+        Process.delete(:sentry_lv_static_render)
+        propagate_context_if_needed(socket)
+      end
+    end
+
+    @doc """
+    Returns true if the current process is handling a static LiveView render.
+    Used by the span processor to filter redundant spans.
+    """
+    def static_render? do
+      Process.get(:sentry_lv_static_render, false)
+    end
+
+    # Check if this is a static render (not connected via WebSocket)
+    defp static_render?(socket) do
+      # During static render, transport_pid is nil
+      socket.transport_pid == nil
+    end
+
+    defp propagate_context_if_needed(socket) do
+      # Try to get trace context from socket's private assigns
+      case get_context_carrier(socket) do
+        nil ->
+          :ok
+
+        carrier when is_map(carrier) and map_size(carrier) > 0 ->
+          # Check if we already have an attached context with a valid span
+          # If so, don't overwrite it (this happens during static renders)
+          current_span_ctx = :otel_tracer.current_span_ctx()
+
+          _ =
+            unless has_valid_span?(current_span_ctx) do
+              # Extract and attach the trace context
+              ctx = :otel_ctx.get_current()
+
+              new_ctx =
+                Sentry.OpenTelemetry.Propagator.extract(
+                  ctx,
+                  carrier,
+                  &map_keys/1,
+                  &map_getter/2,
+                  []
+                )
+
+              :otel_ctx.attach(new_ctx)
+            end
+
+          :ok
+
+        _ ->
+          :ok
+      end
+    end
+
+    # Try to get the carrier from socket private assigns
+    defp get_context_carrier(socket) do
+      session_key = Sentry.Plug.LiveViewContext.session_key()
+
+      # The session can be in different places depending on the connection type:
+      # 1. WebSocket: socket.private.connect_info.session (map)
+      # 2. Static render (test): socket.private.connect_info is a %Plug.Conn{}
+      case socket do
+        # WebSocket connection has session as a map
+        %{private: %{connect_info: %{session: session}}} when is_map(session) ->
+          Map.get(session, session_key)
+
+        # Static render (Phoenix.LiveViewTest) has connect_info as Plug.Conn
+        %{private: %{connect_info: %Plug.Conn{private: %{plug_session: session}}}}
+        when is_map(session) ->
+          Map.get(session, session_key)
+
+        _ ->
+          nil
+      end
+    end
+
+    # Check if span context has a valid (non-zero) trace ID
+    defp has_valid_span?(:undefined), do: false
+
+    defp has_valid_span?(span_ctx) when is_tuple(span_ctx) do
+      case span_ctx do
+        {:span_ctx, trace_id, _span_id, _flags, _tracestate, _is_valid, _is_remote, _is_recording,
+         _sdk}
+        when trace_id != 0 ->
+          true
+
+        _other ->
+          false
+      end
+    end
+
+    defp map_keys(carrier), do: Map.keys(carrier)
+
+    defp map_getter(key, carrier) do
+      case Map.fetch(carrier, key) do
+        {:ok, value} -> value
+        :error -> :undefined
+      end
+    end
+  end
+end