feat(flask): Add sentry_trace() template helper (#1336)

To setup distributed tracing links between a Flask app and a front-end app, one needs to figure out how to get the current hub, safely get the traceparent and then properly pass it into a template and then finally use that properly in a `meta` tag. [The guide](https://docs.sentry.io/platforms/javascript/performance/connect-services/) is woefully inadequete and error-prone so this PR adds a built-in helper `sentry_trace()` to the Flask integration to simplfy this linking.

Author: BYK

examples/tracing/templates/index.html

@@ -1,4 +1,6 @@
-<script src="https://browser.sentry-cdn.com/5.4.1/bundle.js" crossorigin="anonymous"></script>
+<meta name="sentry-trace" content="{{ sentry_trace }}" />
+<script src="https://browser.sentry-cdn.com/6.17.7/bundle.js" crossorigin="anonymous"></script>
+{{ sentry_trace }}
 <!-- TODO: Replace with real tracing integration once it's fixed -->
 <script src="/static/tracing.js" crossorigin="anonymous"></script>
 
@@ -14,14 +16,6 @@
     debug: true
 });
 
-window.setTimeout(function() {
-    const scope = Sentry.getCurrentHub().getScope();
-    // TODO: Wait for Daniel's traceparent API
-    scope.setSpan(scope.getSpan().constructor.fromTraceparent(
-        "00-{{ traceparent['sentry-trace'].strip("-") }}-00"
-    ));
-});
-
 async function compute() {
     const res = await fetch(
         "/compute/" +

sentry_sdk/integrations/flask.py

@@ -27,13 +27,15 @@
 
 try:
     from flask import (  # type: ignore
+        Markup,
         Request,
         Flask,
         _request_ctx_stack,
         _app_ctx_stack,
         __version__ as FLASK_VERSION,
     )
     from flask.signals import (
+        before_render_template,
         got_request_exception,
         request_started,
     )
@@ -77,6 +79,7 @@ def setup_once():
             if version < (0, 10):
                 raise DidNotEnable("Flask 0.10 or newer is required.")
 
+        before_render_template.connect(_add_sentry_trace)
         request_started.connect(_request_started)
         got_request_exception.connect(_capture_exception)
 
@@ -94,6 +97,23 @@ def sentry_patched_wsgi_app(self, environ, start_response):
         Flask.__call__ = sentry_patched_wsgi_app  # type: ignore
 
 
+def _add_sentry_trace(sender, template, context, **extra):
+    # type: (Flask, Any, Dict[str, Any], **Any) -> None
+
+    if "sentry_trace" in context:
+        return
+
+    sentry_span = Hub.current.scope.span
+    context["sentry_trace"] = (
+        Markup(
+            '<meta name="sentry-trace" content="%s" />'
+            % (sentry_span.to_traceparent(),)
+        )
+        if sentry_span
+        else ""
+    )
+
+
 def _request_started(sender, **kwargs):
     # type: (Flask, **Any) -> None
     hub = Hub.current

tests/integrations/flask/test_flask.py

@@ -6,7 +6,14 @@
 
 flask = pytest.importorskip("flask")
 
-from flask import Flask, Response, request, abort, stream_with_context
+from flask import (
+    Flask,
+    Response,
+    request,
+    abort,
+    stream_with_context,
+    render_template_string,
+)
 from flask.views import View
 
 from flask_login import LoginManager, login_user
@@ -365,7 +372,7 @@ def index():
     assert transaction_event["request"]["data"] == data
 
 
-@pytest.mark.parametrize("input_char", [u"a", b"a"])
+@pytest.mark.parametrize("input_char", ["a", b"a"])
 def test_flask_too_large_raw_request(sentry_init, input_char, capture_events, app):
     sentry_init(integrations=[flask_sentry.FlaskIntegration()], request_bodies="small")
 
@@ -737,3 +744,34 @@ def dispatch_request(self):
 
     assert event["message"] == "hi"
     assert event["transaction"] == "hello_class"
+
+
+def test_sentry_trace_context(sentry_init, app, capture_events):
+    sentry_init(integrations=[flask_sentry.FlaskIntegration()])
+    events = capture_events()
+
+    @app.route("/")
+    def index():
+        sentry_span = Hub.current.scope.span
+        capture_message(sentry_span.to_traceparent())
+        return render_template_string("{{ sentry_trace }}")
+
+    with app.test_client() as client:
+        response = client.get("/")
+        assert response.status_code == 200
+        assert response.data.decode(
+            "utf-8"
+        ) == '<meta name="sentry-trace" content="%s" />' % (events[0]["message"],)
+
+
+def test_dont_override_sentry_trace_context(sentry_init, app):
+    sentry_init(integrations=[flask_sentry.FlaskIntegration()])
+
+    @app.route("/")
+    def index():
+        return render_template_string("{{ sentry_trace }}", sentry_trace="hi")
+
+    with app.test_client() as client:
+        response = client.get("/")
+        assert response.status_code == 200
+        assert response.data == b"hi"