sentry: Add the observed user's IP address before forwarding.

The default for Javascript reporting is that Sentry sets the IP
address of the user to the IP address that the report was observed to
come from[^1].  Since all reports come through the Zulip server, this
results in all reports being "from" one IP address, thus undercounting
the number of affected unauthenticated users, and making it difficult
to correlate Sentry reports with server logs.

Consume the Sentry Envelope format[^2] to inject the submitting
client's observed IP address, when possible.  This ensures that Sentry
reports contain the same IP address that Zulip's server logs do.

[^1]: https://docs.sentry.io/platforms/python/guides/logging/enriching-events/identify-user/
[^2]: https://develop.sentry.dev/sdk/envelopes/

zerver/views/sentry.py

@@ -1,5 +1,7 @@
 import urllib
+from contextlib import suppress
 
+import orjson
 from django.conf import settings
 from django.http import HttpRequest, HttpResponse
 from django.utils.translation import gettext as _
@@ -23,9 +25,9 @@ def sentry_tunnel(
     request: HttpRequest,
 ) -> HttpResponse:
     try:
-        envelope = request.body
-        header = to_wild_value("envelope", envelope.split(b"\n")[0].decode("utf-8"))
-        dsn = urllib.parse.urlparse(header["dsn"].tame(check_url))
+        envelope_header_line, envelope_items = request.body.split(b"\n", 1)
+        envelope_header = to_wild_value("envelope_header", envelope_header_line.decode("utf-8"))
+        dsn = urllib.parse.urlparse(envelope_header["dsn"].tame(check_url))
     except Exception:
         raise JsonableError(_("Invalid request format"))
 
@@ -35,7 +37,47 @@ def sentry_tunnel(
     assert dsn.hostname
     project_id = dsn.path.strip("/")
     url = dsn._replace(netloc=dsn.hostname, path=f"/api/{project_id}/envelope/").geturl()
+
+    # Adjust the payload to explicitly contain the IP address of the
+    # user we see.  If left blank, Sentry will assume the IP it
+    # received the request from, which is Zulip's, which can make
+    # debugging more complicated.
+    updated_body = request.body
+    # If we fail to update the body for any reason, leave it as-is; it
+    # is better to mis-report the IP than to drop the report entirely.
+    with suppress(Exception):
+        # This parses the Sentry ingestion format, known as an
+        # Envelope.  See https://develop.sentry.dev/sdk/envelopes/ for
+        # spec.
+        parts = [envelope_header_line, b"\n"]
+        while envelope_items != b"":
+            item_header_line, rest = envelope_items.split(b"\n", 1)
+            parts.append(item_header_line)
+            parts.append(b"\n")
+            item_header = orjson.loads(item_header_line.decode("utf-8"))
+            length = item_header.get("length")
+            if length is None:
+                item_body, envelope_items = ([*rest.split(b"\n", 1), b""])[:2]
+            else:
+                item_body, envelope_items = rest[0:length], rest[length:]
+            if item_header.get("type") in ("transaction", "event"):
+                # Event schema:
+                # https://develop.sentry.dev/sdk/event-payloads/#core-interfaces
+                # https://develop.sentry.dev/sdk/event-payloads/user/
+                #
+                # Transaction schema:
+                # https://develop.sentry.dev/sdk/event-payloads/transaction/#anatomy
+                # Note that "Transactions are Events enriched with Span data."
+                payload_data = orjson.loads(item_body)
+                if "user" in payload_data:
+                    payload_data["user"]["ip_address"] = request.META.get("REMOTE_ADDR")
+                    item_body = orjson.dumps(payload_data)
+            parts.append(item_body)
+            if length is None:
+                parts.append(b"\n")
+        updated_body = b"".join(parts)
+
     SentryTunnelSession().post(
-        url=url, data=envelope, headers={"Content-Type": "application/x-sentry-envelope"}
+        url=url, data=updated_body, headers={"Content-Type": "application/x-sentry-envelope"}
     ).raise_for_status()
     return HttpResponse(status=200)