From 06f703a0b522e9e9b6a092c69dd68b39a7f48caa Mon Sep 17 00:00:00 2001
From: Thomas Eizinger <thomas@eizinger.io>
Date: Fri, 11 Jul 2025 15:32:11 +0200
Subject: [PATCH] feat(telemetry): log use of `map-enobufs-to-wouldblock`
 (#9829)

In order to better track, how well our `ENOBUFS` mitigation is working,
we should log the use of our feature flag to PostHog. This will give us
some stats how often this is happening. That combined with the lack of
error reports should give us good confidence in permanently enabling
this behaviour.
---
 rust/connlib/socket-factory/src/lib.rs |  3 ++
 rust/telemetry/src/analytics.rs        | 38 ++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/rust/connlib/socket-factory/src/lib.rs b/rust/connlib/socket-factory/src/lib.rs
index 7d0d0f910..9189dc20d 100644
--- a/rust/connlib/socket-factory/src/lib.rs
+++ b/rust/connlib/socket-factory/src/lib.rs
@@ -356,6 +356,9 @@ impl UdpSocket {
                         if e.raw_os_error().is_some_and(|e| e == libc::ENOBUFS)
                             && firezone_telemetry::feature_flags::map_enobufs_to_would_block() =>
                     {
+                        firezone_telemetry::analytics::feature_flag_called(
+                            "map-enobufs-to-wouldblock",
+                        );
                         tracing::debug!("Encountered ENOBUFS, treating as WouldBlock");
 
                         Err(io::Error::from(io::ErrorKind::WouldBlock))
diff --git a/rust/telemetry/src/analytics.rs b/rust/telemetry/src/analytics.rs
index 1c165d037..f9c0bc280 100644
--- a/rust/telemetry/src/analytics.rs
+++ b/rust/telemetry/src/analytics.rs
@@ -65,6 +65,36 @@ pub fn identify(release: String, account_slug: Option<String>) {
     });
 }
 
+pub fn feature_flag_called(name: impl Into<String>) {
+    let Some(env) = Telemetry::current_env() else {
+        tracing::debug!("Cannot send $feature_flag_called: Unknown env");
+        return;
+    };
+    let Some(distinct_id) = Telemetry::current_user() else {
+        tracing::debug!("Cannot send $feature_flag_called: Unknown user");
+        return;
+    };
+    let feature_flag = name.into();
+
+    RUNTIME.spawn({
+        async move {
+            if let Err(e) = capture(
+                "$feature_flag_called",
+                distinct_id,
+                env,
+                FeatureFlagCalledProperties {
+                    feature_flag,
+                    feature_flag_response: "true".to_owned(),
+                },
+            )
+            .await
+            {
+                tracing::debug!("Failed to log `$feature_flag_called` event: {e:#}");
+            }
+        }
+    });
+}
+
 async fn capture<P>(
     event: impl Into<String>,
     distinct_id: String,
@@ -137,3 +167,11 @@ struct PersonProperties {
     #[serde(rename = "$os")]
     os: String,
 }
+
+#[derive(serde::Serialize)]
+struct FeatureFlagCalledProperties {
+    #[serde(rename = "$feature_flag")]
+    feature_flag: String,
+    #[serde(rename = "$feature_flag_response")]
+    feature_flag_response: String,
+}