fix: send buffered telemetry on the background worker

CHANGELOG.md

@@ -1,3 +1,9 @@
+## Unreleased
+
+### Bug Fixes 🐛
+
+- Send buffered logs and metrics on the background worker instead of the thread that filled the buffer by @ikraamg in [#3079](https://github.com/getsentry/sentry-ruby/pull/3079)
+
 ## 7.0.0
 
 ### Breaking Changes 🛠

sentry-ruby/lib/sentry/telemetry_event_buffer.rb

@@ -125,7 +125,16 @@ def send_items
         { items: envelope_items }
       )
 
-      @client.send_envelope(envelope)
+      # Off the calling thread: send_items runs under @mutex, so a request thread that filled the
+      # buffer would otherwise hold every other add_item caller behind its HTTP round trip.
+      queued = Sentry.background_worker.perform do
+        @client.send_envelope(envelope)
+      rescue => e
+        log_error("[#{self.class}] Failed to send #{@event_class}", e, debug: @debug)
+      end
+
+      # A full worker queue discards the block without raising, the same way it drops an event.
+      @client.transport.record_lost_event(:queue_overflow, @data_category, num: envelope_items.size) unless queued
     rescue => e
       log_error("[#{self.class}] Failed to send #{@event_class}", e, debug: @debug)
     ensure

sentry-ruby/spec/support/shared_examples_for_telemetry_event_buffers.rb

@@ -58,6 +58,46 @@
     end
   end
 
+  describe "sending" do
+    let(:max_items) { 3 }
+
+    before do
+      Sentry.configuration.background_worker_threads = 1
+      Sentry.background_worker = Sentry::BackgroundWorker.new(Sentry.configuration)
+    end
+
+    it "sends the envelope on the background worker, not on the thread that filled the buffer" do
+      sending_thread = nil
+      allow(client).to receive(:send_envelope) { sending_thread = Thread.current }
+
+      3.times { subject.add_item(event) }
+      Sentry.background_worker.shutdown
+
+      expect(sending_thread).not_to be_nil
+      expect(sending_thread).not_to eq(Thread.current)
+    end
+
+    it "records the batch as lost when the background worker's queue is full" do
+      allow(Sentry.background_worker).to receive(:perform).and_return(false)
+      expect(client.transport).to receive(:record_lost_event).with(:queue_overflow, subject.data_category, num: 3)
+
+      3.times { subject.add_item(event) }
+
+      expect(subject).to be_empty
+    end
+
+    it "flushes on the background worker too" do
+      sending_thread = nil
+      allow(client).to receive(:send_envelope) { sending_thread = Thread.current }
+
+      subject.add_item(event)
+      subject.flush
+      Sentry.background_worker.shutdown
+
+      expect(sending_thread).not_to eq(Thread.current)
+    end
+  end
+
   describe "multi-threaded access" do
     let(:max_items) { 30 }