fix(subscriber): don't let a raising callback kill event dispatch

CHANGELOG.md

@@ -23,6 +23,9 @@
   the browser; a failed `IO.close` is raised to the caller.
 
 ### Fixed
+- A raising callback killed `Client::Subscriber`'s dispatch thread and with it every event for the rest of the
+  process: pages created later raised `NoSuchTargetError`, open ones lost their execution contexts. Callbacks are
+  now rescued one by one and reported to stderr [#470]
 - `create_page` raised `Failed to find browser context with id` against a browser that came up with its own startup
   window (browserless). That window lives in the browser's implicit context, which Chrome below 145
   won't address by id. Ferrum now detects it and exposes as `Ferrum::Context#implicit?`. It's never

lib/ferrum/client/subscriber.rb

@@ -133,6 +133,11 @@ def call(message)
         @on[event]&.each_with_index do |block, index|
           # In case of multiple callbacks we provide current index and total
           block.call(params, index, total)
+        rescue StandardError => e
+          # A raising callback used to terminate the dispatch thread, and with it every
+          # event for the rest of the browser's life: targets stopped being registered
+          # and any page created later raised `NoSuchTargetError`. Report and carry on.
+          warn("Ferrum: #{event} callback raised #{e.class}: #{e.message}\n  #{e.backtrace&.first}")
         end
       end
     end

spec/client_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+describe Ferrum::Client do
+  describe "event callbacks" do
+    let(:remote) { Ferrum::Browser.new(base_url: base_url, timeout: 3, protocol_timeout: 3) }
+
+    after { remote.quit }
+
+    it "keeps dispatching events when a callback raises" do
+      page = remote.create_page
+
+      expect do
+        remote.client.on("Target.targetCreated") { raise Ferrum::TimeoutError }
+
+        2.times { remote.create_page }
+        page.go_to("/")
+      end.to output(/Target.targetCreated callback raised Ferrum::TimeoutError/).to_stderr
+
+      expect(page.body).to include("Hello world!")
+    end
+  end
+end