Fix Health Monitor JSON plugin + add error logging

src/bosh-monitor/lib/bosh/monitor.rb

@@ -20,6 +20,7 @@ module Monitor
 require 'async'
 require 'async/http'
 require 'async/io'
+require 'async/io/stream'
 require 'logging'
 require 'nats/io/client'
 require 'puma'

src/bosh-monitor/lib/bosh/monitor/plugins/json.rb

@@ -72,7 +72,10 @@ def restart_process(bin)
 
     def start_process(bin)
       process = Bosh::Monitor::Plugins::DeferrableChildProcess.open(bin)
-      process.errback do
+      process.errback do |exception|
+        if exception
+          @logger.error("JSON Plugin: Unexpected error occurred for #{bin}: #{exception} - #{exception.backtrace.join("\n")}")
+        end
         Async do
           sleep(@restart_wait)
           restart_process bin
@@ -117,12 +120,12 @@ def run
         status = @wait_thr.value
         unless status.success?
           @errback.each do |errback|
-            errback.call
+            errback.call(nil)
           end
         end
       rescue => e
         @errback.each do |errback|
-          errback.call
+          errback.call(e)
         end
       ensure
         @stdin.close if @stdin

src/bosh-monitor/spec/unit/bosh/monitor/plugins/json_spec.rb

@@ -35,6 +35,17 @@
   context 'when the event loop is running' do
     include_context Async::RSpec::Reactor
 
+    it 'works' do
+      allow(Dir).to receive(:[]).with('/*/json-plugin/*').and_return(['echo "hello"'])
+      expect(Bosh::Monitor::Plugins::DeferrableChildProcess).to receive(:open).once.with('echo "hello"').and_call_original
+      expect(Open3).to receive(:popen3).with('echo "hello"').and_call_original
+
+      process_manager.start
+
+      # Wait to ensure that the process isn't restarted due to an internal error
+      sleep restart_wait * 2
+    end
+
     it 'starts processes that match the glob' do
       allow(Dir).to receive(:[]).with('/*/json-plugin/*').and_return(['/plugin'])