(PUP-11897) Fix excessive cpu usage after EOF from an exec

lib/puppet/util/execution.rb

@@ -223,8 +223,12 @@ def self.execute(command, options = NoOptionsSpecified)
             # Use non-blocking read to check for data. After each attempt,
             # check whether the child is done. This is done in case the child
             # forks and inherits stdout, as happens in `foo &`.
-
-            until results = Process.waitpid2(child_pid, Process::WNOHANG) #rubocop:disable Lint/AssignmentInCondition
+            # If we encounter EOF, though, then switch to a blocking wait for
+            # the child; after EOF, IO.select will never block and the loop
+            # below will use maximum CPU available.
+
+            wait_flags = Process::WNOHANG
+            until results = Process.waitpid2(child_pid, wait_flags) #rubocop:disable Lint/AssignmentInCondition
 
               # If not done, wait for data to read with a timeout
               # This timeout is selected to keep activity low while waiting on
@@ -235,6 +239,7 @@ def self.execute(command, options = NoOptionsSpecified)
                 output << reader.read_nonblock(4096) if ready
               rescue Errno::EAGAIN
               rescue EOFError
+                wait_flags &= ~Process::WNOHANG
               end
             end
 

spec/integration/type/exec_spec.rb

@@ -75,6 +75,19 @@
     end
   end
 
+  context 'when an exec sends an EOF' do
+    let(:command) { ["/bin/bash", "-c", "exec /bin/sleep 1 >/dev/null 2>&1"] }
+
+    it 'should not take significant user time' do
+      exec = described_class.new :command => command, :path => ENV['PATH']
+      catalog.add_resource exec
+      timed_apply = Benchmark.measure { catalog.apply }
+      # In testing I found the user time before the patch in 4f35fd262e to be above
+      # 0.3, after the patch it was consistently below 0.1 seconds.
+      expect(timed_apply.utime).to be < 0.3
+    end
+  end
+
   context 'when command is a string' do
     let(:command) { "ruby -e 'File.open(\"#{path}\", \"w\") { |f| f.print \"foo\" }'" }
 

spec/unit/util/execution_spec.rb

@@ -29,6 +29,7 @@ def stub_process_wait(exitstatus)
         allow(FFI::WIN32).to receive(:CloseHandle).with(thread_handle)
       else
         allow(Process).to receive(:waitpid2).with(pid, Process::WNOHANG).and_return(nil, [pid, double('child_status', :exitstatus => exitstatus)])
+        allow(Process).to receive(:waitpid2).with(pid, 0).and_return(nil, [pid, double('child_status', :exitstatus => exitstatus)])
         allow(Process).to receive(:waitpid2).with(pid).and_return([pid, double('child_status', :exitstatus => exitstatus)])
       end
     end