Do not hang in communicate if spawned process forks

Currently we only exit the `communicate` loop when we get an EOF from
stdout and stderr. If the child process forks before exiting, the fork
will inherit those file descriptors and `communicate` will hang until
the child exits. I believe it's preferable to drain stdout/stderr after
the process exits and then break out of the `communicate` loop.

Note that Open3 similarly hangs waiting for EOF. There is a risk that
existing software depends on this behavior and that output could be
non-deterministic if the forked grandchild is writing to stdout/stderr.

lib/subprocess.rb

@@ -414,13 +414,17 @@ def communicate(input=nil, timeout_s=nil)
       self.class.catching_sigchld(pid) do |global_read, self_read|
         wait_r = [@stdout, @stderr, self_read, global_read].compact
         wait_w = [input && @stdin].compact
-        loop do
-          # If the process has exited and we're not waiting to read anything
-          # other than the self pipe, then we're done.
-          break if poll && wait_r == [self_read, global_read]
-
-          ready_r, ready_w = select_until(wait_r, wait_w, [], timeout_at)
-          raise CommunicateTimeout.new(@command, stdout, stderr) if ready_r.nil?
+        done = false
+        while !done
+          # If the process has exited, we want to drain any remaining output before returning
+          if poll
+            ready_r = wait_r - [self_read, global_read]
+            ready_w = []
+            done = true
+          else
+            ready_r, ready_w = select_until(wait_r, wait_w, [], timeout_at)
+            raise CommunicateTimeout.new(@command, stdout, stderr) if ready_r.nil?
+          end
 
           if ready_r.include?(@stdout)
             if drain_fd(@stdout, stdout)

lib/subprocess/version.rb

@@ -1,3 +1,3 @@
 module Subprocess
-  VERSION = '1.5.2'
+  VERSION = '1.5.3'
 end

test/test_subprocess.rb

@@ -294,6 +294,16 @@ def call_multiwrite_script(&block)
       end
     end
 
+    it 'does not deadlock when communicating with a process that forks' do
+      script = <<EOF
+  echo "fork"
+  (echo "me" && sleep 10) &
+EOF
+      p = Subprocess::Process.new(['bash', '-c', script], stdout: Subprocess::PIPE)
+      stdout, _stderr = p.communicate(nil, 5)
+      stdout.must_include("fork\n")
+    end
+
     it 'should not raise an error when the process closes stdin before we finish writing' do
       script = File.join(File.dirname(__FILE__), 'bin', 'closer.rb')
       Subprocess.check_call(['bash', '-c', '<&-; echo -n "foo"; sleep 1'], :stdin => Subprocess::PIPE,