Handle long reads and use IO#readpartial

The complete read might not show up in the first read() call,
so continue looping until the target length is reached.

Use IO#readpartial instead of looping with IO#sysread; the difference is
that readpartial loops on EAGAIN and EINTR, but it uses select() to wait
for a readable handle instead of doing a busy retry.

code/protocol.rb

@@ -91,14 +91,15 @@ def do_write(client, buff)
       defined?(nwritten) ? nwritten : 0
     end
 
-    def do_read(client, numr = 32768)
-      client.sysread(numr)
-    rescue Errno::EAGAIN, Errno::EINTR  # Ruby threading can cause an alarm/timer interrupt on a syscall
-      sleep 0.001 # A tiny pause to prevent consuming all CPU
-      retry
+    def do_read(client, size)
+      out = ""
+      while out.bytesize < size
+        remain = size - out.bytesize
+        out << client.readpartial(remain)
+      end
+      out
     rescue EOFError
-      $log.debug("Got an EOF from socket read")
-      return nil
+      out
     rescue Errno::ECONNRESET,Errno::EPIPE,Errno::EINVAL,Errno::EBADF
       raise "Got an #{$!} from socket read"
     end

code/queue.rb

@@ -1071,7 +1071,7 @@ def handle_status_read(msg)
       child_pid = msg['child_pid']
 
       $log.debug("#{child_pid}: Reading status from child")
-      data = do_read(child_io, 4096)
+      data = child_io.readpartial(4096) rescue nil
       return false unless data
 
       child_msgs = data.split("\n")