(MODULES-8748) Improve pipe reading in the PowerShell Manager

Previously the powershell manager was modified to read in data from the pipe
until all data was read, in 8K chunks.  However it was noted that this would be
causing more IO calls than needed due to the 8K chunks.

In this commit, the reader will attempt to read all bytes in a single call and
use the string concatenation method to reduce the number the potential number
of internal objects being created.

Also the is_readable? method was updated to also check that the stream is not
EOF. In odd cases the stream can show that it's ready to read, but a sysread
call will throw an EOF, for example this could occur when data is buffered in
a slow way.

Author: glennsarti

lib/puppet_x/puppetlabs/powershell/powershell_manager.rb

@@ -274,7 +274,7 @@ def self.instance_key(cmd, args, options)
       def self.is_readable?(stream, timeout = 0.5)
         raise Errno::EPIPE if !is_stream_valid?(stream)
         read_ready = IO.select([stream], [], [], timeout)
-        read_ready && stream == read_ready[0][0]
+        read_ready && stream == read_ready[0][0] && !stream.eof?
       end
 
       # when a stream has been closed by handle, but Ruby still has a file
@@ -395,17 +395,16 @@ def read_streams
           # read a Little Endian 32-bit integer for length of response
           expected_response_length = pipe.sysread(4).unpack('V').first
 
-          if expected_response_length == 0
-            nil
-          else
-            # reads the expected bytes as a binary string or fails
-            buffer = ""
-            # Reads in the pipe data 8K, or less, at a time
-            while (buffer.length < expected_response_length)
-              buffer << pipe.sysread([expected_response_length - buffer.length, 8192].min)
-            end
-            buffer
+          next nil if expected_response_length == 0
+          # reads the expected bytes as a binary string or fails
+          buffer = ""
+          # sysread may not return all of the requested bytes due to buffering or the
+          # underlying IO system. Keep reading from the pipe until all the bytes are read
+          loop do
+            buffer.concat(pipe.sysread(expected_response_length - buffer.length))
+            break if buffer.length >= expected_response_length
           end
+          buffer
         end
 
         Puppet.debug "Waited #{Time.now - start_time} total seconds."