ensure that the shell provisioner only emits complete output lines to the ui

partial lines are buffered until they are complete (have a line ending)

see https://github.com/hashicorp/vagrant/issues/11047

lib/vagrant/util/line_buffer.rb

@@ -0,0 +1,35 @@
+require 'stringio'
+
+module Vagrant
+  module Util
+    class LineBuffer
+      def initialize
+        @buffer = StringIO.new
+      end
+
+      def lines(data, &block)
+        if data == nil
+          return
+        end
+        remaining_buffer = StringIO.new
+        @buffer << data
+        @buffer.string.each_line do |line|
+          if line.end_with? "\n"
+            block.call(line.rstrip)
+          else
+            remaining_buffer << line
+            break
+          end
+        end
+        @buffer = remaining_buffer
+      end
+
+      def remaining(&block)
+        if @buffer.length > 0
+          block.call(@buffer.string.rstrip)
+          @buffer = StringIO.new
+        end
+      end
+    end
+  end
+end

plugins/provisioners/shell/provisioner.rb

@@ -2,6 +2,7 @@ require "pathname"
 require "tempfile"
 
 require "vagrant/util/downloader"
+require "vagrant/util/line_buffer"
 require "vagrant/util/retryable"
 
 module VagrantPlugins
@@ -65,15 +66,42 @@ module VagrantPlugins
 
       protected
 
-      # This handles outputting the communication data back to the UI
-      def handle_comm(type, data)
-        if [:stderr, :stdout].include?(type)
-          # Output the data with the proper color based on the stream.
-          color = type == :stdout ? :green : :red
+      # This buffers a communicator output data (which can have partial
+      # lines) into full lines and emits them back to the UI.
+      def comm_line_buffer(&block)
+        stdout = Vagrant::Util::LineBuffer.new
+        stderr = Vagrant::Util::LineBuffer.new
 
-          # Clear out the newline since we add one
-          data = data.chomp
-          return if data.empty?
+        line_buffer_handle_comm = Proc.new do |type, data|
+          case type
+          when :stdout
+            stdout.lines data do |line|
+              handle_comm(type, line)
+            end
+          when :stderr
+            stderr.lines data do |line|
+              handle_comm(type, line)
+            end
+          end
+        end
+
+        # execute the block and emit complete lines to the UI.
+        block.call(line_buffer_handle_comm)
+
+        # emit the remaining as incomplete/partial lines to the UI.
+        stdout.remaining do |line|
+          handle_comm(:stdout, line)
+        end
+        stderr.remaining do |line|
+          handle_comm(:stderr, line)
+        end
+      end
+
+      # This handles outputting the communication line back to the UI
+      def handle_comm(type, line)
+        if [:stderr, :stdout].include?(type)
+          # Output the line with the proper color based on the stream.
+          color = type == :stdout ? :green : :red
 
           options = {}
           options[:color] = color if !config.keep_color
@@ -121,12 +149,14 @@ module VagrantPlugins
             end
 
             # Execute it with sudo
-            comm.execute(
-              command,
-              sudo: config.privileged,
-              error_key: :ssh_bad_exit_status_muted
-            ) do |type, data|
-              handle_comm(type, data)
+            comm_line_buffer do |handle_comm|
+              comm.execute(
+                command,
+                sudo: config.privileged,
+                error_key: :ssh_bad_exit_status_muted
+              ) do |type, data|
+                handle_comm.call(type, data)
+              end
             end
           end
         end
@@ -176,12 +206,14 @@ module VagrantPlugins
             end
 
             # Execute it with sudo
-            comm.execute(
-              command,
-              shell: :powershell,
-              error_key: :ssh_bad_exit_status_muted
-            ) do |type, data|
-              handle_comm(type, data)
+            comm_line_buffer do |handle_comm|
+              comm.execute(
+                command,
+                shell: :powershell,
+                error_key: :ssh_bad_exit_status_muted
+              ) do |type, data|
+                handle_comm.call(type, data)
+              end
             end
           end
         end
@@ -245,8 +277,10 @@ module VagrantPlugins
             end
 
             # Execute it with sudo
-            comm.sudo(command, { elevated: config.privileged, interactive: config.powershell_elevated_interactive }) do |type, data|
-              handle_comm(type, data)
+            comm_line_buffer do |handle_comm|
+              comm.sudo(command, { elevated: config.privileged, interactive: config.powershell_elevated_interactive }) do |type, data|
+                handle_comm.call(type, data)
+              end
             end
           end
         end