[win/bootstrap] Use "for" instead of "set".

Using "set /p" to read a file fails when STDOUT is not connected.
Replace it with a method of reading using "for".

In the Python batch file, we eat up some of our sled buffer in order to
accommodate currently-running Python scripts. They should now resume
past the end of the file without issue. The sled was oversized, so this
shouldn't affect upgrade paths.

BUG=chromium:746602
TEST=local
  - On Windows machine, tested upgrade from pre-rework and current
    bootstraps. This code works on both, and no running Python
    processes were disrupted on exit, suggesting the sled change is
    sound.
R=iannucci@chromium.org, smut@chromium.org

Change-Id: Ica51999d672dd4e448fdad797bffc06ec9e9febf
Reviewed-on: https://chromium-review.googlesource.com/578725
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: smut <smut@google.com>

bootstrap/win/python27.bleeding_edge.bat

+@echo off
+:: Copyright 2017 The Chromium Authors. All rights reserved.
+:: Use of this source code is governed by a BSD-style license that can be
+:: found in the LICENSE file.
+
+setlocal
+set PYTHON_BAT_RUNNER=1
+
+::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+
+:: This file is automatically generated by "bootstrap\win\win_tools.py", and
+:: should not be modified.
+::
+:: The previous "::" block acts as a nop-sled. Each time a batch file executes
+:: a command, it reloads itself and seeks to its previous execution offset to
+:: begin execution. Updating this batch file is, therefore, risky, since any
+:: running Python instance that is using the old batch file will reload the new
+:: batch file once the Python command terminates and resume at some unknown
+:: offset.
+::
+:: With the sled in place, a previous instance will resume mid-label. We are
+:: assuming that the offset of the Python invocation is greater than the
+:: PYTHON_BAT_RUNNER set command, which is the case since the old instance has
+:: a large PATH set block before the Python execution. Old instances will hit
+:: the next block of code without PYTHON_BAT_RUNNER set. New instances will have
+:: it set.
+::
+:: We remedy this in the future by having the batch file load its core paths
+:: from an external file with for/set, removing the need to modify "python.bat"
+:: during upgrade.
+::
+:: After all of the old batch files are believed to be replaced, we can remove
+:: the PYTHON_BAT_RUNNER block and the sled. For this update, old instances
+:: will resume past the end of the file and terminate.
+
+if not "%PYTHON_BAT_RUNNER%" == "1" goto :END
+
+for /f %%i in (%~dp0python_bin_reldir.txt) do set PYTHON_BIN_RELDIR=%%i
+set PATH=%~dp0%PYTHON_BIN_RELDIR%;%~dp0%PYTHON_BIN_RELDIR%\Scripts;%PATH%
+"%~dp0%PYTHON_BIN_RELDIR%\python.exe" %*
+
+:END

bootstrap/win/win_tools.py

@@ -320,11 +320,14 @@ def main(argv):
       template.PYTHON_BIN_RELDIR,
       os.path.join(ROOT_DIR, 'python_bin_reldir.txt'))
 
+  python_bat_template = ('python27.new.bat' if not args.bleeding_edge
+                         else 'python27.bleeding_edge.bat')
+
   # Re-evaluate and regenerate our root templated files.
   for src_name, dst_name in (
       ('git-bash.template.sh', 'git-bash'),
       ('pylint.new.bat', 'pylint.bat'),
-      ('python27.new.bat', 'python.bat'),
+      (python_bat_template, 'python.bat'),
       ):
     template.maybe_install(src_name, os.path.join(ROOT_DIR, dst_name))
 

cipd.bat

@@ -17,7 +17,7 @@ if not exist "%~dp0.cipd_client.exe" (
   if not errorlevel 0 goto :END
 )
 
-set /p CIPD_CLIENT_VER=<%~dp0cipd_client_version
+for /f %%i in (%~dp0cipd_client_version) do set CIPD_CLIENT_VER=%%i
 "%~dp0.cipd_client.exe" selfupdate -version "%CIPD_CLIENT_VER%"
 if not errorlevel 0 goto :END