From 100d335e9d6fd8fc980e6f28c23f1d4836c0cb82 Mon Sep 17 00:00:00 2001
From: Ned Deily <nad@python.org>
Date: Mon, 24 May 2021 00:16:15 -0400
Subject: [PATCH] bpo-41282: Fix broken ``make install``

A previous commit broke a check in sysconfig when building cpython itself.
This caused builds of the standard library modules to search a wrong
location (the installed location rather than the source directory) for
header files with the net effect that a ``make install``
incorrectly caused all extension modules to be rebuilt again and
with incorrect include file paths.
---
 Lib/distutils/command/install.py                            | 3 +++
 Lib/sysconfig.py                                            | 6 ++++++
 .../next/Build/2021-05-24-03-31-17.bpo-41282.L8nP44.rst     | 3 +++
 3 files changed, 12 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Build/2021-05-24-03-31-17.bpo-41282.L8nP44.rst

diff --git a/Lib/distutils/command/install.py b/Lib/distutils/command/install.py
index d33a889afe4a81..8fa2a3adf2feac 100644
--- a/Lib/distutils/command/install.py
+++ b/Lib/distutils/command/install.py
@@ -316,6 +316,9 @@ def finalize_options(self):
             self.config_vars['userbase'] = self.install_userbase
             self.config_vars['usersite'] = self.install_usersite
 
+        if sysconfig.is_python_build(True):
+            self.config_vars['srcdir'] = sysconfig.get_config_var('srcdir')
+
         self.expand_basedirs()
 
         self.dump_dirs("post-expand_basedirs()")
diff --git a/Lib/sysconfig.py b/Lib/sysconfig.py
index e8869af0b5cc94..2e8d6d9ac32e2f 100644
--- a/Lib/sysconfig.py
+++ b/Lib/sysconfig.py
@@ -182,6 +182,12 @@ def is_python_build(check_home=False):
 
 _PYTHON_BUILD = is_python_build(True)
 
+if _PYTHON_BUILD:
+    for scheme in ('posix_prefix', 'posix_home'):
+        _INSTALL_SCHEMES[scheme]['include'] = '{srcdir}/Include'
+        _INSTALL_SCHEMES[scheme]['platinclude'] = '{projectbase}/.'
+
+
 def _subst_vars(s, local_vars):
     try:
         return s.format(**local_vars)
diff --git a/Misc/NEWS.d/next/Build/2021-05-24-03-31-17.bpo-41282.L8nP44.rst b/Misc/NEWS.d/next/Build/2021-05-24-03-31-17.bpo-41282.L8nP44.rst
new file mode 100644
index 00000000000000..cc6eadefc6cba7
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2021-05-24-03-31-17.bpo-41282.L8nP44.rst
@@ -0,0 +1,3 @@
+Fix broken ``make install`` that caused standard library extension modules
+to be unnecessarily and incorrectly rebuilt during the install phase of
+cpython.