From f092670a89ce4eb213cf295406996fe97d257d21 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Fri, 16 Apr 2021 10:26:01 +0200
Subject: Handle hermetic configurations in ~host and ~build2

Specifically, we keep config.config.environment but strip
config.config.hermetic. Also update the INSTALL file.
---
 INSTALL.cli         | 20 +++++++++++++++-----
 libbuild2/buildfile | 23 ++++++++++++++---------
 2 files changed, 29 insertions(+), 14 deletions(-)

diff --git a/INSTALL.cli b/INSTALL.cli
index 7150edc..dd29c17 100644
--- a/INSTALL.cli
+++ b/INSTALL.cli
@@ -2,9 +2,10 @@
 // license   : MIT; see accompanying LICENSE file
 
 "
-Unless you specifically only need the \c{build2} build system and not the
-complete \c{build2} toolchain, you should use the \c{build2-toolchain}
-distribution instead.
+Unless you specifically only need the \c{build2} build system, you should
+install the entire \c{build2} toolchain instead, either using the
+\l{https://build2.org/install.xhtml install script} or the
+\c{build2-toolchain} distribution.
 
 The instructions outlined below are essentially a summary of the first three
 steps of the manual bootstrap process described in \c{build2-toolchain} with a
@@ -12,7 +13,7 @@ few extra examples that would primarily be useful for distribution packaging.
 
 Also, below we only show commands for UNIX-like operating systems. For other
 operating systems and for more details on each step, refer to the
-\c{build2-toolchain} documentation.
+\c{build2-toolchain} installation documentation.
 
 \c{build2} requires a C++14 compiler. GCC 4.9, Clang 3.7, and MSVC 14 (2015)
 Update 3 or any later versions of these compilers are known to work. The build
@@ -106,6 +107,7 @@ using shared libraries:
 
 \
 $ build2/b-boot configure         \
+  config.config.hermetic=true     \
   config.cxx=g++                  \
   config.cc.coptions=-O3          \
   config.bin.rpath=/usr/local/lib \
@@ -115,6 +117,13 @@ $ build2/b-boot configure         \
 $ build2/b-boot
 \
 
+\N|The \c{config.config.hermetic=true} configuration variable in the first
+command makes sure the embedded \c{~host} and \c{~build2} configurations
+include the current environment. This is especially important for \c{~build2}
+which is used to dynamically build and load ad hoc recipes and build system
+modules and must therefore match the environment that was used to build the
+build system itself.|
+
 If you are only interested in installing the result, then you can avoid
 building tests by specifying the \c{update-for-install} operation in the last
 command:
@@ -126,7 +135,6 @@ $ build2/b-boot update-for-install
 On the other hand, if I you are not planning to install the result, then you
 can omit the \c{config.install.*} values as well as \c{.rpath}.
 
-
 To install:
 
 \
@@ -146,6 +154,7 @@ Or, alternatively, for an out of tree build:
 
 \
 $ build2-static/build2/b configure: build2-X.Y.Z/@build2-shared/ \
+  config.config.hermetic=true     \
   config.cxx=g++                  \
   config.cc.coptions=-O3          \
   config.bin.rpath=/usr/local/lib \
@@ -167,6 +176,7 @@ configuration variable, for example:
 
 \
 $ build2-static/build2/b configure: build2-X.Y.Z/@build2-shared/ \
+  config.config.hermetic=true        \
   config.cxx=g++                     \
   config.cc.coptions=-O3             \
   config.install.root=/usr           \
diff --git a/libbuild2/buildfile b/libbuild2/buildfile
index 28adbdd..96fcfd2 100644
--- a/libbuild2/buildfile
+++ b/libbuild2/buildfile
@@ -66,18 +66,23 @@ config/cxx{host-config}: config/in{host-config}
   # For ~build2 also filter out config.install.chroot -- we definitely don't
   # want it carried through.
   #
-  build2_config = $regex.replace_lines(                 \
-    $config.save(),                                     \
-    '^ *(#|config\.dist\.|config\.install\.chroot).*$', \
-    [null],                                             \
+  # Finally, for both ~host and ~build2 we keep config.config.environment
+  # but strip config.config.hermetic* (we shouldn't be forcing hermiticity
+  # on the users of ~host/~build2; they can decide for themselves if they
+  # want it).
+  #
+  build2_config = $regex.replace_lines(                              \
+    $config.save(),                                                  \
+    '^ *(#|(config\.(dist\.|install\.chroot|config\.hermetic))).*$', \
+    [null],                                                          \
     return_lines)
 
-  # Also preserve blank lines between groups of options.
+  # Also preserve config.version and blank lines between groups of options.
   #
-  host_config = $regex.replace_lines(                \
-    $build2_config,                                  \
-    '^( *config\.(c[. ]|cxx[. ]|cc[.]|bin[.]).*|)$', \
-    '$&',                                            \
+  host_config = $regex.replace_lines(                                             \
+    $build2_config,                                                               \
+    '^( *config\.(c[. ]|cxx[. ]|cc[.]|bin[.]|config.environment |version ).*|)$', \
+    '$&',                                                                         \
     format_no_copy return_lines)
 }
 
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