From b1f13bce62ff666f59286e8325011d8e9e0ddca7 Mon Sep 17 00:00:00 2001
From: Hood Chatham <roberthoodchatham@gmail.com>
Date: Tue, 29 Oct 2024 22:45:49 +0100
Subject: [PATCH] gh-124932: Distinguish build prefix from host prefix in cross
 builds (#124933)

In Emscripten and other cross builds, the build file system and the host file
system look different. For instance, we may want to install into
`cross-build/$TARGET/lib`, and then mount that as `/lib` in the host file
system. This change adds a distinction between:

* `prefix` -- the path in the build file system where we want to install the files
* `host_prefix` -- the path in the host file system where getpath.c will look for the files

And similarly for `exec_prefix` and `host_exec_prefix`. At present, this is only
used for Emscripten.
---
 Makefile.pre.in                               | 11 ++++++--
 ...-10-25-17-20-50.gh-issue-124932.F-aNuS.rst |  4 +++
 configure                                     | 25 +++++++++++++++++++
 configure.ac                                  | 19 ++++++++++++++
 4 files changed, 57 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Build/2024-10-25-17-20-50.gh-issue-124932.F-aNuS.rst

diff --git a/Makefile.pre.in b/Makefile.pre.in
index 751e7ab8b427f5..db5bad8752e541 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -141,6 +141,13 @@ prefix=		@prefix@
 # Install prefix for architecture-dependent files
 exec_prefix=	@exec_prefix@
 
+# For cross compilation, we distinguish between "prefix" (where we install the
+# files) and "host_prefix" (where getpath.c expects to find the files at
+# runtime)
+host_prefix= 	@host_prefix@
+host_exec_prefix= 	@host_exec_prefix@
+
+
 # Install prefix for data files
 datarootdir=    @datarootdir@
 
@@ -1740,8 +1747,8 @@ Modules/getbuildinfo.o: $(PARSER_OBJS) \
 
 Modules/getpath.o: $(srcdir)/Modules/getpath.c Python/frozen_modules/getpath.h Makefile $(PYTHON_HEADERS)
 	$(CC) -c $(PY_CORE_CFLAGS) -DPYTHONPATH='"$(PYTHONPATH)"' \
-		-DPREFIX='"$(prefix)"' \
-		-DEXEC_PREFIX='"$(exec_prefix)"' \
+		-DPREFIX='"$(host_prefix)"' \
+		-DEXEC_PREFIX='"$(host_exec_prefix)"' \
 		-DVERSION='"$(VERSION)"' \
 		-DVPATH='"$(VPATH)"' \
 		-DPLATLIBDIR='"$(PLATLIBDIR)"' \
diff --git a/Misc/NEWS.d/next/Build/2024-10-25-17-20-50.gh-issue-124932.F-aNuS.rst b/Misc/NEWS.d/next/Build/2024-10-25-17-20-50.gh-issue-124932.F-aNuS.rst
new file mode 100644
index 00000000000000..10c4171dc14a64
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2024-10-25-17-20-50.gh-issue-124932.F-aNuS.rst
@@ -0,0 +1,4 @@
+For cross builds, there is now support for having a different install
+``prefix`` than the ``host_prefix`` used by ``getpath.py``. This is set to ``/`` by
+default for Emscripten, on other platforms the default behavior is the same
+as before.
diff --git a/configure b/configure
index 13c7d8a734ccf5..7a778b414e9139 100755
--- a/configure
+++ b/configure
@@ -1003,6 +1003,8 @@ LIPO_INTEL64_FLAGS
 LIPO_32BIT_FLAGS
 ARCH_RUN_32BIT
 UNIVERSALSDK
+host_exec_prefix
+host_prefix
 MACHDEP
 PKG_CONFIG_LIBDIR
 PKG_CONFIG_PATH
@@ -4105,6 +4107,29 @@ fi
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: \"$MACHDEP\"" >&5
 printf "%s\n" "\"$MACHDEP\"" >&6; }
 
+
+if test -z "$host_prefix"; then
+  case $ac_sys_system in #(
+  Emscripten) :
+    host_prefix=/ ;; #(
+  *) :
+    host_prefix='${prefix}'
+   ;;
+esac
+fi
+
+
+if test -z "$host_exec_prefix"; then
+  case $ac_sys_system in #(
+  Emscripten) :
+    host_exec_prefix=$host_prefix ;; #(
+  *) :
+    host_exec_prefix='${exec_prefix}'
+   ;;
+esac
+fi
+
+
 # On cross-compile builds, configure will look for a host-specific compiler by
 # prepending the user-provided host triple to the required binary name.
 #
diff --git a/configure.ac b/configure.ac
index a470fd425880c5..42daf1c4c5adfd 100644
--- a/configure.ac
+++ b/configure.ac
@@ -379,6 +379,25 @@ then
 fi
 AC_MSG_RESULT(["$MACHDEP"])
 
+dnl For cross compilation, we distinguish between "prefix" (where we install the
+dnl files) and "host_prefix" (where we expect to find the files at runtime)
+
+if test -z "$host_prefix"; then
+  AS_CASE([$ac_sys_system],
+    [Emscripten], [host_prefix=/],
+    [host_prefix='${prefix}']
+  )
+fi
+AC_SUBST([host_prefix])
+
+if test -z "$host_exec_prefix"; then
+  AS_CASE([$ac_sys_system],
+    [Emscripten], [host_exec_prefix=$host_prefix],
+    [host_exec_prefix='${exec_prefix}']
+  )
+fi
+AC_SUBST([host_exec_prefix])
+
 # On cross-compile builds, configure will look for a host-specific compiler by
 # prepending the user-provided host triple to the required binary name.
 #