unix: define and use CROSS_COMPILING environment variable

This will help pave the road for x86-64 triple variants.

Author: indygreg

cpython-unix/build-cpython.sh

@@ -62,7 +62,7 @@ rm -rf pip-tmp
 
 # If we are cross-compiling, we need to build a host Python to use during
 # the build.
-if [ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" ]; then
+if [ -n "${CROSS_COMPILING}" ]; then
   pushd "Python-${PYTHON_VERSION}"
 
   # Same patch as below. See comment there.
@@ -312,7 +312,7 @@ fi
 
 # Configure nerfs RUNSHARED when cross-compiling, which prevents i386 PGO from
 # running from an x86_64 environment. Undo that, as we can run i386 from x86_64.
-if [[ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" && "${TARGET_TRIPLE}" = "i686-unknown-linux-gnu" ]]; then
+if [[ -n "${CROSS_COMPILING}" && "${TARGET_TRIPLE}" = "i686-unknown-linux-gnu" ]]; then
     patch -p1 << "EOF"
 diff --git a/configure b/configure
 index 1252335472..33c11fbade 100755
@@ -719,7 +719,7 @@ if [ "${PYBUILD_PLATFORM}" = "macos" ]; then
         CONFIGURE_FLAGS="${CONFIGURE_FLAGS} ac_cv_func_pwritev=no"
     fi
 
-    if [ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" ]; then
+    if [ -n "${CROSS_COMPILING}" ]; then
         # Python's configure doesn't support cross-compiling on macOS. So we need
         # to explicitly set MACHDEP to avoid busted checks. The code for setting
         # MACHDEP also sets ac_sys_system/ac_sys_release, so we have to set
@@ -764,7 +764,7 @@ if [ "${PYBUILD_PLATFORM}" = "macos" ]; then
     export MACOSX_DEPLOYMENT_TARGET="${APPLE_MIN_DEPLOYMENT_TARGET}"
 fi
 
-if [ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" ]; then
+if [ -n "${CROSS_COMPILING}" ]; then
     # configure doesn't like a handful of scenarios when cross-compiling.
     #
     # getaddrinfo buggy test fails for some reason. So we short-circuit it.
@@ -851,7 +851,7 @@ fi
 # we have the Makefile emit a script which sets some environment
 # variables that force the invoked Python to pick up the configuration
 # of the target Python but invoke the host binary.
-if [ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" ]; then
+if [ -n "${CROSS_COMPILING}" ]; then
     make write-python-for-build
     BUILD_PYTHON=$(pwd)/python-for-build
 else

cpython-unix/build-libX11.sh

@@ -43,7 +43,7 @@ fi
 
 # configure doesn't support cross-compiling in malloc(0) returns null test.
 # So we have to force a value.
-if [ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" ]; then
+if [ -n "${CROSS_COMPILING}" ]; then
   case "${TARGET_TRIPLE}" in
     aarch64-unknown-linux-gnu)
       EXTRA_FLAGS="${EXTRA_FLAGS} --enable-malloc0returnsnull"

cpython-unix/build-ncurses.sh

@@ -16,7 +16,7 @@ tar -xf ncurses-${NCURSES_VERSION}.tar.gz
 # ncurses version. Our workaround is to build ncurses for the host when
 # cross-compiling then make its `tic` available to the target ncurses
 # build.
-if [[ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" && "${PYBUILD_PLATFORM}" != "macos" ]]; then
+if [[ -n "${CROSS_COMPILING}" && "${PYBUILD_PLATFORM}" != "macos" ]]; then
   echo "building host ncurses to provide modern tic for cross-compile"
 
   pushd ncurses-${NCURSES_VERSION}
@@ -51,7 +51,7 @@ CONFIGURE_FLAGS="
 # ncurses wants --with-build-cc when cross-compiling. But it insists on CC
 # and this value not being equal, even though using the same binary with
 # different compiler flags is doable!
-if [[ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" && "${PYBUILD_PLATFORM}" != "macos" ]]; then
+if [[ -n "${CROSS_COMPILING}" && "${PYBUILD_PLATFORM}" != "macos" ]]; then
   CONFIGURE_FLAGS="${CONFIGURE_FLAGS} --with-build-cc=$(which "${HOST_CC}")"
 fi
 

cpython-unix/build.py

@@ -99,10 +99,14 @@ def add_target_env(env, build_platform, target_triple, build_env):
         env["BUILD_TRIPLE"] = "x86_64-unknown-linux-gnu"
 
         # TODO should the musl target be normalized?
-        if target_triple == "x86_64-unknown-linux-musl":
-            env["TARGET_TRIPLE"] = "x86_64-unknown-linux-gnu"
-        else:
-            env["TARGET_TRIPLE"] = target_triple
+        env["TARGET_TRIPLE"] = target_triple.replace(
+            "-unknown-linux-musl", "-unknown-linux-gnu"
+        )
+
+        if env["BUILD_TRIPLE"] != target_triple or target_triple.endswith(
+            "-unknown-linux-musl"
+        ):
+            env["CROSS_COMPILING"] = "1"
 
     if build_platform == "macos":
         machine = platform.machine()
@@ -130,6 +134,9 @@ def add_target_env(env, build_platform, target_triple, build_env):
 
         env["TARGET_TRIPLE"] = target_triple
 
+        if env["BUILD_TRIPLE"] != env["TARGET_TRIPLE"]:
+            env["CROSS_COMPILING"] = "1"
+
         # We don't have build isolation on macOS. We nerf PATH to prevent
         # non-system (e.g. Homebrew) executables from being used.
         env["PATH"] = "/usr/bin:/bin"