Python: improve cross-compilation

This changeset allows for cross-compilation of Python packages. Packages
built with buildPythonPackage are not allowed to refer to the build
machine. Executables that have shebangs will refer to the host.

2 files changed, 24 insertions, 8 deletions

diff --git a/pkgs/development/interpreters/python/cpython/2.7/default.nix b/pkgs/development/interpreters/python/cpython/2.7/default.nix
index 23b88b168b5b..249c4ac9cf79 100644
--- a/pkgs/development/interpreters/python/cpython/2.7/default.nix
+++ b/pkgs/development/interpreters/python/cpython/2.7/default.nix
@@ -32,6 +32,9 @@ assert x11Support -> tcl != null
 with stdenv.lib;
 
 let
+
+  pythonForBuild = buildPackages.${"python${sourceVersion.major}${sourceVersion.minor}"};
+
   passthru = passthruFun rec {
     inherit self sourceVersion packageOverrides;
     implementation = "cpython";
diff --git a/pkgs/development/interpreters/python/cpython/default.nix b/pkgs/development/interpreters/python/cpython/default.nix
index 0d1794f04fab..6e738a598dc4 100644
--- a/pkgs/development/interpreters/python/cpython/default.nix
+++ b/pkgs/development/interpreters/python/cpython/default.nix
@@ -21,6 +21,7 @@
 , sourceVersion
 , sha256
 , passthruFun
+, bash
 }:
 
 assert x11Support -> tcl != null
@@ -46,7 +47,8 @@ let
   nativeBuildInputs = [
     nukeReferences
   ] ++ optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
-    buildPackages.stdenv.cc crossPython
+    buildPackages.stdenv.cc
+    pythonForBuild
   ];
 
   buildInputs = filter (p: p != null) [
@@ -56,11 +58,11 @@ let
 
   hasDistutilsCxxPatch = !(stdenv.cc.isGNU or false);
 
-  crossPython = buildPackages.${"python${sourceVersion.major}${sourceVersion.minor}"};
+  pythonForBuild = buildPackages.${"python${sourceVersion.major}${sourceVersion.minor}"};
 
-  pythonForBuild = if stdenv.hostPlatform == stdenv.buildPlatform then
+  pythonForBuildInterpreter = if stdenv.hostPlatform == stdenv.buildPlatform then
     "$out/bin/python"
-  else crossPython.interpreter;
+  else pythonForBuild.interpreter;
 
 in with passthru; stdenv.mkDerivation {
   pname = "python3";
@@ -215,14 +217,25 @@ in with passthru; stdenv.mkDerivation {
     # We rebuild three times, once for each optimization level
     # Python 3.7 implements PEP 552, introducing support for deterministic bytecode.
     # This is automatically used when `SOURCE_DATE_EPOCH` is set.
-    find $out -name "*.py" | ${pythonForBuild}     -m compileall -q -f -x "lib2to3" -i -
-    find $out -name "*.py" | ${pythonForBuild} -O  -m compileall -q -f -x "lib2to3" -i -
-    find $out -name "*.py" | ${pythonForBuild} -OO -m compileall -q -f -x "lib2to3" -i -
+    find $out -name "*.py" | ${pythonForBuildInterpreter}     -m compileall -q -f -x "lib2to3" -i -
+    find $out -name "*.py" | ${pythonForBuildInterpreter} -O  -m compileall -q -f -x "lib2to3" -i -
+    find $out -name "*.py" | ${pythonForBuildInterpreter} -OO -m compileall -q -f -x "lib2to3" -i -
+  '';
+
+  preFixup = stdenv.lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) ''
+    # Ensure patch-shebangs uses shebangs of host interpreter.
+    export PATH=${stdenv.lib.makeBinPath [ "$out" bash ]}:$PATH
   '';
 
   # Enforce that we don't have references to the OpenSSL -dev package, which we
   # explicitly specify in our configure flags above.
-  disallowedReferences = [ openssl.dev ];
+  disallowedReferences = [
+    openssl.dev
+  ] ++ stdenv.lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
+    # Ensure we don't have references to build-time packages.
+    # These typically end up in shebangs.
+    pythonForBuild buildPackages.bash
+  ];
 
   inherit passthru;