bazel: fix python stub paths.

Since the 0.21 upgrade, the host `$PATH` is not forwarded anymore by
default to the sandboxes in charge to realize Bazel actions. This
default change broke the `py_binary` rule among other things.

Every python binary is wrapped in a stub in charge to setup the
execution environment. Currently, this stub's shebang points to a
`/usr/bin/env python` which cannot be resolved with the current
`$PATH`.
This results in breaking any build pipeline requiring the use of
python at some point. On top of the incorrect shebang, the stub
template is unable to find the actual python binary using
`SearchPath`.

This PR fixes those two things by re-writing the stub template shebang
to the actual python binary and by substituting the faulty default
python binary lookup to the right one.

pkgs/development/tools/build-managers/bazel/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, lib, fetchurl, fetchpatch, runCommand, makeWrapper
+{ stdenv, callPackage, lib, fetchurl, fetchpatch, runCommand, makeWrapper
 , jdk, zip, unzip, bash, writeCBin, coreutils
 , which, python, perl, gnused, gnugrep, findutils
 # Apple dependencies
@@ -38,6 +38,11 @@ stdenv.mkDerivation rec {
     platforms = platforms.linux ++ platforms.darwin;
   };
 
+  # additional tests that check bazel’s functionality
+  passthru.tests = {
+    python_bin_path = callPackage ./python-bin-path-test.nix {};
+  };
+
   name = "bazel-${version}";
 
   src = fetchurl {
@@ -47,8 +52,10 @@ stdenv.mkDerivation rec {
 
   sourceRoot = ".";
 
-  patches =
-    lib.optional enableNixHacks ./nix-hacks.patch;
+  patches = [
+    (lib.optional enableNixHacks ./nix-hacks.patch)
+    ./python-stub-path-fix.patch
+    ];
 
   # Bazel expects several utils to be available in Bash even without PATH. Hence this hack.
 
@@ -118,6 +125,10 @@ stdenv.mkDerivation rec {
     '';
 
     genericPatches = ''
+      # Substitute python's stub shebang to plain python path. (see TODO add pr URL)
+      substituteInPlace src/main/java/com/google/devtools/build/lib/bazel/rules/python/python_stub_template.txt\
+          --replace "/usr/bin/env python" "${python}/bin/python" \
+          --replace "NIX_STORE_PYTHON_PATH" "${python}/bin/python" \
       # substituteInPlace is rather slow, so prefilter the files with grep
       grep -rlZ /bin src/main/java/com/google/devtools | while IFS="" read -r -d "" path; do
         # If you add more replacements here, you must change the grep above!
@@ -137,6 +148,7 @@ stdenv.mkDerivation rec {
         --replace '"jvm_opts": JDK9_JVM_OPTS' \
                   '"jvm_opts": JDK8_JVM_OPTS'
 
+
       # add nix environment vars to .bazelrc
       cat >> .bazelrc <<EOF
       build --experimental_distdir=${distDir}

pkgs/development/tools/build-managers/bazel/python-bin-path-test.nix

@@ -0,0 +1,55 @@
+{ stdenv, lib, writeText, runCommandCC, bazel }:
+
+let
+  WORKSPACE = writeText "WORKSPACE" ''
+    workspace(name = "our_workspace")
+  '';
+
+  pythonLib = writeText "lib.py" ''
+    def foo():
+      return 43
+  '';
+
+  pythonBin = writeText "bin.py" ''
+    from lib import foo
+
+    assert foo() == 43
+  '';
+
+  pythonBUILD = writeText "BUILD" ''
+    py_library(
+      name = "lib",
+      srcs = [ "lib.py" ],
+    )
+
+    py_test(
+      name = "bin",
+      srcs = [ "bin.py" ],
+      deps = [ ":lib" ],
+    )
+  '';
+
+  runLocal = name: script: runCommandCC name { preferLocalBuild = true; } script;
+
+  workspaceDir = runLocal "our_workspace" ''
+    mkdir $out
+    cp ${WORKSPACE} $out/WORKSPACE
+    mkdir $out/python
+    cp ${pythonLib} $out/python/lib.py
+    cp ${pythonBin} $out/python/bin.py
+    cp ${pythonBUILD} $out/python/BUILD.bazel
+  '';
+
+  testBazel = runLocal "bazel-test-builtin-rules" ''
+    export HOME=$(mktemp -d)
+    cp -r ${workspaceDir}/* .
+    ${bazel}/bin/bazel --output_base=/tmp/bazel-tests/wd\
+      test \
+        --test_output=errors \
+        --host_javabase='@local_jdk//:jdk' \
+        //...
+
+    touch $out
+  '';
+
+in testBazel

pkgs/development/tools/build-managers/bazel/python-stub-path-fix.patch

@@ -0,0 +1,13 @@
+diff --git a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/python_stub_template.txt b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/python_stub_template.txt
+index dac21c9a83..69b11c283f 100644
+--- a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/python_stub_template.txt
++++ b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/python_stub_template.txt
+@@ -67,7 +67,7 @@ def FindPythonBinary(module_space):
+     return os.path.join(module_space, PYTHON_BINARY)
+   else:
+     # Case 4: Path has to be looked up in the search path.
+-    return SearchPath(PYTHON_BINARY)
++    return "NIX_STORE_PYTHON_PATH"
+
+ def CreatePythonPathEntries(python_imports, module_space):
+   parts = python_imports.split(':');