Ensure nixpkgs will work with Bazel build from source

This commit make sure, that the checks placed for certain Bazel version will treat Bazel build from source gracefuly

core/util.bzl

@@ -1,5 +1,6 @@
 load("@bazel_tools//tools/cpp:lib_cc_configure.bzl", "get_cpu_value")
 load("@bazel_skylib//lib:paths.bzl", "paths")
+load("@bazel_skylib//lib:versions.bzl", "versions")
 
 def is_supported_platform(repository_ctx):
     return repository_ctx.which("nix-build") != None
@@ -287,3 +288,28 @@ def expand_location(repository_ctx, string, labels, attr = None):
             fail("Internal error: Unknown location expansion command '{}'.".format(command), attr)
 
     return result
+
+def is_bazel_version_at_least(threshold):
+    """ Check if current bazel version is higer or equals to a threshold.
+
+    Args:
+        threshold: string: minimum desired version of Bazel
+
+    Returns:
+        treshold_met, from_source_version: bool, bool: tuple where
+        first item states if the treshold was met, the second indicates
+        if obtained bazel version is empty string (indicating from source build)
+    """
+    treshold_met = False
+    from_source_version = False
+
+    bazel_version = versions.get()
+    if not bazel_version:
+        from_source_version = True
+    else:
+        treshold_met = versions.is_at_least(threshold, bazel_version)
+
+    return (
+        treshold_met,
+        from_source_version,
+    )

toolchains/cc/cc.bzl

@@ -15,10 +15,10 @@ load(
     "write_builtin_include_directory_paths",
 )
 load("@bazel_skylib//lib:sets.bzl", "sets")
-load("@bazel_skylib//lib:versions.bzl", "versions")
 load("@rules_nixpkgs_core//:nixpkgs.bzl", "nixpkgs_package")
 load(
     "@rules_nixpkgs_core//:util.bzl",
+    "is_bazel_version_at_least",
     "ensure_constraints",
     "execute_or_fail",
 )
@@ -134,7 +134,8 @@ def _nixpkgs_cc_toolchain_config_impl(repository_ctx):
 
     # A module map is required for clang starting from Bazel version 3.3.0.
     # https://github.com/bazelbuild/bazel/commit/8b9f74649512ee17ac52815468bf3d7e5e71c9fa
-    needs_module_map = info.is_clang and versions.is_at_least("3.3.0", versions.get())
+    bazel_version_match, bazel_from_source  = is_bazel_version_at_least("3.3.0")
+    needs_module_map = info.is_clang and (bazel_version_match or bazel_from_source)
     if needs_module_map:
         generate_system_module_map = [
             repository_ctx.path(repository_ctx.attr._generate_system_module_map),

toolchains/python/python.bzl

@@ -7,13 +7,13 @@ Rules for importing a Python toolchain from Nixpkgs.
 * [nixpkgs_python_configure](#nixpkgs_python_configure)
 """
 
-load("@bazel_skylib//lib:versions.bzl", "versions")
 load(
     "@rules_nixpkgs_core//:nixpkgs.bzl",
     "nixpkgs_package",
 )
 load(
     "@rules_nixpkgs_core//:util.bzl",
+    "is_bazel_version_at_least",
     "ensure_constraints",
     "label_string",
 )
@@ -56,7 +56,8 @@ _nixpkgs_python_toolchain = repository_rule(
 )
 
 def _python_nix_file_content(attribute_path, bin_path, version):
-    add_shebang = versions.is_at_least("4.2.0", versions.get())
+    bazel_version_match, bazel_from_source = is_bazel_version_at_least("4.2.0")
+    add_shebang = bazel_version_match or bazel_from_source
 
     return """
 with import <nixpkgs> {{ config = {{}}; overlays = []; }};