feat: Introduces a new target_tar_toolchain_type (#65)

This PR is a follow-up to
bazel-contrib/bazel-lib#1180 which is supposed
to help with the issue described in
bazel-contrib/rules_oci#769. The description
of the issue and proposal are in
bazel-contrib/rules_oci#844 PR.

CC @alexeagle @fmeum

Author: sfc-gh-dbieliaiev

tar/tests/BUILD

@@ -46,6 +46,33 @@ assert_archive_contains(
     ],
 )
 
+# Test that resolved_target_toolchain works similarly to resolved_toolchain.
+# This demonstrates target-platform-based toolchain resolution.
+genrule(
+    name = "tar_genrule_target_toolchain",
+    srcs = [
+        ":fixture1",
+        "src_file",
+    ],
+    outs = ["1_target.tar"],
+    cmd = "$(BSDTAR_BIN) --create --dereference --file $@ -s '#$(BINDIR)##' $(execpath :fixture1) $(execpath src_file)",
+    target_compatible_with = select({
+        # bsdtar.exe: -s is not supported by this version of bsdtar
+        "@platforms//os:windows": ["@platforms//:incompatible"],
+        "//conditions:default": [],
+    }),
+    toolchains = ["@bsd_tar_toolchains//:resolved_target_toolchain"],
+)
+
+assert_archive_contains(
+    name = "test_genrule_target_toolchain",
+    archive = "1_target.tar",
+    expected = [
+        "tar/tests/generated.txt",
+        "tar/tests/src_file",
+    ],
+)
+
 #############
 # Example 2: exact control of the resulting tar file, using a custom specification in the "mtree" format.
 # Copied from the output of `man tar`:

tar/toolchain/BUILD

@@ -4,4 +4,6 @@ package(default_visibility = ["//visibility:public"])
 
 toolchain_type(name = "type")
 
+toolchain_type(name = "target_type")
+
 utf8_environment(name = "utf8_environment")

tar/toolchain/toolchain.bzl

@@ -84,13 +84,29 @@ resolved_toolchain = rule(
     implementation = _resolved_toolchain_impl,
     toolchains = ["@tar.bzl//tar/toolchain:type"],
 )
+
+def _resolved_target_toolchain_impl(ctx):
+    toolchain_info = ctx.toolchains["@tar.bzl//tar/toolchain:target_type"]
+    return [
+        toolchain_info,
+        toolchain_info.default,
+        toolchain_info.tarinfo,
+        toolchain_info.template_variables,
+    ]
+
+resolved_target_toolchain = rule(
+    implementation = _resolved_target_toolchain_impl,
+    toolchains = ["@tar.bzl//tar/toolchain:target_type"],
+)
 """
     rctx.file("defs.bzl", starlark_content)
 
     build_content = """# @generated by @tar.bzl//tar/toolchain:toolchain.bzl
-load(":defs.bzl", "resolved_toolchain")
+load(":defs.bzl", "resolved_toolchain", "resolved_target_toolchain")
 
-resolved_toolchain(name = "resolved_toolchain", visibility = ["//visibility:public"])"""
+resolved_toolchain(name = "resolved_toolchain", visibility = ["//visibility:public"])
+
+resolved_target_toolchain(name = "resolved_target_toolchain", visibility = ["//visibility:public"])"""
 
     for [platform, meta] in BSDTAR_PLATFORMS.items():
         build_content += """
@@ -100,6 +116,13 @@ toolchain(
     toolchain = "@{user_repository_name}_{platform}//:bsdtar_toolchain",
     toolchain_type = "@tar.bzl//tar/toolchain:type",
 )
+
+toolchain(
+    name = "{platform}_target_toolchain",
+    target_compatible_with = {compatible_with},
+    toolchain = "@{user_repository_name}_{platform}//:bsdtar_toolchain",
+    toolchain_type = "@tar.bzl//tar/toolchain:target_type",
+)
 """.format(
             platform = platform,
             user_repository_name = rctx.attr.user_repository_name,