feat(toolchain): add xz_target for remote execution

Copied what is currently done for gzip_target.

README.md

@@ -139,6 +139,9 @@ docker_toolchain_configure(
   # OPTIONAL: Path to the xz binary.
   # Should be set explicitly for remote execution.
   xz_path="<enter absolute path to the xz binary (in the remote exec env) here>",
+  # OPTIONAL: Bazel target for the xz tool.
+  # Either xz_path or xz_target should be set explicitly for remote execution.
+  xz_target="<enter absolute path (i.e., must start with repo name @...//:...) to an executable xz target>",
   # OPTIONAL: List of additional flags to pass to the docker command.
   docker_flags = [
     "--tls",

container/layer.bzl

@@ -116,8 +116,17 @@ def build_layer(
         args.add("--mtime=portable")
     if ctx.attr.enable_mtime_preservation:
         args.add("--enable_mtime_preservation=true")
-    if toolchain_info.xz_path != "":
-        args.add(toolchain_info.xz_path, format = "--xz_path=%s")
+
+    xz_path = toolchain_info.xz_path
+    xz_tools = []
+    xz_input_manifests = []
+    if toolchain_info.xz_target:
+        xz_path = toolchain_info.xz_target.files_to_run.executable.path
+        xz_tools, _, xz_input_manifests = ctx.resolve_command(tools = [toolchain_info.xz_target])
+    elif toolchain_info.xz_path == "":
+        print("WARNING: xz could not be found. Make sure it is in the path or set it " +
+              "explicitly in the docker_toolchain_configure")
+    args.add(xz_path, format = "--xz_path=%s")
 
     # Windows layer.tar require two separate root directories instead of just 1
     # 'Files' is the equivalent of '.' in Linux images.
@@ -151,7 +160,8 @@ def build_layer(
     ctx.actions.run(
         executable = build_layer_exec,
         arguments = [args],
-        tools = files + file_map.values() + tars + debs + [manifest_file],
+        input_manifests = xz_input_manifests,
+        tools = files + file_map.values() + tars + debs + [manifest_file] + xz_tools,
         outputs = [layer],
         execution_requirements = {
             # This action produces large output files, so it's not

testing/default_toolchain/WORKSPACE

@@ -21,13 +21,23 @@ local_repository(
     path = "replace/this/path",
 )
 
+load(
+    "//:local_tool.bzl",
+    "local_tool",
+)
+
+local_tool(
+    name = "xz",
+)
+
 load(
     "@io_bazel_rules_docker//toolchains/docker:toolchain.bzl",
     docker_toolchain_configure = "toolchain_configure",
 )
 
 docker_toolchain_configure(
     name = "docker_config",
+    xz_target = "@xz//:xz",
 )
 
 load(

testing/default_toolchain/local_tool.bzl

@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-"""Rule to load a local tool. Used to test gzip_target."""
+"""Rule to load a local tool. Used to test gzip_target and xz_target."""
 
 _local_tool_build_template = """
 sh_binary(

toolchains/docker/BUILD.tpl

@@ -24,5 +24,5 @@ docker_toolchain(
     %{GZIP_ATTR}
     tool_path = "%{DOCKER_TOOL}",
     docker_flags = ["%{DOCKER_FLAGS}"],
-    xz_path = "%{XZ_TOOL_PATH}",
+    %{XZ_ATTR}
 )

toolchains/docker/toolchain.bzl

@@ -31,6 +31,8 @@ DockerToolchainInfo = provider(
         "xz_path": "Optional path to the xz binary. This is used by " +
                    "build_tar.py when the Python lzma module is unavailable. " +
                    "If not set found via which.",
+        "xz_target": "Optional Bazel target for the xz tool. " +
+                     "Should only be set if xz_path is unset.",
     },
 )
 
@@ -43,6 +45,7 @@ def _docker_toolchain_impl(ctx):
             gzip_target = ctx.attr.gzip_target,
             tool_path = ctx.attr.tool_path,
             xz_path = ctx.attr.xz_path,
+            xz_target = ctx.attr.xz_target,
         ),
     )
     return [toolchain_info]
@@ -81,23 +84,34 @@ docker_toolchain = rule(
             doc = "Optional path to the xz binary. This is used by " +
                   "build_tar.py when the Python lzma module is unavailable.",
         ),
+        "xz_target": attr.label(
+            allow_files = True,
+            doc = "Bazel target for the xz tool. " +
+                  "Should only be set if xz_path is unset.",
+            cfg = "host",
+            executable = True,
+        ),
     },
 )
 
 def _toolchain_configure_impl(repository_ctx):
     if repository_ctx.attr.gzip_target and repository_ctx.attr.gzip_path:
         fail("Only one of gzip_target or gzip_path can be set.")
+    if repository_ctx.attr.xz_target and repository_ctx.attr.xz_path:
+        fail("Only one of xz_target or xz_path can be set.")
     tool_path = ""
     if repository_ctx.attr.docker_path:
         tool_path = repository_ctx.attr.docker_path
     elif repository_ctx.which("docker"):
         tool_path = repository_ctx.which("docker")
 
-    xz_path = ""
-    if repository_ctx.attr.xz_path:
-        xz_path = repository_ctx.attr.xz_path
+    xz_attr = ""
+    if repository_ctx.attr.xz_target:
+        xz_attr = "xz_target = \"%s\"," % repository_ctx.attr.xz_target
+    elif repository_ctx.attr.xz_path:
+        xz_attr = "xz_path = \"%s\"," % repository_ctx.attr.xz_path
     elif repository_ctx.which("xz"):
-        xz_path = repository_ctx.which("xz")
+        xz_attr = "xz_path = \"%s\"," % repository_ctx.which("xz")
 
     gzip_attr = ""
     if repository_ctx.attr.gzip_target:
@@ -118,7 +132,7 @@ def _toolchain_configure_impl(repository_ctx):
             "%{DOCKER_FLAGS}": "%s" % "\", \"".join(docker_flags),
             "%{DOCKER_TOOL}": "%s" % tool_path,
             "%{GZIP_ATTR}": "%s" % gzip_attr,
-            "%{XZ_TOOL_PATH}": "%s" % xz_path,
+            "%{XZ_ATTR}": "%s" % xz_attr,
         },
         False,
     )
@@ -176,6 +190,14 @@ toolchain_configure = repository_rule(
                   "be searched for in the path. If not available, running commands " +
                   "that use xz will fail.",
         ),
+        "xz_target": attr.label(
+            executable = True,
+            cfg = "host",
+            allow_files = True,
+            mandatory = False,
+            doc = "The bazel target for the xz tool. " +
+                  "Can only be set if xz_path is not set.",
+        ),
     },
     environ = [
         "PATH",