Allow downloading prebuilt binary in kubectl toolchain (#253)

* Allow downloading prebuilt binary in kubectl toolchain

Addresses customer request to download v1.10.0 in #245 because
v1.10.0 can't be built from source.

* Address review comments

* Doc fix

toolchains/kubectl/README.md

@@ -6,7 +6,9 @@ rule. At the moment, the tool's configuration does one of the following:
 
 1. Detect the path to the `kubectl` tool (default).
 2. Build the `kubectl` tool from source.
+3. Download a `kubectl` prebuilt binary not on the system path.
 
+### Use a kubectl built from source
 If you want to build the `kubectl` tool from source you will
 need to add to your `WORKSPACE` file the following lines (Note:
 The call to `kubectl_configure` must be before the call to
@@ -91,6 +93,56 @@ source repository is compatible with. Look at the `http_archive` invocation in
 https://github.com/kubernetes/kubernetes/blob/{k8s_commit}/build/root/WORKSPACE
 for the `@io_kubernetes_build` to get the commit pin, sha256 and prefix values.*
 
+### Download a custom kubectl binary
+
+If you want to download a standard binary released by the kubernetes project,
+get the URL and SHA256 for the binary for your platform from [here](https://kubernetes.io/docs/tasks/tools/install-kubectl/#install-kubectl-binary-using-curl).
+For example, if you want to use kubectl v1.10.0 on a x86 64 bit Linux platform,
+add to your `WORKSPACE` file the following lines (Note: The call to
+`kubectl_configure` must be before the  call to `k8s_repositories`):
+
+```python
+load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
+
+git_repository(
+    name = "io_bazel_rules_docker",
+    commit = "{HEAD}",
+    remote = "https://github.com/bazelbuild/rules_docker.git",
+)
+
+load(
+  "@io_bazel_rules_docker//container:container.bzl",
+  container_repositories = "repositories",
+)
+
+container_repositories()
+
+# This requires rules_docker to be fully instantiated before
+# it is pulled in.
+git_repository(
+    name = "io_bazel_rules_k8s",
+    commit = "{HEAD}",
+    remote = "https://github.com/bazelbuild/rules_k8s.git",
+)
+
+load("@io_bazel_rules_k8s//toolchains/kubectl:kubectl_configure.bzl", "kubectl_configure")
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_file")
+# Download the v1.10.0 kubectl binary for the Linux x86 64 bit platform.
+http_file(
+    name="k8s_binary",
+    downloaded_file_path = "kubectl",
+    sha256="49f7e5791d7cd91009c728eb4dc1dbf9ee1ae6a881be6b970e631116065384c3",
+    executable=True,
+    urls=["https://storage.googleapis.com/kubernetes-release/release/v1.10.0/bin/linux/amd64/kubectl"],
+)
+# Configure the kubectl toolchain to use the downloaded prebuilt v1.10.0
+# kubectl binary.
+kubectl_configure(name="k8s_config", kubectl_path="@k8s_binary//file")
+k8s_repositories()
+```
+
+
+
 ## Using the kubectl toolchain
 
 The information below will be helpful if:

toolchains/kubectl/kubectl_configure.bzl

@@ -30,11 +30,13 @@ load(":defaults.bzl",
 
 def _impl(repository_ctx):
     substitutions = None
-    label = None
     if repository_ctx.attr.build_srcs:
         kubectl_target = "@io_kubernetes//cmd/kubectl:kubectl"
         substitutions = {"%{KUBECTL_TARGET}": "%s" % kubectl_target}
         template = Label("@io_bazel_rules_k8s//toolchains/kubectl:BUILD.target.tpl")
+    elif repository_ctx.attr.kubectl_path != None:
+         substitutions = {"%{KUBECTL_TARGET}": "%s" % repository_ctx.attr.kubectl_path}
+         template = Label("@io_bazel_rules_k8s//toolchains/kubectl:BUILD.target.tpl")
     else:
         kubectl_tool_path = repository_ctx.which("kubectl") or ""
         substitutions = {"%{KUBECTL_TOOL}": "%s" % kubectl_tool_path}
@@ -44,12 +46,18 @@ def _impl(repository_ctx):
         "BUILD",
         template,
         substitutions,
-        False,
+        False
     )
 
 _kubectl_configure = repository_rule(
     implementation = _impl,
     attrs = {
+        "kubectl_path": attr.label(
+            allow_single_file=True,
+            mandatory=False,
+            doc = "Optional. Path to a prebuilt custom kubectl binary file or" +
+                  " label. Can't be used together with attribute 'build_srcs'.",
+        ),
         "build_srcs": attr.bool(
             doc = "Optional. Set to true to build kubectl from sources.",
             default = False,
@@ -96,18 +104,24 @@ def kubectl_configure(name, **kwargs):
       Required Args
         name: A unique name for this rule.
       Default Args:
-        build_srcs: Optional. Set to true to build kubectl from sources. Default: False
-        k8s_commit: Otional. Commit / release tag at which to build kubectl
+        build_srcs: Optional. Set to true to build kubectl from sources. Default: False.
+                    Can't be specified if kubectl_path is specified.
+        k8s_commit: Optional. Commit / release tag at which to build kubectl
           from. Default is defined as k8s_tag in :defaults.bzl.
         k8s_sha256: Optional. sha256 of commit at which to build kubectl from.
           Default is defined as k8s_sha256 in :defaults.bzl.
+        kubectl_path: Optional. Use the kubectl binary at the given path or label.
+        This can't be used with 'build_srcs'.
       Note: Not all versions/commits of kubernetes project can be used to compile
       kubectl from an external repo. Notably, we have only tested with v1.13.0-beta.1
       or above. Note this rule has a hardcoded pointer to io_kubernetes_build repo
       if your commit (above v1.13.0-beta.1) does not work due to problems,
       related to @io_kubernetes_build repo, please send a PR to update these values.
     """
     build_srcs = False
+    if "build_srcs" in kwargs and "kubectl_path" in kwargs:
+        fail("Attributes 'build_srcs' and 'kubectl_path' can't be specified at"+
+             " the same time")
     if "build_srcs" in kwargs and kwargs["build_srcs"]:
         build_srcs = True
         _ensure_all_provided("kubectl_configure",
@@ -143,4 +157,7 @@ def kubectl_configure(name, **kwargs):
                 k8s_repo_tools_commit
             )],
         )
-    _kubectl_configure(name = name, build_srcs = build_srcs)
+    if "kubectl_path" in kwargs:
+        _kubectl_configure(name = name, kubectl_path=kwargs["kubectl_path"])
+    else:
+        _kubectl_configure(name = name, build_srcs = build_srcs)

toolchains/kubectl/kubectl_toolchain.bzl

@@ -19,7 +19,7 @@ KubectlInfo = provider(
     doc = "Information about how to invoke the kubectl tool.",
     fields = {
         "tool_path": "Path to the kubectl executable",
-        "tool_target": "Target to build kubectl executable",
+        "tool_target": "A kubectl executable target built from source or downloaded.",
     },
 )
 
@@ -42,7 +42,7 @@ kubectl_toolchain = rule(
             mandatory = False,
         ),
         "tool_target": attr.label(
-            doc = "Target to build kubectl from source.",
+            doc = "Target to build kubectl from source or a downloaded kubectl binary.",
             mandatory = False,
         ),
     },