Add launcher,launcher_args attrs to container_image (#616)

* Add launcher,launcher_args attrs to container_image

* Add tests for 'launcher,launcher_args' attrs of container_image rule.

* Better document `container_image.launcher` restrictions.

README.md

@@ -1798,6 +1798,25 @@ container_image(name, base, data_path, directory, files, legacy_repository_namin
         syntax, e.g. <code>foo{BUILD_USER}bar</code>.</p>
       </td>
     </tr>
+    <tr>
+      <td><code>launcher</code></td>
+      <td>
+        <p><code>Label; optional</code></p>
+        <p>If present, prefix the image's ENTRYPOINT with this file.
+        Note that the launcher should be a container-compatible (OS & Arch)
+        single executable file without any runtime dependencies (as none
+        of its runfiles will be included in the image).
+        </p>
+      </td>
+    </tr>
+    <tr>
+      <td><code>launcher_args</code></td>
+      <td>
+        <p><code>String list; optional</code></p>
+        <p>Optional arguments for the <code>launcher</code> attribute.
+        Only valid when <code>launcher</code> is specified.</p>
+      </td>
+    </tr>
   </tbody>
 </table>
 

container/create_image_config.py

@@ -86,6 +86,9 @@
                     choices=['linux', 'windows'],
                     help=('Operating system to create docker image for, e.g. {linux}'))
 
+parser.add_argument('--entrypoint_prefix', action='append', default=[],
+                    help='Prefix the "Entrypoint" with the specified arguments.')
+
 _PROCESSOR_ARCHITECTURE = 'amd64'
 
 def KeyValueToDict(pair):
@@ -195,6 +198,10 @@ def Stamp(inp):
       args.null_entrypoint == "True"):
     del (output['config']['Entrypoint'])
 
+  if args.entrypoint_prefix:
+    output['config']['Entrypoint'] = (args.entrypoint_prefix +
+                                      output['config'].get('Entrypoint', []))
+
   with open(args.output, 'w') as fp:
     json.dump(output, fp, sort_keys=True)
     fp.write('\n')

container/image.bzl

@@ -186,6 +186,14 @@ def _image_config(
         args += ["--stamp-info-file=%s" % f.path for f in stamp_inputs]
         inputs += stamp_inputs
 
+    if ctx.attr.launcher_args and not ctx.attr.launcher:
+        fail("launcher_args does nothing when launcher is not specified.", attr = "launcher_args")
+    if ctx.attr.launcher:
+        args += [
+            "--entrypoint_prefix=%s" % x
+            for x in ["/" + ctx.file.launcher.basename] + ctx.attr.launcher_args
+        ]
+
     ctx.actions.run(
         executable = ctx.executable.create_image_config,
         arguments = args,
@@ -302,6 +310,11 @@ def _impl(
         legacy_run_behavior = ctx.attr.base[ImageInfo].legacy_run_behavior
         docker_run_flags = ctx.attr.base[ImageInfo].docker_run_flags
 
+    if ctx.attr.launcher:
+        if not file_map:
+            file_map = {}
+        file_map["/" + ctx.file.launcher.basename] = ctx.file.launcher
+
     # composite a layer from the container_image rule attrs,
     image_layer = _layer.implementation(
         ctx = ctx,
@@ -457,6 +470,8 @@ _attrs = dict(_layer.attrs.items() + {
     "layers": attr.label_list(providers = [LayerInfo]),
     "repository": attr.string(default = "bazel"),
     "stamp": attr.bool(default = False),
+    "launcher": attr.label(allow_single_file = True),
+    "launcher_args": attr.string_list(default = []),
     # Implicit/Undocumented dependencies.
     "label_files": attr.label_list(
         allow_files = True,

testdata/BUILD

@@ -970,6 +970,23 @@ go_image(
     ],
 )
 
+load("@io_bazel_rules_go//go:def.bzl", "go_binary")
+
+go_binary(
+    name = "launcher",
+    srcs = ["launcher_main.go"],
+    goarch = "amd64",
+    goos = "linux",
+    pure = "on",
+)
+
+container_image(
+    name = "launcher_image",
+    base = ":go_image",
+    launcher = ":launcher",
+    launcher_args = ["-env=CUSTOM_MESSAGE=Launched via launcher!"],
+)
+
 load("//rust:image.bzl", "rust_image")
 
 rust_image(

testdata/launcher_main.go

@@ -0,0 +1,46 @@
+// Copyright 2017 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// 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.
+
+// This file is used to test the launcher attr of container_image
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"syscall"
+)
+
+func main() {
+	var extraEnv stringSlice
+	flag.Var(&extraEnv, "env", "Append to the environment of the launched binary. May be specified multiple times. (eg --env=VAR_NAME=value)")
+	flag.Parse()
+	envv := append(os.Environ(), extraEnv...)
+	argv := flag.Args()
+	err := syscall.Exec(argv[0], argv, envv)
+	if err != nil {
+		panic(err)
+	}
+}
+
+type stringSlice []string
+
+func (i *stringSlice) String() string {
+	return fmt.Sprintf("%s", *i)
+}
+
+func (i *stringSlice) Set(v string) error {
+	*i = append(*i, v)
+	return nil
+}

testdata/main.go

@@ -15,8 +15,15 @@
 // Package main tells the linter to shut up.
 package main
 
-import "fmt"
+import (
+    "fmt"
+    "os"
+)
 
 func main() {
-    fmt.Println("Hello, world!")
+    message := "Hello, world!"
+    if v, ok := os.LookupEnv("CUSTOM_MESSAGE"); ok {
+        message = v
+    }
+    fmt.Println(message)
 }

testing/e2e.sh

@@ -251,6 +251,12 @@ function test_cc_image_wrapper() {
   EXPECT_CONTAINS "$(bazel run "$@" testdata:cc_image_wrapper)" "Hello World"
 }
 
+function test_launcher_image() {
+  cd "${ROOT}"
+  clear_docker
+  EXPECT_CONTAINS "$(bazel run "$@" testdata:launcher_image)" "Launched via launcher!"
+}
+
 function test_go_image() {
   cd "${ROOT}"
   clear_docker
@@ -517,3 +523,4 @@ test_rust_image -c dbg
 test_nodejs_image -c opt
 test_nodejs_image -c dbg
 test_container_push
+test_launcher_image