Properly set cmd from args in lang images. (#624)

Fixes #618.

lang/image.bzl

@@ -196,6 +196,9 @@ def app_layer_impl(ctx, runfiles = None, emptyfiles = None):
         # we should use the "exec" (list) form of entrypoint.
         entrypoint = entrypoint,
         cmd = args,
+        # If `args` is set to [], None or not set, Docker config will have
+        # a null `Cmd` value.
+        null_cmd = args == [],
     )
 
 _app_layer = rule(

tests/docker/nodejs/BUILD

@@ -16,6 +16,7 @@ package(default_visibility = ["//visibility:public"])
 load("//nodejs:image.bzl", "nodejs_image")
 load("//contrib:test.bzl", "container_test")
 
+# Docker Cmd value should be `["arg0", "arg1"]`.
 nodejs_image(
     name = "nodejs_image",
     args = [
@@ -27,23 +28,67 @@ nodejs_image(
     node_modules = "@npm_deps//:node_modules",
 )
 
+# Docker Cmd value should be `[""]`.
 nodejs_image(
-    name = "nodejs_image_no_args",
-    # this should work without having to set args to empty list
+    name = "nodejs_image_list_with_empty_string_args",
     args = [""],
     data = ["//testdata:nodejs_image.js"],
     entry_point = "io_bazel_rules_docker/testdata/nodejs_image.js",
     node_modules = "@npm_deps//:node_modules",
 )
 
+# Docker Cmd value should be `null`.
+nodejs_image(
+    name = "nodejs_image_no_args",
+    data = ["//testdata:nodejs_image.js"],
+    entry_point = "io_bazel_rules_docker/testdata/nodejs_image.js",
+    node_modules = "@npm_deps//:node_modules",
+)
+
+# Docker Cmd value should be `null`.
+nodejs_image(
+    name = "nodejs_image_empty_list_args",
+    args = [],
+    data = ["//testdata:nodejs_image.js"],
+    entry_point = "io_bazel_rules_docker/testdata/nodejs_image.js",
+    node_modules = "@npm_deps//:node_modules",
+)
+
+# Docker Cmd value should be `null`.
+nodejs_image(
+    name = "nodejs_image_none_args",
+    args = None,
+    data = ["//testdata:nodejs_image.js"],
+    entry_point = "io_bazel_rules_docker/testdata/nodejs_image.js",
+    node_modules = "@npm_deps//:node_modules",
+)
+
 container_test(
     name = "nodejs_image_test",
     configs = ["//tests/docker/nodejs/configs:nodejs_image.yaml"],
     image = ":nodejs_image",
 )
 
+container_test(
+    name = "nodejs_image_list_with_empty_string_args_test",
+    configs = ["//tests/docker/nodejs/configs:nodejs_image_list_with_empty_string_args.yaml"],
+    image = ":nodejs_image_list_with_empty_string_args",
+)
+
 container_test(
     name = "nodejs_image_no_args_test",
     configs = ["//tests/docker/nodejs/configs:nodejs_image_no_args.yaml"],
     image = ":nodejs_image_no_args",
 )
+
+container_test(
+    name = "nodejs_image_empty_list_args_test",
+    configs = ["//tests/docker/nodejs/configs:nodejs_image_empty_list_args.yaml"],
+    image = ":nodejs_image_empty_list_args",
+)
+
+container_test(
+    name = "nodejs_image_none_args_test",
+    configs = ["//tests/docker/nodejs/configs:nodejs_image_none_args.yaml"],
+    image = ":nodejs_image_none_args",
+)

tests/docker/nodejs/configs/nodejs_image_empty_list_args.yaml

@@ -0,0 +1,13 @@
+schemaVersion: 2.0.0
+
+metadataTest:
+  cmd: Null
+  env:
+    - key: PORT
+      value: "8080"
+    - key: DEBIAN_FRONTEND
+      value: "noninteractive"
+    - key: PATH
+      value: "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+  entrypoint: ['/app/tests/docker/nodejs/nodejs_image_empty_list_args.binary']
+  workdir: "/app/tests/docker/nodejs/nodejs_image_empty_list_args.binary.runfiles/io_bazel_rules_docker"

tests/docker/nodejs/configs/nodejs_image_list_with_empty_string_args.yaml

@@ -0,0 +1,13 @@
+schemaVersion: 2.0.0
+
+metadataTest:
+  cmd: [""]
+  env:
+    - key: PORT
+      value: "8080"
+    - key: DEBIAN_FRONTEND
+      value: "noninteractive"
+    - key: PATH
+      value: "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+  entrypoint: ['/app/tests/docker/nodejs/nodejs_image_list_with_empty_string_args.binary']
+  workdir: "/app/tests/docker/nodejs/nodejs_image_list_with_empty_string_args.binary.runfiles/io_bazel_rules_docker"

tests/docker/nodejs/configs/nodejs_image_no_args.yaml

@@ -1,7 +1,7 @@
 schemaVersion: 2.0.0
 
 metadataTest:
-  cmd: [""]
+  cmd: Null
   env:
     - key: PORT
       value: "8080"

tests/docker/nodejs/configs/nodejs_image_none_args.yaml

@@ -0,0 +1,13 @@
+schemaVersion: 2.0.0
+
+metadataTest:
+  cmd: Null
+  env:
+    - key: PORT
+      value: "8080"
+    - key: DEBIAN_FRONTEND
+      value: "noninteractive"
+    - key: PATH
+      value: "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+  entrypoint: ['/app/tests/docker/nodejs/nodejs_image_none_args.binary']
+  workdir: "/app/tests/docker/nodejs/nodejs_image_none_args.binary.runfiles/io_bazel_rules_docker"