image.bzl: implement tag_name (#2205)

rules_docker decides the name of the resulting docker image:

    bazel/{target}:{name}

The repository (`bazel/` by default) and `{target}` are fine. However,
we want to customize the tag part -- the thing after the colon. This
patch allows customizing the last part.

Why?
====

In our case, we are always building two images for two architectures
using transitions. Here is an extract of a macro that builds two
containers:

Then we build two containers using outgoing transitions:

    container_image(
        name = "{name}-post_transition".format(name = name),
        ...,
    )

    multiplatform_image(
        name = "{name}-amd64".format(name),
        src_image = ":{name}-post_transition".format(name = name),
        ...,
    )

    multiplatform_image(
        name = "{name}-arm64".format(name = name),
        src_image = ":{name}-post_transition".format(name = name),
        ...,
    )

Both images (`-amd64` and `-arm64`) build correctly, but the resulting
image name, according to Docker, happens to be
`bazel/{target}:{name}-post_transition`. So the internal implementation
details of the transitions leak to the resulting images.

This patch makes it possible to rename the post-transition image name to
something we like more, e.g. the architecture name, or remove the suffix
altogether.

container/image.bzl

@@ -288,7 +288,8 @@ def _impl(
         workdir = None,
         user = None,
         null_cmd = None,
-        null_entrypoint = None):
+        null_entrypoint = None,
+        tag_name = None):
     """Implementation for the container_image rule.
 
     You can write a customized container_image rule by writing something like:
@@ -346,6 +347,7 @@ def _impl(
         user: str, overrides ctx.attr.user
         null_cmd: bool, overrides ctx.attr.null_cmd
         null_entrypoint: bool, overrides ctx.attr.null_entrypoint
+        tag_name: str, overrides ctx.attr.tag_name
     """
     name = name or ctx.label.name
     base = base or ctx.attr.base
@@ -367,6 +369,7 @@ def _impl(
     build_script = ctx.outputs.build_script
     null_cmd = null_cmd or ctx.attr.null_cmd
     null_entrypoint = null_entrypoint or ctx.attr.null_entrypoint
+    tag_name = tag_name or ctx.attr.tag_name
 
     # If this target specifies docker_run_flags, they are always used.
     # Fall back to the base image's run flags if present, otherwise use the default value.
@@ -459,8 +462,10 @@ def _impl(
             null_cmd = null_cmd,
         )
 
-    # Construct a temporary name based on the build target.
-    tag_name = "{}:{}".format(_repository_name(ctx), name)
+    # Construct a temporary name based on the build target. This is the name
+    # of the docker container.
+    final_tag = tag_name if tag_name else name
+    container_name = "{}:{}".format(_repository_name(ctx), final_tag)
 
     # These are the constituent parts of the Container image, which each
     # rule in the chain must preserve.
@@ -497,7 +502,7 @@ def _impl(
     # We support incrementally loading or assembling this single image
     # with a temporary name given by its build rule.
     images = {
-        tag_name: container_parts,
+        container_name: container_parts,
     }
 
     _incr_load(
@@ -744,6 +749,9 @@ _attrs = dicts.add(_layer.attrs, {
 
         This field supports stamp variables.""",
     ),
+    "tag_name": attr.string(
+        doc = """Override final tag name. If unspecified, is set to name.""",
+    ),
     "_allowlist_function_transition": attr.label(
         default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
     ),

docs/container.md

@@ -404,7 +404,7 @@ image.implementation(<a href="#image.implementation-ctx">ctx</a>, <a href="#imag
                      <a href="#image.implementation-compression_options">compression_options</a>, <a href="#image.implementation-experimental_tarball_format">experimental_tarball_format</a>, <a href="#image.implementation-debs">debs</a>, <a href="#image.implementation-tars">tars</a>, <a href="#image.implementation-architecture">architecture</a>,
                      <a href="#image.implementation-operating_system">operating_system</a>, <a href="#image.implementation-os_version">os_version</a>, <a href="#image.implementation-output_executable">output_executable</a>, <a href="#image.implementation-output_tarball">output_tarball</a>, <a href="#image.implementation-output_config">output_config</a>,
                      <a href="#image.implementation-output_config_digest">output_config_digest</a>, <a href="#image.implementation-output_digest">output_digest</a>, <a href="#image.implementation-output_layer">output_layer</a>, <a href="#image.implementation-workdir">workdir</a>, <a href="#image.implementation-user">user</a>, <a href="#image.implementation-null_cmd">null_cmd</a>,
-                     <a href="#image.implementation-null_entrypoint">null_entrypoint</a>)
+                     <a href="#image.implementation-null_entrypoint">null_entrypoint</a>, <a href="#image.implementation-tag_name">tag_name</a>)
 </pre>
 
 Implementation for the container_image rule.
@@ -469,5 +469,6 @@ You can write a customized container_image rule by writing something like:
 | <a id="image.implementation-user"></a>user |  str, overrides ctx.attr.user   |  <code>None</code> |
 | <a id="image.implementation-null_cmd"></a>null_cmd |  bool, overrides ctx.attr.null_cmd   |  <code>None</code> |
 | <a id="image.implementation-null_entrypoint"></a>null_entrypoint |  bool, overrides ctx.attr.null_entrypoint   |  <code>None</code> |
+| <a id="image.implementation-tag_name"></a>tag_name |  str, overrides ctx.attr.tag_name   |  <code>None</code> |
 
 

testdata/BUILD

@@ -97,6 +97,12 @@ container_image(
     repository = "gcr.io/dummy",
 )
 
+container_image(
+    name = "tag_name",
+    files = ["foo"],
+    tag_name = "this-tag-instead",
+)
+
 container_image(
     name = "no_data_path_image",
     files = ["//testdata/test:test-data"],

tests/container/BUILD

@@ -935,6 +935,7 @@ TEST_TARGETS = [
     ":absolute_data_path_image",
     ":root_data_path_image",
     ":dummy_repository",
+    ":tag_name",
     # TODO(mattmoor): Re-enable once archive is visible
     # "extras_with_deb",
     ":bundle_test",

tests/container/image_test.py

@@ -301,6 +301,13 @@ def test_dummy_repository(self):
             self.assertEqual(1, len(img.fs_layers()))
             self.assertTopLayerContains(img, ['.', './foo'])
 
+    def test_tag_name(self):
+        # users may override the tag that comes after ':'
+        name = 'bazel/%s:this-tag-instead' % TEST_DATA_TARGET_BASE
+        with TestBundleImage('tag_name', name) as img:
+            self.assertEqual(1, len(img.fs_layers()))
+            self.assertTopLayerContains(img, ['.', './foo'])
+
     def test_with_double_env(self):
         with TestImage('with_double_env') as img:
             self.assertEqual(3, len(img.fs_layers()))