docker_image: The image_tags field must not be empty. (pantsbuild…

…#19980)

Closes pantsbuild#19979

src/python/pants/backend/docker/goals/package_image.py

@@ -45,10 +45,10 @@
 from pants.engine.fs import CreateDigest, Digest, FileContent
 from pants.engine.process import FallibleProcessResult, Process, ProcessExecutionFailure
 from pants.engine.rules import Get, MultiGet, collect_rules, rule
-from pants.engine.target import Target, WrappedTarget, WrappedTargetRequest
+from pants.engine.target import InvalidFieldException, Target, WrappedTarget, WrappedTargetRequest
 from pants.engine.unions import UnionMembership, UnionRule
 from pants.option.global_options import GlobalOptions, KeepSandboxes
-from pants.util.strutil import bullet_list
+from pants.util.strutil import bullet_list, softwrap
 from pants.util.value_interpolation import InterpolationContext, InterpolationError
 
 logger = logging.getLogger(__name__)
@@ -414,6 +414,16 @@ async def build_docker_image(
         )
     )
     tags = tuple(tag.full_name for registry in image_refs for tag in registry.tags)
+    if not tags:
+        raise InvalidFieldException(
+            softwrap(
+                f"""
+                The `{DockerImageTagsField.alias}` field in target {field_set.address} must not be
+                empty, unless there is a custom plugin providing additional tags using the
+                `DockerImageTagsRequest` union type.
+                """
+            )
+        )
 
     # Mix the upstream image ids into the env to ensure that Pants invalidates this
     # image-building process correctly when an upstream image changes, even though the

src/python/pants/backend/docker/goals/package_image_test.py

@@ -1846,18 +1846,40 @@ def test_image_ref_formatting(test: ImageRefTest) -> None:
         assert tuple(image_refs) == test.expect_refs
 
 
-def test_docker_image_tags_from_plugin_hook(rule_runner: RuleRunner) -> None:
-    rule_runner.write_files({"docker/test/BUILD": 'docker_image(name="plugin")'})
+@pytest.mark.parametrize(
+    "BUILD, plugin_tags, tag_flags",
+    [
+        (
+            'docker_image(name="plugin")',
+            ("1.2.3",),
+            (
+                "--tag",
+                "plugin:latest",
+                "--tag",
+                "plugin:1.2.3",
+            ),
+        ),
+        (
+            'docker_image(name="plugin", image_tags=[])',
+            ("1.2.3",),
+            (
+                "--tag",
+                "plugin:1.2.3",
+            ),
+        ),
+    ],
+)
+def test_docker_image_tags_from_plugin_hook(
+    rule_runner: RuleRunner, BUILD: str, plugin_tags: tuple[str, ...], tag_flags: tuple[str, ...]
+) -> None:
+    rule_runner.write_files({"docker/test/BUILD": BUILD})
 
     def check_docker_proc(process: Process):
         assert process.argv == (
             "/dummy/docker",
             "build",
             "--pull=False",
-            "--tag",
-            "plugin:latest",
-            "--tag",
-            "plugin:1.2.3",
+            *tag_flags,
             "--file",
             "docker/test/Dockerfile",
             ".",
@@ -1867,10 +1889,21 @@ def check_docker_proc(process: Process):
         rule_runner,
         Address("docker/test", target_name="plugin"),
         process_assertions=check_docker_proc,
-        plugin_tags=("1.2.3",),
+        plugin_tags=plugin_tags,
     )
 
 
+def test_docker_image_tags_defined(rule_runner: RuleRunner) -> None:
+    rule_runner.write_files({"docker/test/BUILD": 'docker_image(name="no-tags", image_tags=[])'})
+
+    err = "The `image_tags` field in target docker/test:no-tags must not be empty, unless"
+    with pytest.raises(InvalidFieldException, match=err):
+        assert_build(
+            rule_runner,
+            Address("docker/test", target_name="no-tags"),
+        )
+
+
 def test_docker_info_serialize() -> None:
     image_id = "abc123"
     # image refs with unique strings (i.e. not actual templates/names etc.), to make sure they're