fix: scope --add-host to the target's dependency closure (#55)

Author: lesnik512

architecture/supported-subset.md

@@ -281,12 +281,20 @@ flags. compose2pod hoists them onto `podman pod create` instead
   unioned by key, and two closure services setting the same key to
   different values is refused (`conflicting sysctl ...`) rather than
   resolved last-writer-wins. `--add-host` is seeded from the alias/hostname
-  set (document-wide, not closure-scoped — see `hostname`/`container_name`/
-  `networks`, above), then layered with each closure service's
+  set of the closure's services, then layered with each closure service's
   `extra_hosts`; a host name landing on two different addresses is refused
   the same way (`conflicting host ...`). An alias/hostname `--add-host`
   entry stays a plain unquoted token; an `extra_hosts` entry is
   `${VAR}`-live.
+
+  Only the closure joins the pod, so only the closure is resolvable: a
+  service outside it contributes no name and cannot conflict with an
+  `extra_hosts` entry. Resolving a never-run service's name to `127.0.0.1`
+  would point it at a port where nothing listens, turning an honest
+  resolution failure into a connection-refused. Shape validation of
+  `hostname`/`container_name`/`networks` stays document-wide at the gate
+  (`hostnames` in `parsing.py`), so a malformed value is still rejected on a
+  service the target never reaches.
 - **Pod-wide divergence:** unlike every other service key, these apply to
   every container in the pod once emitted — including services that never
   declared them — because the pod shares one `/etc/resolv.conf`, sysctl

compose2pod/emit.py

@@ -272,8 +272,11 @@ def _plan(compose: dict[str, Any], options: EmitOptions) -> PlannedScript:
         msg = f"invalid pod name {options.pod!r}"
         raise UnsupportedComposeError(msg)
     services = compose["services"]
-    hosts = hostnames(services)
     order = startup_order(services, options.target)
+    # Only the closure joins the pod, so only the closure is resolvable. A name
+    # pointing at 127.0.0.1 for a service that never runs would turn an honest
+    # resolution failure into a connection-refused.
+    hosts = hostnames({name: services[name] for name in order})
     completion_gated = {
         dep
         for svc in services.values()

planning/changes/2026-07-14.02-add-host-closure-scope.md

@@ -0,0 +1,69 @@
+---
+summary: The emit-side --add-host set is scoped to the target's dependency closure, so a service that never runs no longer resolves to 127.0.0.1 and can no longer veto another service's extra_hosts.
+---
+
+# Change: Scope --add-host to the target's closure
+
+**Lane:** lightweight — one-line change in `emit._plan`, plus tests.
+
+## Goal
+
+`--add-host` is the one aggregate in the emit path that is built document-wide
+instead of closure-scoped, and the seam shows. `emit._plan` seeds hosts from
+`graph.hostnames(services)` — every service in the *document* — while
+`extra_hosts` is layered per service in `order`, the target's dependency
+closure. `pod._add_host_flags` then conflict-checks the two against each other,
+so a service that **never runs** can veto a valid configuration:
+
+```yaml
+services:
+  app:   {image: i, extra_hosts: ["db:1.2.3.4"]}
+  other: {image: i, hostname: db}          # not in app's closure; never started
+```
+→ `UnsupportedComposeError: service 'app': conflicting host 'db'
+('127.0.0.1' vs '1.2.3.4')`
+
+The second symptom is quieter: a never-run service still gets an `--add-host`
+entry pointing its name at `127.0.0.1`, where nothing is listening. That turns
+an honest name-resolution failure into a connection-refused.
+
+Every other aggregate in the emit path — `dns`, `dns_search`, `dns_opt`,
+`sysctls`, secrets, configs — is closure-scoped. This makes `--add-host` agree.
+
+## Approach
+
+`emit._plan` passes only the closure's services to `hostnames()`:
+
+```python
+hosts = hostnames({name: services[name] for name in order})
+```
+
+`hostnames()` has exactly two callers, and only this one changes:
+
+- `parsing.py` calls it document-wide to shape-check every service's
+  `hostname`/`container_name`/`networks` at the gate. That stays — validation
+  is target-agnostic, and scoping it would stop rejecting a malformed
+  `hostname` on a service outside the closure.
+- `emit.py` calls it to build the `--add-host` set. That is the one that must
+  match the pod's actual contents.
+
+Truth home: `architecture/supported-subset.md`'s Pod-level options section,
+which currently documents the document-wide behavior as "pre-existing,
+orthogonal".
+
+## Files
+
+- `compose2pod/emit.py` — scope the `hostnames()` argument to `order`
+- `architecture/supported-subset.md` — Pod-level options: `--add-host` is
+  closure-scoped like the rest
+- `tests/test_emit.py` — tests added
+
+## Verification
+
+- [ ] Failing test first: an out-of-closure `hostname` colliding with an
+      in-closure `extra_hosts` must NOT raise; a never-run service must NOT
+      appear in `--add-host`.
+- [ ] Apply the change.
+- [ ] Tests pass.
+- [ ] `just test-ci` — full suite green at 100%.
+- [ ] `just lint-ci`, `just check-planning` — clean.

tests/test_emit.py

@@ -995,3 +995,59 @@ def test_referenced_variables_is_equally_guarded(self) -> None:
         # and must reject malformed input identically.
         with pytest.raises(UnsupportedComposeError):
             referenced_variables({}, self._options())
+
+
+class TestAddHostClosureScope:
+    """--add-host covers the target's closure, like every other emit-path aggregate."""
+
+    def _options(self, target: str) -> EmitOptions:
+        return EmitOptions(
+            target=target,
+            ci_image="ci:latest",
+            command="",
+            pod="test-pod",
+            project_dir=".",
+            artifacts=[],
+            allow_exit_codes=[],
+        )
+
+    def test_service_outside_the_closure_is_not_resolvable(self) -> None:
+        # A never-run service pointed its name at 127.0.0.1, where nothing listens.
+        compose = {"services": {"app": {"image": "x"}, "never_run": {"image": "x"}}}
+        script = emit_script(compose=compose, options=self._options("app"))
+        assert "--add-host app:127.0.0.1" in script
+        assert "never_run" not in script
+
+    def test_out_of_closure_hostname_does_not_veto_extra_hosts(self) -> None:
+        # 'other' is not in app's closure, so its hostname cannot conflict with app's extra_hosts.
+        compose = {
+            "services": {
+                "app": {"image": "x", "extra_hosts": ["db:1.2.3.4"]},
+                "other": {"image": "x", "hostname": "db"},
+            }
+        }
+        script = emit_script(compose=compose, options=self._options("app"))
+        assert '--add-host "db:1.2.3.4"' in script
+
+    def test_in_closure_hostname_still_conflicts_with_extra_hosts(self) -> None:
+        # The conflict rule still holds for services that actually run.
+        compose = {
+            "services": {
+                "app": {"image": "x", "extra_hosts": ["db:1.2.3.4"], "depends_on": ["db_svc"]},
+                "db_svc": {"image": "x", "hostname": "db"},
+            }
+        }
+        with pytest.raises(UnsupportedComposeError, match="conflicting host"):
+            emit_script(compose=compose, options=self._options("app"))
+
+    def test_dependency_hostnames_and_aliases_still_resolve(self) -> None:
+        # Everything inside the closure keeps its add-host entry.
+        compose = {
+            "services": {
+                "app": {"image": "x", "depends_on": ["db"]},
+                "db": {"image": "x", "hostname": "db-host", "networks": {"default": {"aliases": ["db-alias"]}}},
+            }
+        }
+        script = emit_script(compose=compose, options=self._options("app"))
+        for host in ("app", "db", "db-host", "db-alias"):
+            assert f"--add-host {host}:127.0.0.1" in script