fix(delete-previous-head-image): keep referrers of retained manifests

.github/actions/delete-previous-head-image/README.md

@@ -15,9 +15,9 @@ removes exactly that image after the new one is pushed:
   and each manifest in it
 
 It keeps anything that is still reachable from another tag, such as a release
-whose image shares a manifest, together with that manifest's signatures and
-attestations, and any version that carries a tag it did not
-expect. If the previous image is still tagged (for example, promoted to a
+whose image shares a manifest, and any version that carries a tag it did not
+expect. Every manifest it keeps also keeps its signatures and attestations,
+whichever reason kept it. If the previous image is still tagged (for example, promoted to a
 release), it deletes nothing. It checks that against both the registry tag list
 and the packages API, and if either shows a tag, it deletes nothing. Every deleted
 and kept version is logged and written to the step summary.

.github/actions/delete-previous-head-image/src/action.py

@@ -240,11 +240,6 @@ def delete(owner, name, token, previous, current, dry_run):
     if previous in protected:
         log(f"{previous} is still tagged {protected[previous]}; nothing to delete")
         return
-    # A kept manifest keeps its signatures and attestations too.
-    for digest, subject in signs.items():
-        if subject in protected:
-            protected.setdefault(digest, protected[subject])
-
     packages = Packages(owner, name, token)
     versions = packages.versions(set(tree))
     # The registry and the packages API are separate reads; trust the stricter one.
@@ -255,17 +250,22 @@ def delete(owner, name, token, previous, current, dry_run):
         tags = ", ".join(sorted(versions[previous][1]))
         log(f"{previous} is still tagged {tags}; nothing to delete")
         return
-    plan, kept = [], []
+    kept = {}
     for digest, allowed in tree.items():
         if digest in protected:
-            kept.append(f"{digest}: also part of tag {protected[digest]}")
+            kept[digest] = f"also part of tag {protected[digest]}"
         elif digest not in versions:
-            kept.append(f"{digest}: no package version found")
+            kept[digest] = "no package version found"
         elif not versions[digest][1] <= allowed:
             extra = ", ".join(sorted(versions[digest][1] - allowed))
-            kept.append(f"{digest}: carries tag {extra}")
-        else:
-            plan.append((digest, versions[digest][0]))
+            kept[digest] = f"carries tag {extra}"
+
+    # A kept manifest keeps its signatures and attestations too, including when
+    # only the packages API revealed the reason to keep it.
+    for digest, subject in signs.items():
+        if subject in kept:
+            kept.setdefault(digest, f"refers to kept manifest {subject}")
+    plan = [(digest, versions[digest][0]) for digest in tree if digest not in kept]
 
     verb = "would delete" if dry_run else "deleting"
     lines = [f"### Previous head image {previous}"]
@@ -274,9 +274,9 @@ def delete(owner, name, token, previous, current, dry_run):
         lines.append(f"- {verb} `{digest}` (version {version_id})")
         if not dry_run:
             packages.delete(version_id)
-    for reason in kept:
-        log(f"keeping {reason}")
-        lines.append(f"- keeping {reason}")
+    for digest, reason in kept.items():
+        log(f"keeping {digest}: {reason}")
+        lines.append(f"- keeping {digest}: {reason}")
     summary(lines)
 
 

.github/actions/delete-previous-head-image/test/test_action.py

@@ -321,6 +321,33 @@ def test_version_with_an_unexpected_tag_is_kept(fake, tmp_path):
     assert "carries tag debug" in result.stdout
 
 
+@pytest.mark.parametrize("dry_run", [False, True])
+def test_packages_only_child_tag_keeps_its_referrers(fake, tmp_path, dry_run):
+    old, new, manifests, tags, versions = candy()
+    child = d("1")
+    referrer_tag = "sha256-" + child[7:]
+    manifests[d("9")] = index(d("8"))
+    manifests[d("8")] = image()
+    tags[referrer_tag] = d("9")
+    server = fake(manifests=manifests, tags=tags)
+    server.versions[1] = version(2, child, "debug")
+    server.versions += [version(13, d("9"), referrer_tag), version(14, d("8"))]
+
+    result = run(
+        server, tmp_path, mode="delete", previous_digest=old, current_digest=new,
+        dry_run=str(dry_run),
+    )
+
+    assert result.returncode == 0, result.stdout
+    assert sorted(server.deleted) == ([] if dry_run else [1, 3, 4, 5])
+    verb = "would delete" if dry_run else "deleting"
+    for digest in (old, d("2"), d("3"), d("e")):
+        assert f"{verb} {digest}" in result.stdout
+    for digest in (child, d("9"), d("8")):
+        assert f"{verb} {digest}" not in result.stdout
+    assert "carries tag debug" in result.stdout
+
+
 def test_no_previous_digest_is_a_no_op(fake, tmp_path):
     server = fake()
     result = run(