ansible-galaxy - fix fallback for AH searches (#70957) (#70983)

(cherry picked from commit b1cb2553af)

changelogs/fragments/galaxy-collection-fallback.yml

@@ -0,0 +1,2 @@
+bugfixes:
+- ansible-galaxy collection install - fix fallback mechanism if the AH server did not have the collection requested - https://github.com/ansible/ansible/issues/70940

lib/ansible/galaxy/collection.py

@@ -349,11 +349,17 @@ class CollectionRequirement:
                     # StrictVersion (x.y.z) and only support pre-releases if an explicit version was set (done above).
                     versions = [v for v in resp if StrictVersion.version_re.match(v)]
             except GalaxyError as err:
-                if err.http_code == 404:
-                    display.vvv("Collection '%s' is not available from server %s %s"
-                                % (collection, api.name, api.api_server))
-                    continue
-                raise
+                if err.http_code != 404:
+                    raise
+
+                versions = []
+
+            # Automation Hub doesn't return a 404 but an empty version list so we check that to align both AH and
+            # Galaxy when the collection is not available on that server.
+            if not versions:
+                display.vvv("Collection '%s' is not available from server %s %s" % (collection, api.name,
+                                                                                    api.api_server))
+                continue
 
             display.vvv("Collection '%s' obtained from server %s %s" % (collection, api.name, api.api_server))
             break

test/integration/targets/ansible-galaxy-collection/tasks/main.yml

@@ -153,3 +153,20 @@
     server: '{{ fallaxy_galaxy_server }}'
   - name: automation_hub
     server: '{{ fallaxy_ah_server }}'
+
+# fake.fake does not exist but we check the output to ensure it checked all 3
+# servers defined in the config. We hardcode to -vvv as that's what level the
+# message is shown
+- name: test install fallback on server list
+  command: ansible-galaxy collection install fake.fake -vvv
+  ignore_errors: yes
+  environment:
+    ANSIBLE_CONFIG: '{{ galaxy_dir }}/ansible.cfg'
+  register: missing_fallback
+
+- name: assert test install fallback on server list
+  assert:
+    that:
+    - missing_fallback.rc == 1
+    - '"Collection ''fake.fake'' is not available from server galaxy" in missing_fallback.stdout'
+    - '"Collection ''fake.fake'' is not available from server automation_hub" in missing_fallback.stdout'