fix: prevent lost YAML targets during parallel writes

The YAML strategy performed a separate read-modify-write operation for
every managed host. Because these tasks were delegated to the same
Prometheus host, parallel Ansible forks could read the same original
file before any fork wrote its update.

Each fork then generated YAML containing only its own target. Although
the copy module atomically replaced the file, it did not make the full
read-modify-write sequence atomic. The last fork to write therefore
overwrote targets added by earlier forks.

Collect exporter updates from all hosts through hostvars, process them
in one run_once operation, and write each distinct target file once.
This preserves every target without requiring users to set serial.

Add parallel coverage for multiple target files and label groups, and
reset the fixtures on every convergence so stale files cannot hide the
race.

1 files changed, 17 insertions, 0 deletions

diff --git a/molecule/default/tests/test_yaml_strategy.py b/molecule/default/tests/test_yaml_strategy.py
index 89dbdeb..7ba834e 100644
--- a/molecule/default/tests/test_yaml_strategy.py
+++ b/molecule/default/tests/test_yaml_strategy.py
@@ -44,6 +44,22 @@ def test_yaml_parallel_writes_keep_all_hosts_in_same_group(host):
         "application4:9100",
     ]
 
+    assert get_group_by_labels(groups, {"job": "alternate"})["targets"] == [
+        "application:9300",
+        "application2:9300",
+        "application3:9300",
+        "application4:9300",
+    ]
+
+    second_groups = read_yaml_file(host, "/opt/yaml_parallel_second.yml")
+    assert get_group_by_labels(second_groups, {"job": "node"})["targets"] == [
+        "existing:9100",
+        "application:9200",
+        "application2:9200",
+        "application3:9200",
+        "application4:9200",
+    ]
+
 
 def test_yaml_user_edited_file_is_reparsed_and_keeps_expected_semantics(host):
     groups = read_yaml_file(host, "/opt/yaml_user_edited.yml")
@@ -70,6 +86,7 @@ def test_yaml_output_remains_parseable_after_all_operations(host):
     for path in [
         "/opt/yaml_bootstrap.yml",
         "/opt/yaml_parallel.yml",
+        "/opt/yaml_parallel_second.yml",
         "/opt/yaml_user_edited.yml",
         "/opt/yaml_move_remove_last.yml",
         "/opt/yaml_branch_matrix.yml",