Implement ZFS dataset management and snapshot configuration

- Introduced a new `datasets.nix` file to define ZFS datasets and their properties, allowing for centralized management of dataset configurations.
- Updated `default.nix` to import and enable the `zfs_manager` service, integrating it into the system configuration.
- Refactored `zfs.sh` to remove dataset creation commands, delegating dataset management to the `zfs_manager` service.
- Removed the legacy `snapshot_config.toml` file, as snapshot configurations are now handled within `datasets.nix`.
- Modified `vars.nix` to derive mountpoint paths from `datasets.nix`, ensuring consistency across the configuration.
- Created a new `zfs.nix` file to define the `zfs_manager` service and its dependencies.
- Added comprehensive tests for the `zfs_manager` functionality, covering dataset creation, property management, and error handling.

python/tools/snapshot_manager.py

@@ -23,6 +23,7 @@ def main(config_file: Path) -> None:
     """Main."""
     configure_logger(level="DEBUG")
     logger.info("Starting snapshot_manager")
+    failures: list[str] = []
 
     try:
         time_stamp = get_time_stamp()
@@ -34,16 +35,23 @@ def main(config_file: Path) -> None:
                 msg = f"{dataset.name} failed to create snapshot {time_stamp}"
                 logger.error(msg)
                 signal_alert(msg)
+                failures.append(msg)
                 continue
             count_lookup = get_count_lookup(config_file, dataset.name)
             logger.info(f"using {count_lookup} for {dataset.name}")
-            get_snapshots_to_delete(dataset, count_lookup)
+            failures.extend(get_snapshots_to_delete(dataset, count_lookup))
     except Exception:
         logger.exception("snapshot_manager failed")
         signal_alert("snapshot_manager failed")
         sys.exit(1)
-    else:
-        logger.info("snapshot_manager completed")
+
+    if failures:
+        logger.error(f"snapshot_manager completed with {len(failures)} errors")
+        for failure in failures:
+            logger.error(f"  {failure}")
+        sys.exit(1)
+
+    logger.info("snapshot_manager completed")
 
 
 def get_count_lookup(config_file: Path, dataset_name: str) -> dict[str, int]:
@@ -92,19 +100,29 @@ def load_config_data(config_file: Path) -> dict[str, dict[str, int]]:
 def get_snapshots_to_delete(
     dataset: Dataset,
     count_lookup: dict[str, int],
-) -> None:
+) -> list[str]:
     """Get snapshots to delete.
 
     Args:
         dataset (Dataset): the dataset
         count_lookup (dict[str, int]): the count lookup
+
+    Returns:
+        list[str]: Snapshot deletion failures encountered while pruning.
     """
+    for retention_class in ("15_min", "hourly", "daily", "monthly"):
+        count = count_lookup.get(retention_class)
+        if not isinstance(count, int) or isinstance(count, bool) or count < 0:
+            error = f"{retention_class} retention must be a non-negative integer, got {count!r}"
+            raise ValueError(error)
+
+    failures: list[str] = []
     snapshots = dataset.get_snapshots()
 
     logger.info(f"calculating snapshots for {dataset.name} to be deleted")
     if not snapshots:
         logger.info(f"{dataset.name} has no snapshots")
-        return
+        return failures
 
     filters = (
         ("15_min", re_compile(r"auto_\d{10}(?:15|30|45)")),
@@ -129,6 +147,9 @@ def get_snapshots_to_delete(
                 error_message = f"{dataset.name}@{snapshot} failed to delete: {error}"
                 signal_alert(error_message)
                 logger.error(error_message)
+                failures.append(error_message)
+
+    return failures
 
 
 def get_time_stamp() -> str: