Actually activate inventory caching

The inventory plugin advertises support for caching, but the
implementation was missing the required pieces to make it work
properly.

This commit split the inventory construction into two stages. The
first stage is responsible for fetching remote data (interacting with
the DigitalOcean's API). The second stage extracts the relevant data
from the API responses and populated the inventory.

Caching support then came almost for free. All we had to do is only
fetch data if:

 1. User does not want to use cache at all.
 2. User does use cache and wants data refreshed.
 3. User does use cache but the cache is empty.

There is some additional logic that makes sure we write the data to
cache if data refresh is requested, but nothing too complex.

changelogs/fragments/97-wire-up-inventory-cache.yml

@@ -0,0 +1,3 @@
+bugfixes:
+  - digitalocean inventory plugin - wire up advertised caching functionality
+    (https://github.com/ansible-collections/community.digitalocean/pull/97).

plugins/inventory/digitalocean.py

@@ -170,12 +170,12 @@ def _get_payload(self):
 
         return payload
 
-    def _populate(self):
+    def _populate(self, records):
         attributes = self.get_option('attributes')
         var_prefix = self.get_option('var_prefix')
         strict = self.get_option('strict')
         host_filters = self.get_option('filters')
-        for record in self._get_payload():
+        for record in records:
 
             host_name = record.get('name')
             if not host_name:
@@ -220,22 +220,24 @@ def _passes_filters(self, filters, variables, host, strict=False):
     def parse(self, inventory, loader, path, cache=True):
         super(InventoryModule, self).parse(inventory, loader, path)
 
-        # cache settings
         self._read_config_data(path)
-        self.cache_key = self.get_cache_key(path)
 
-        self.use_cache = self.get_option('cache') and cache
-        self.update_cache = self.get_option('cache') and not cache
+        # cache settings
+        cache_key = self.get_cache_key(path)
+        use_cache = self.get_option('cache') and cache
+        update_cache = self.get_option('cache') and not cache
 
-        results = []
-        if not self.update_cache:
+        records = None
+        if use_cache:
             try:
-                results = self._cache[self.cache_key]['digitalocean']
+                records = self._cache[cache_key]
             except KeyError:
-                pass
+                update_cache = True
+
+        if records is None:
+            records = self._get_payload()
 
-        if not results:
-            if self.cache_key not in self._cache:
-                self._cache[self.cache_key] = {'digitalocean': ''}
+        if update_cache:
+            self._cache[cache_key] = records
 
-        self._populate()
+        self._populate(records)

tests/unit/plugins/inventory/test_digitalocean.py

@@ -28,8 +28,17 @@ def test_verify_file_bad_config(inventory):
     assert inventory.verify_file('digitalocean_foobar.yml') is False
 
 
-def get_payload():
-    return [
+def get_option(option):
+    options = {
+        'attributes': ['id', 'size_slug'],
+        'var_prefix': 'do_',
+        'strict': False,
+    }
+    return options.get(option)
+
+
+def test_populate_hostvars(inventory, mocker):
+    records = [
         {
             "id": 3164444,
             "name": "foo",
@@ -125,21 +134,9 @@ def get_payload():
             "vpc_uuid": "f9b0769c-e118-42fb-a0c4-fed15ef69662"
         }
     ]
-
-
-def get_option(option):
-    options = {
-        'attributes': ['id', 'size_slug'],
-        'var_prefix': 'do_',
-        'strict': False,
-    }
-    return options.get(option)
-
-
-def test_populate_hostvars(inventory, mocker):
-    inventory._get_payload = mocker.MagicMock(side_effect=get_payload)
     inventory.get_option = mocker.MagicMock(side_effect=get_option)
-    inventory._populate()
+
+    inventory._populate(records)
 
     host_foo = inventory.inventory.get_host('foo')
     host_bar = inventory.inventory.get_host('bar')