check_redfish: stop syncing the live fan reading

module/sources/check_redfish/import_inventory.py

@@ -446,23 +446,16 @@ def update_fan(self):
             health_status = get_string_or_none(grab(fan, "health_status"))
             physical_context = get_string_or_none(grab(fan, "physical_context"))
             fan_id = get_string_or_none(grab(fan, "id"))
-            reading = get_string_or_none(grab(fan, "reading"))
-            reading_unit = get_string_or_none(grab(fan, "reading_unit"))
-
             description = list()
-            speed = None
             if physical_context is not None:
                 description.append(f"Context: {physical_context}")
 
-            if reading is not None and reading_unit is not None:
-                reading_unit = "%" if reading_unit.lower() == "percent" else reading_unit
-                speed = f"{reading}{reading_unit}"
-
+            # a fan's reading is a live measurement, not something the inventory describes.
+            # Writing it would update this object in NetBox on every single run
             items.append({
                 "description": description,
                 "full_name": f"{fan_name} (ID: {fan_id})",
-                "health": health_status,
-                "speed": speed
+                "health": health_status
             })
 
         self.update_all_items(items, "Fan")

tests/test_check_redfish_fan_speed.py

@@ -0,0 +1,40 @@
+"""
+A fan's reading is a live measurement. Syncing it means NetBox records a change on
+every run, for every fan of every server (reported by @marcinpsk on #473).
+"""
+from module.sources.check_redfish.import_inventory import CheckRedfish
+
+
+def _fan(reading):
+    return {"inventory": {"fan": [{
+        "id": "Fan.Embedded.1", "name": "Fan1A", "health_status": "OK",
+        "physical_context": "SystemBoard", "reading": reading, "reading_unit": "RPM",
+    }]}}
+
+
+def _collect(inventory, reading):
+    source = object.__new__(CheckRedfish)
+    source.inventory = inventory
+    source.name = "redfish"
+    source.source_tag = "Source: redfish"
+    source.inventory_file_content = _fan(reading)
+    collected = []
+    source.update_all_items = lambda items, inventory_type: collected.extend(items)
+    source.update_fan()
+    return collected
+
+
+def test_a_spinning_fan_produces_the_same_item(inventory):
+    slow = _collect(inventory, 7015)
+    fast = _collect(inventory, 9120)
+
+    assert slow == fast, "the fan item changes with its rpm, so NetBox is written on every run"
+
+
+def test_the_fan_is_still_described(inventory):
+    # control: dropping the reading must not empty the item
+    item = _collect(inventory, 8280)[0]
+
+    assert item["full_name"] == "Fan1A (ID: Fan.Embedded.1)"
+    assert item["health"] == "OK"
+    assert "Context: SystemBoard" in item["description"]