storcon: handle pageserver disk loss (#12667)

NB: effectively a no-op in the neon env since the handling is config
gated
in storcon

## Problem

When a pageserver suffers from a local disk/node failure and restarts,
the storage controller will receive a re-attach call and return all the
tenants the pageserver is suppose to attach, but the pageserver will not
act on any tenants that it doesn't know about locally. As a result, the
pageserver will not rehydrate any tenants from remote storage if it
restarted following a local disk loss, while the storage controller
still thinks that the pageserver have all the tenants attached. This
leaves the system in a bad state, and the symptom is that PG's
pageserver connections will fail with "tenant not found" errors.

## Summary of changes

Made a slight change to the storage controller's `re_attach` API:
* The pageserver will set an additional bit `empty_local_disk` in the
reattach request, indicating whether it has started with an empty disk
or does not know about any tenants.
* Upon receiving the reattach request, if this `empty_local_disk` bit is
set, the storage controller will go ahead and clear all observed
locations referencing the pageserver. The reconciler will then discover
the discrepancy between the intended state and observed state of the
tenant and take care of the situation.

To facilitate rollouts this extra behavior in the `re_attach` API is
guarded by the `handle_ps_local_disk_loss` command line flag of the
storage controller.

---------

Co-authored-by: William Huang <william.huang@databricks.com>

libs/pageserver_api/src/upcall_api.rs

@@ -21,6 +21,14 @@ pub struct ReAttachRequest {
     /// if the node already has a node_id set.
     #[serde(skip_serializing_if = "Option::is_none", default)]
     pub register: Option<NodeRegisterRequest>,
+
+    /// Hadron: Optional flag to indicate whether the node is starting with an empty local disk.
+    /// Will be set to true if the node couldn't find any local tenant data on startup, could be
+    /// due to the node starting for the first time or due to a local SSD failure/disk wipe event.
+    /// The flag may be used by the storage controller to update its observed state of the world
+    /// to make sure that it sends explicit location_config calls to the node following the
+    /// re-attach request.
+    pub empty_local_disk: Option<bool>,
 }
 
 #[derive(Serialize, Deserialize, Debug)]