pageserver: require stripe size (#12257)

## Problem

In #12217, we began passing the stripe size in reattach responses, and
persisting it in the on-disk state. This is necessary to ensure the
storage controller and Pageserver have a consistent view of the intended
stripe size of unsharded tenants, which will be used for splits that do
not specify a stripe size. However, for backwards compatibility, these
stripe sizes were optional.

## Summary of changes

Make the stripe sizes required for reattach responses and on-disk
location configs. These will always be provided by the previous
(current) release.

libs/pageserver_api/src/upcall_api.rs

@@ -23,22 +23,12 @@ pub struct ReAttachRequest {
     pub register: Option<NodeRegisterRequest>,
 }
 
-fn default_mode() -> LocationConfigMode {
-    LocationConfigMode::AttachedSingle
-}
-
 #[derive(Serialize, Deserialize, Debug)]
 pub struct ReAttachResponseTenant {
     pub id: TenantShardId,
     /// Mandatory if LocationConfigMode is None or set to an Attached* mode
     pub r#gen: Option<u32>,
-
-    /// Default value only for backward compat: this field should be set
-    #[serde(default = "default_mode")]
     pub mode: LocationConfigMode,
-
-    // Default value only for backward compat: this field should be set
-    #[serde(default = "ShardStripeSize::default")]
     pub stripe_size: ShardStripeSize,
 }
 #[derive(Serialize, Deserialize)]

pageserver/src/tenant/config.rs

@@ -61,8 +61,10 @@ pub(crate) struct LocationConf {
     /// The detailed shard identity.  This structure is already scoped within
     /// a TenantShardId, but we need the full ShardIdentity to enable calculating
     /// key->shard mappings.
-    // TODO(vlad): Remove this default once all configs have a shard identity on disk.
-    #[serde(default = "ShardIdentity::unsharded")]
+    ///
+    /// NB: we store this even for unsharded tenants, so that we agree with storcon on the intended
+    /// stripe size. Otherwise, a split request that does not specify a stripe size may use a
+    /// different default than storcon, which can lead to incorrect stripe sizes and corruption.
     pub(crate) shard: ShardIdentity,
 
     /// The pan-cluster tenant configuration, the same on all locations