Make `update` a bit more like compare-and-swap

2 files changed, 23 insertions, 19 deletions

diff --git a/lib/kernel/BranchManager.ml b/lib/kernel/BranchManager.ml
index 71a25b3..80e32a4 100644
--- a/lib/kernel/BranchManager.ml
+++ b/lib/kernel/BranchManager.ml
@@ -1,21 +1,26 @@
-module BranchValue = Merkle.StringKey (* FIXME *)
-
 type branch = string
 
 module Error = struct
   open Concepts.Condition
 
-  let invalid_root head = condition "invalid-root" "The root hash for the database state is missing from the backend. Either your storage is corrupted, or this is a bug in RNT!"
-                            ("hash" |=| (Concepts.Value.String (Concepts.Hash.to_hum_string head)))
+  let invalid_root head =
+    condition "invalid-root" "The root hash for the database state is missing from the backend. Either your storage is corrupted, or this is a bug in RNT!"
+      ("hash" |=| (Concepts.Value.String (Concepts.Hash.to_hum_string head)))
+
+  let comparison_failed branch_name reference head =
+    condition "comparison-failed" "The HEAD of the branch did not match what was expected"
+      ("branch"    |=| Concepts.Value.String branch_name &
+       "reference" |=| Concepts.Value.String (Concepts.Hash.to_hum_string reference) &
+       "head"      |=| Concepts.Value.String (Concepts.Hash.to_hum_string head))
 end
 
 module Make (S : Abstract.Storage.STORAGE) = struct
-  module I = Merkle.Interface (S) (Merkle.StringKey) (BranchValue)
+  module M = Merkle.Make (S) (Merkle.StringKey)
   module SI = Storage.Make (S)
 
   type manager = { storage: S.connection;
                    label : string;
-                   head: I.node Atomic.t }
+                   head: M.node Atomic.t }
 
   let root_for tx label =
     let open Utilities.Result in
@@ -23,8 +28,6 @@ module Make (S : Abstract.Storage.STORAGE) = struct
     let label = Option.map Concepts.Hash.hash_of_blob label in
     Ok label
 
-  let persist_root tx label new_branch = failwith "TODO"
-
   let initialize storage label =
     let open Utilities.Result in
     SI.with_transaction storage (fun tx ->
@@ -32,22 +35,25 @@ module Make (S : Abstract.Storage.STORAGE) = struct
         match addr with
         | None -> failwith "TODO"
         | Some addr ->
-           let* head = I.find tx addr in
+           let* head = M.find tx addr in
            let* head = Option.to_result ~none:(Error.invalid_root addr) head in
            Ok { storage; label; head = Atomic.make head })
 
   let fetch { storage; head; _ } branch_name =
     SI.with_transaction storage (fun tx ->
         let head = Atomic.get head in
-        I.lookup tx branch_name head)
+        M.lookup tx branch_name head)
 
-  let update ({ storage; head; label } as manager) branch_name f =
+  let update ({ storage; head; label } as manager) branch_name reference new_branch =
     let open Utilities.Result in
     let* _ = SI.with_transaction storage (fun tx ->
                  Utilities.Atomic.mswap head (fun head ->
-                     let* branch = I.lookup tx branch_name head in
-                     let new_branch = f branch in
-                     let* () = persist_root tx label new_branch in
-                     new_branch)) in
+                     let* branch = M.lookup tx branch_name head in
+                     if branch = reference then
+                       let* new_head = M.insert tx label new_branch head in
+                       new_head
+                     else
+                       Error.comparison_failed branch_name reference branch)) in
     Ok ()
+
 end
diff --git a/lib/kernel/BranchManager.mli b/lib/kernel/BranchManager.mli
index 063f8a7..052f9a1 100644
--- a/lib/kernel/BranchManager.mli
+++ b/lib/kernel/BranchManager.mli
@@ -1,11 +1,9 @@
-type branch = string            (* FIXME *)
-
 module Make (S : Abstract.Storage.STORAGE) : sig
   type manager
 
   val initialize : S.connection -> string -> (manager, Concepts.Condition.condition) result
-  val fetch : manager -> string -> (branch option, Concepts.Condition.condition) result
-  val update : manager -> string -> (branch -> branch) -> (manager, Concepts.Condition.condition) result
+  val fetch : manager -> string -> (Concepts.Hash.hash option, Concepts.Condition.condition) result
+  val update : manager -> string -> Concepts.Hash.hash -> Concepts.Hash.hash -> (unit, Concepts.Condition.condition) result
 
   (* TODO: fork, join *)
 end