gc-based snapshots

src/irmin-pack/unix/ext.ml

@@ -248,7 +248,8 @@ module Maker (Config : Conf.S) = struct
                 cancelled
             | None -> false
 
-          let start ~unlink ~use_auto_finalisation t commit_key =
+          let start ~unlink ~use_auto_finalisation ~path_new_files t commit_key
+              =
             let open Result_syntax in
             [%log.info "GC: Starting on %a" pp_key commit_key];
             let* () =
@@ -276,19 +277,21 @@ module Maker (Config : Conf.S) = struct
             let gc =
               Gc.v ~root ~generation:next_generation ~unlink
                 ~dispatcher:t.dispatcher ~fm:t.fm ~contents:t.contents
-                ~node:t.node ~commit:t.commit commit_key
+                ~node:t.node ~commit:t.commit ~path_new_files commit_key
             in
             t.running_gc <- Some { gc; use_auto_finalisation };
             Ok ()
 
-          let start_exn ?(unlink = true) ~use_auto_finalisation t commit_key =
+          let start_exn ?(unlink = true) ~use_auto_finalisation ~path_new_files
+              t commit_key =
             match t.running_gc with
             | Some _ ->
                 [%log.info "Repo is alreadying running GC. Skipping."];
                 Lwt.return false
             | None -> (
                 let result =
-                  start ~unlink ~use_auto_finalisation t commit_key
+                  start ~unlink ~use_auto_finalisation ~path_new_files t
+                    commit_key
                 in
                 match result with
                 | Ok _ -> Lwt.return true
@@ -351,6 +354,31 @@ module Maker (Config : Conf.S) = struct
                       Pack_key.v_direct ~offset ~length ~hash:entry.hash
                     in
                     Some key)
+
+          let create_one_commit_store t commit_key path =
+            let () = Io.mkdir path |> Errs.raise_if_error in
+            let* _launched =
+              start_exn ~use_auto_finalisation:false ~path_new_files:path t
+                commit_key
+            in
+            let generation = File_manager.generation t.fm + 1 in
+            let* () =
+              match t.running_gc with
+              | None -> assert false
+              | Some { gc; _ } -> Gc.finalise_without_swap gc
+            in
+            let () =
+              File_manager.create_one_commit_store t.config ~generation
+                ~new_store_root:path commit_key
+              |> Errs.raise_if_error
+            in
+            let branch_path = Irmin_pack.Layout.V4.branch ~root:path in
+            let* branch_store =
+              Branch.v ~fresh:true ~readonly:false branch_path
+            in
+            let* () = Branch.close branch_store in
+            (* TODO fsync dir.*)
+            Lwt.return_unit
         end
 
         let batch t f =
@@ -538,6 +566,7 @@ module Maker (Config : Conf.S) = struct
     let flush = X.Repo.flush
     let fsync = X.Repo.fsync
     let split = X.Repo.split_exn
+    let create_one_commit_store = X.Repo.Gc.create_one_commit_store
 
     module Gc = struct
       type msg = [ `Msg of string ]
@@ -568,13 +597,18 @@ module Maker (Config : Conf.S) = struct
         `Msg err_msg
 
       let finalise_exn = X.Repo.Gc.finalise_exn
-      let start_exn = X.Repo.Gc.start_exn ~use_auto_finalisation:false
+
+      let start_exn ?unlink t =
+        let root = Irmin_pack.Conf.root t.X.Repo.config in
+        X.Repo.Gc.start_exn ?unlink ~use_auto_finalisation:false
+          ~path_new_files:root t
 
       let start repo commit_key =
+        let root = Irmin_pack.Conf.root repo.X.Repo.config in
         try
           let* started =
-            X.Repo.Gc.start_exn ~unlink:true ~use_auto_finalisation:true repo
-              commit_key
+            X.Repo.Gc.start_exn ~unlink:true ~use_auto_finalisation:true
+              ~path_new_files:root repo commit_key
           in
           Lwt.return_ok started
         with exn -> catch_errors "Start GC" exn

src/irmin-pack/unix/file_manager.ml

@@ -817,4 +817,72 @@ struct
     let chunk_start_idx = pl.chunk_start_idx in
     let chunk_num = pl.chunk_num in
     cleanup ~root ~generation ~chunk_start_idx ~chunk_num
+
+  let create_one_commit_store config ~generation ~new_store_root
+      latest_gc_target =
+    let open Result_syntax in
+    let root = Irmin_pack.Conf.root config in
+    let commit_key, latest_gc_target_offset, suffix_start_offset =
+      match Pack_key.inspect latest_gc_target with
+      | Direct { offset; length; _ } as commit_key ->
+          let suffix_start_offset =
+            Int63.Syntax.(offset + Int63.of_int length)
+          in
+          (commit_key, offset, suffix_start_offset)
+      | Indexed _ ->
+          (* The caller of this function lifted the key to a direct one. *)
+          assert false
+    in
+    (* Step 1. Copy the dict *)
+    let src_dict = Irmin_pack.Layout.V4.dict ~root in
+    let dst_dict = Irmin_pack.Layout.V4.dict ~root:new_store_root in
+    let* () = Io.cp_file ~src:src_dict ~dst:dst_dict in
+    (* Step 2. Create an empty suffix and close it. *)
+    let* suffix =
+      Suffix.create_rw ~root:new_store_root ~overwrite:false
+        ~auto_flush_threshold:1_000_000 ~auto_flush_procedure:`Internal
+        ~start_idx:generation
+    in
+    let* () = Suffix.close suffix in
+    (* Step 3. Create the control file and close it. *)
+    let status =
+      Payload.Gced
+        {
+          suffix_start_offset;
+          generation;
+          latest_gc_target_offset;
+          suffix_dead_bytes = Int63.zero;
+        }
+    in
+    let dict_end_poff = Io.size_of_path dst_dict |> Errs.raise_if_error in
+    let pl =
+      {
+        Payload.dict_end_poff;
+        suffix_end_poff = Int63.zero;
+        checksum = Int63.zero;
+        status;
+        upgraded_from_v3_to_v4 = false;
+        chunk_num = 1;
+        chunk_start_idx = generation;
+      }
+    in
+    let path = Irmin_pack.Layout.V4.control ~root:new_store_root in
+    let* control = Control.create_rw ~path ~overwrite:false pl in
+    let* () = Control.close control in
+    (* Step 4. Create the index. *)
+    let* index =
+      let log_size = Conf.index_log_size config in
+      let throttle = Conf.merge_throttle config in
+      Index.v ~fresh:true ~flush_callback:Fun.id ~readonly:false ~throttle
+        ~log_size new_store_root
+    in
+    (* Step 5. Add the commit to the index, close the index. *)
+    let () =
+      match commit_key with
+      | Direct { hash; offset; length } ->
+          Index.add index hash (offset, length, Pack_value.Kind.Commit_v2)
+      | Indexed _ -> assert false
+    in
+    let* () = Index.close index in
+    Ok ()
 end

src/irmin-pack/unix/file_manager_intf.ml

@@ -253,6 +253,13 @@ module type S = sig
   val generation : t -> int
   val gc_allowed : t -> bool
   val split : t -> (unit, [> Errs.t ]) result
+
+  val create_one_commit_store :
+    Irmin.Backend.Conf.t ->
+    generation:int ->
+    new_store_root:string ->
+    Index.key Pack_key.t ->
+    (unit, [> open_rw_error | close_error ]) result
 end
 
 module type Sigs = sig

src/irmin-pack/unix/gc.ml

@@ -42,8 +42,8 @@ module Make (Args : Gc_args.S) = struct
     latest_gc_target_offset : int63;
   }
 
-  let v ~root ~generation ~unlink ~dispatcher ~fm ~contents ~node ~commit
-      commit_key =
+  let v ~root ~path_new_files ~generation ~unlink ~dispatcher ~fm ~contents
+      ~node ~commit commit_key =
     let offset, latest_gc_target_offset =
       let state : _ Pack_key.state = Pack_key.inspect commit_key in
       match state with
@@ -89,7 +89,8 @@ module Make (Args : Gc_args.S) = struct
     (* start worker task *)
     let task =
       Async.async (fun () ->
-          Worker.run_and_output_result root commit_key offset ~generation)
+          Worker.run_and_output_result root commit_key offset ~generation
+            ~path_new_files)
     in
     let partial_stats =
       Gc_stats.Main.finish_current_step partial_stats "before finalise"
@@ -314,6 +315,15 @@ module Make (Args : Gc_args.S) = struct
           | `Running -> Lwt.return_ok `Running
           | #Async.outcome as status -> go status)
 
+  let finalise_without_swap t =
+    let* status = Async.await t.task in
+    match status with
+    | `Success -> Lwt.return_unit
+    | _ ->
+        let gc_output = read_gc_output ~root:t.root ~generation:t.generation in
+        let r = gc_errors status gc_output |> Errs.raise_if_error in
+        Lwt.return r
+
   let on_finalise t f =
     (* Ignore returned promise since the purpose of this
        function is to add asynchronous callbacks to the GC

src/irmin-pack/unix/gc.mli

@@ -25,6 +25,7 @@ module Make (Args : Gc_args.S) : sig
 
   val v :
     root:string ->
+    path_new_files:string ->
     generation:int ->
     unlink:bool ->
     dispatcher:Args.Dispatcher.t ->
@@ -51,5 +52,6 @@ module Make (Args : Gc_args.S) : sig
       finalises. *)
 
   val cancel : t -> bool
+  val finalise_without_swap : t -> unit Lwt.t
 end
 with module Args = Args

src/irmin-pack/unix/gc_worker.ml

@@ -122,7 +122,7 @@ module Make (Args : Gc_args.S) = struct
     stats := Gc_stats.Worker.add_file_size !stats "old_prefix" prefix_size;
     stats := Gc_stats.Worker.add_file_size !stats "old_mapping" mapping_size
 
-  let run ~generation root commit_key new_prefix_end_offset =
+  let run ~generation ~path_new_files root commit_key new_prefix_end_offset =
     let open Result_syntax in
     let config =
       Irmin_pack.Conf.init ~fresh:false ~readonly:true ~lru_size:0 root
@@ -167,7 +167,7 @@ module Make (Args : Gc_args.S) = struct
         stats := Gc_stats.Worker.add_file_size !stats "mapping" mapping_size
       in
       (fun f ->
-        Mapping_file.create ~report_file_sizes ~root ~generation
+        Mapping_file.create ~report_file_sizes ~root:path_new_files ~generation
           ~register_entries:f ()
         |> Errs.raise_if_error)
       @@ fun ~register_entry ->
@@ -218,7 +218,9 @@ module Make (Args : Gc_args.S) = struct
       (* Step 4. Create the new prefix. *)
       stats := Gc_stats.Worker.finish_current_step !stats "prefix: start";
       let prefix =
-        let path = Irmin_pack.Layout.V4.prefix ~root ~generation in
+        let path =
+          Irmin_pack.Layout.V4.prefix ~root:path_new_files ~generation
+        in
         Ao.create_rw_exn ~path
       in
       let () =
@@ -252,7 +254,9 @@ module Make (Args : Gc_args.S) = struct
         Dispatcher.read_exn dispatcher accessor buf
       in
       let prefix =
-        let path = Irmin_pack.Layout.V4.prefix ~root ~generation in
+        let path =
+          Irmin_pack.Layout.V4.prefix ~root:path_new_files ~generation
+        in
         Io.open_ ~path ~readonly:false |> Errs.raise_if_error
       in
       Errors.finalise_exn (fun _outcome ->
@@ -330,10 +334,11 @@ module Make (Args : Gc_args.S) = struct
 
   (* No one catches errors when this function terminates. Write the result in a
      file and terminate. *)
-  let run_and_output_result ~generation root commit_key new_prefix_end_offset =
+  let run_and_output_result ~generation ~path_new_files root commit_key
+      new_prefix_end_offset =
     let result =
       Errs.catch (fun () ->
-          run ~generation root commit_key new_prefix_end_offset)
+          run ~generation ~path_new_files root commit_key new_prefix_end_offset)
     in
     let write_result = write_gc_output ~root ~generation result in
     write_result |> Errs.log_if_error "writing gc output"

src/irmin-pack/unix/gc_worker.mli

@@ -23,7 +23,12 @@ module Make (Args : Gc_args.S) : sig
   module Args : Gc_args.S
 
   val run_and_output_result :
-    generation:int -> string -> Args.key -> int63 -> unit
+    generation:int ->
+    path_new_files:string ->
+    string ->
+    Args.key ->
+    int63 ->
+    unit
 
   type gc_output = (Stats.Latest_gc.worker, Args.Errs.t) result
   [@@deriving irmin]

src/irmin-pack/unix/io.ml

@@ -267,6 +267,12 @@ module Unix = struct
       Ok ()
     with Sys_error msg -> Error (`Sys_error msg)
 
+  let cp_file ~src ~dst =
+    let cmd = Filename.quote_command "cp" [ "-p"; src; dst ] in
+    match Sys.command cmd with
+    | 0 -> Ok ()
+    | n -> Error (`Sys_error (Int.to_string n))
+
   let mkdir path =
     match (classify_path (Filename.dirname path), classify_path path) with
     | `Directory, `No_such_file_or_directory -> (

src/irmin-pack/unix/io_intf.ml

@@ -78,6 +78,9 @@ module type S = sig
   val move_file :
     src:string -> dst:string -> (unit, [> `Sys_error of string ]) result
 
+  val cp_file :
+    src:string -> dst:string -> (unit, [> `Sys_error of string ]) result
+
   val mkdir : string -> (unit, [> mkdir_error ]) result
   val unlink : string -> (unit, [> `Sys_error of string ]) result
 

src/irmin-pack/unix/s.ml

@@ -53,6 +53,15 @@ module type S = sig
 
       TODO: Detail exceptions raised. *)
 
+  val create_one_commit_store : repo -> commit_key -> string -> unit Lwt.t
+  (** [create_one_commit_store t key path] creates a new store at [path] from
+      the existing one, containing only one commit, specified by the [key]. Note
+      that this operation is blocking.
+
+      It requires that the files existing on disk when the operation is
+      launched, remain on disk until the operation completes. In particular, a
+      Gc running in a different process could remove files from disk. *)
+
   module Gc : sig
     (** GC *)