irmin-pack: Rewrite pack_store using dispatcher's accessor

This commit is meant to better handle objects that are still live in
index but that have been GCed.

CHANGES.md

@@ -4,11 +4,17 @@
 
 - **irmin**
   - Add `Storage` module for creating custom storage layers (#2047, @metanivek)
-  
+
 ### Changed
 
 ### Fixed
 
+- **irmin-pack**
+  - Fix the behaviour of irmin-pack regarding hashes and keys to GCed objects.
+    It used to not correctly ignore these entries, which could have resulted in
+    various bugs. E.g. the impossibility to append an object that used to be
+    dead and that has its hash in index. (#2070, @Ngoguey42)
+
 ## 3.4.0 (2022-08-25)
 
 ### Added

src/irmin-pack/unix/dispatcher.ml

@@ -240,4 +240,8 @@ module Make (Fm : File_manager.S with module Io = Io.Unix) :
 
   let create_accessor_exn = Accessor.v_exn
   let create_accessor_from_range_exn = Accessor.v_range_exn
+
+  let shrink_accessor_exn a ~new_len =
+    if new_len > a.len then failwith "shrink_accessor_exn to larger accessor";
+    { a with len = new_len }
 end

src/irmin-pack/unix/dispatcher_intf.ml

@@ -46,6 +46,10 @@ module type S = sig
       except that the precise length of the span will be decided during the
       call. *)
 
+  val shrink_accessor_exn : accessor -> new_len:int -> accessor
+  (** [shrink_accessor_exn a ~new_len] is [a] where the length is smaller than
+      in [a].*)
+
   val read_exn : t -> accessor -> bytes -> unit
   (** [read_exn] either reads in the prefix or the suffix file, depending on
       [accessor]. *)