pkg: Show git error output when Rev_store's fetch fails

Closes #12610

Signed-off-by: Puneeth Chaganti <[email protected]>

Author: punchagan

src/dune_pkg/rev_store.ml

@@ -725,15 +725,16 @@ let fetch_allow_failure repo ~url obj =
     | true -> Fiber.return `Fetched
     | false ->
       run_with_exit_code
-        ~allow_codes:(fun x -> x = 0 || x = 128)
+        ~allow_codes:(Int.equal 0)
         repo
         ~display:!Dune_engine.Clflags.display
         [ "fetch"; "--no-write-fetch-head"; url; Object.to_hex obj ]
       >>| (function
-       | Ok 128 -> `Not_found
        | Ok 0 ->
          Table.set repo.present_objects obj ();
          `Fetched
+       | Error { Git_error.exit_code; output; _ } when exit_code = 128 ->
+         `Not_found output
        | Error git_error -> Git_error.raise_code_error git_error
        | _ -> assert false))
 ;;
@@ -742,8 +743,14 @@ let fetch repo ~url obj =
   fetch_allow_failure repo ~url obj
   >>| function
   | `Fetched -> ()
-  | `Not_found ->
-    User_error.raise [ Pp.textf "unable to fetch %S from %S" (Object.to_hex obj) url ]
+  | `Not_found output ->
+    User_error.raise
+      ([ Pp.textf
+           "Dune was unable to fetch %S from %S due to the following git fetch error:"
+           (Object.to_hex obj)
+           url
+       ]
+       @ List.map ~f:Pp.verbatim output)
 ;;
 
 module Debug = struct
@@ -1156,7 +1163,7 @@ let resolve_revision t (remote : Remote.t) ~revision =
 let fetch_object t (remote : Remote.t) revision =
   fetch_allow_failure t ~url:remote.url revision
   >>= function
-  | `Not_found -> Fiber.return None
+  | `Not_found _ -> Fiber.return None
   | `Fetched -> At_rev.of_rev t ~revision >>| Option.some
 ;;
 

test/blackbox-tests/test-cases/pkg/commit-hash-references.t

@@ -32,5 +32,5 @@ Depend on foo from the repo
 
 Which foo will we get?
 
-  $ dune pkg lock 2>&1 | head -1 | sed "s/$AMBIGUOUS_REF/\$AMBIGUOUS_REF/g"
+  $ dune pkg lock 2>&1 | grep "not found" | sed "s/$AMBIGUOUS_REF/\$AMBIGUOUS_REF/g"
   revision "$AMBIGUOUS_REF" not found in