ocaml-lsp-server crash in Emacs

[mariusdkm]

I use ocaml-lsp-server installed through opam in my Emacs(GNU Emacs 28.2 running on a M1 Mac)

For some reason if I start the lsp through M-x lsp it just crashes with the error message:
(Side note I have dune build --watch running in the project, but this also happens without running it)

Process ocaml-lsp-server stderr finished
/-----------------------------------------------------------------------
| Internal error: Uncaught exception.
| Failure("Fiber.Ivar.fill")
| Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
| Called from Fiber__Scheduler.exec in file "src/fiber/scheduler.ml", line 87, characters 32-61
| Called from Fiber__Scheduler.advance.(fun) in file "src/fiber/scheduler.ml", line 195, characters 2-58
| Called from Fiber.run.loop in file "src/fiber/fiber.ml", line 15, characters 30-61
| Called from Fiber.run.(fun) in file "src/fiber/fiber.ml" (inlined), line 17, characters 17-47
| Called from Lev_fiber.run in file "submodules/lev/lev-fiber/src/lev_fiber.ml", line 1264, characters 10-59
| Re-raised at Stdune__Exn.raise_with_backtrace in file "otherlibs/stdune/exn.ml" (inlined), line 36, characters 27-56
| Called from Stdune__Exn_with_backtrace.reraise in file "otherlibs/stdune/exn_with_backtrace.ml", line 18, characters 33-71
| Called from Stdune__Exn_with_backtrace.try_with in file "otherlibs/stdune/exn_with_backtrace.ml", line 9, characters 8-12
\-----------------------------------------------------------------------

And this is the lsp-log

Command "ocaml-language-server --stdio" is not present on the path.
Command "ocamllsp" is present on the path.
Command "ocaml-language-server --stdio" is not present on the path.
Command "ocamllsp" is present on the path.
Found the following clients for /Users/_myuser_/Documents/uni/fpv/_school_project_/src/assignment.ml: (server-id ocaml-lsp-server, priority 0)
The following clients were selected based on priority: (server-id ocaml-lsp-server, priority 0)
Connected to dune /Users/_myuser_/Documents/uni/fpv/_school_project_ (/Users/_myuser_/Documents/uni/fpv/_school_project_/_build/.rpc/dune)
client 0: connecting...
client 0: connected to dune at unix:///Users/_myuser_/Documents/uni/fpv/_school_project_/_build/.rpc/dune

My installed ocaml-lsp-server version:

➜  ~ opam info ocaml-lsp-server

<><> ocaml-lsp-server: information on all versions ><><><><><><><><><><><><>  🐫 
name                   ocaml-lsp-server
all-installed-versions 1.15.1-5.0 [5.0.0]

My emacs config for ocaml

(add-to-list 'exec-path "/Users/mariusdekuthymeurers/.opam/5.0.0/bin")

;; Major mode for OCaml programming
(use-package tuareg
  :ensure t
  :mode (("\\.ocamlinit\\'" . tuareg-mode)))

;; Major mode for editing Dune project files
(use-package dune
  :ensure t)

(use-package utop
  :ensure t)

;; Merlin provides advanced IDE features
(use-package merlin
  :ensure t
  :config
  (add-hook 'tuareg-mode-hook #'merlin-mode)
  (add-hook 'merlin-mode-hook #'company-mode)
  ;; we're using flycheck instead
  (setq merlin-error-after-save nil))

(use-package merlin-eldoc
  :ensure t
  :hook ((tuareg-mode) . merlin-eldoc-setup))

;; This uses Merlin internally
(use-package flycheck-ocaml
  :ensure t
  :config
  (flycheck-ocaml-setup))

[rgrinberg]

Can you apply the following patch to fiber and then try reproducing again:

diff --git a/otherlibs/fiber/src/core.ml b/otherlibs/fiber/src/core.ml
index a571e69e4..f005839a4 100644
--- a/otherlibs/fiber/src/core.ml
+++ b/otherlibs/fiber/src/core.ml
@@ -32,7 +32,10 @@ and eff =
   | Toplevel_exception : Exn_with_backtrace.t -> eff
   | Done of value
 
-and 'a ivar = { mutable state : ('a, [ `Full | `Empty ]) ivar_state }
+and 'a ivar =
+  { mutable state : ('a, [ `Full | `Empty ]) ivar_state
+  ; source : Printexc.raw_backtrace
+  }
 
 and ('a, _) ivar_state =
   | Full : 'a -> ('a, [> `Full ]) ivar_state
@@ -210,7 +213,9 @@ let reraise_all l _k =
 module Ivar = struct
   type 'a t = 'a ivar
 
-  let create () = { state = Empty }
+  let create () =
+    let source = Printexc.get_callstack 20 in
+    { state = Empty; source }
 
   let read t k = Read_ivar (t, k)
 
diff --git a/otherlibs/fiber/src/scheduler.ml b/otherlibs/fiber/src/scheduler.ml
index f3c2ac679..d279614f1 100644
--- a/otherlibs/fiber/src/scheduler.ml
+++ b/otherlibs/fiber/src/scheduler.ml
@@ -22,7 +22,10 @@ module Jobs = struct
 
   let fill_ivar ivar x jobs =
     match ivar.state with
-    | Full _ -> failwith "Fiber.Ivar.fill"
+    | Full _ ->
+      let exn = Failure "Ivar.fill" in
+      let backtrace = ivar.source in
+      Exn_with_backtrace.reraise { Exn_with_backtrace.backtrace; exn }
     | Empty ->
       ivar.state <- Full x;
       jobs

[mariusdkm]

How do I apply these patches?
I found the scheduler.ml and core.ml files in my opam lib directoy (~./.opam/5.0.0/lib/fiber/)
But if I apply the patches there nothing happens, and if I do opam reinstall it redownloads them and overrides my patches.

[rgrinberg]

You would have to clone the repe of fiber, checkout the appropriate version, apply the patch, and then pin the repo (see opam pin --help).

[mariusdkm]

Ok, this took way longer to do then I expected to install the fiber package with the specific patches.
But here is the new stack trace:

/-----------------------------------------------------------------------
| Internal error: Uncaught exception.
| Failure("Ivar.fill")
| Raised by primitive operation at Fiber__Core.Ivar.create in file "fiber/src/core.ml", line 217, characters 17-42
| Called from Ocaml_lsp_server__Ocamlformat_rpc.create_state in file "ocaml-lsp-server/src/ocamlformat_rpc.ml", line 188, characters 17-37
| Called from Ocaml_lsp_server__Ocamlformat_rpc.create in file "ocaml-lsp-server/src/ocamlformat_rpc.ml" (inlined), line 190, characters 53-68
| Called from Ocaml_lsp_server.start in file "ocaml-lsp-server/src/ocaml_lsp_server.ml", line 753, characters 24-49
| Called from Fiber__Scheduler.exec in file "fiber/src/scheduler.ml", line 76, characters 8-11
| Called from Fiber__Scheduler.start in file "fiber/src/scheduler.ml", line 219, characters 2-42
| Called from Fiber.run.(fun) in file "fiber/src/fiber.ml" (inlined), line 17, characters 28-47
| Called from Lev_fiber.run in file "submodules/lev/lev-fiber/src/lev_fiber.ml", line 1264, characters 10-59
| Called from Ocaml_lsp_server.run in file "ocaml-lsp-server/src/ocaml_lsp_server.ml", line 868, characters 2-151
| Called from Stdune__Exn_with_backtrace.try_with in file "otherlibs/stdune/exn_with_backtrace.ml", line 9, characters 8-12
| Called from Dune__exe__Main in file "ocaml-lsp-server/bin/main.ml", line 40, characters 6-106
| Re-raised at Stdune__Exn.raise_with_backtrace in file "otherlibs/stdune/exn.ml" (inlined), line 36, characters 27-56
| Called from Stdune__Exn_with_backtrace.reraise in file "otherlibs/stdune/exn_with_backtrace.ml", line 18, characters 33-71
| Called from Fiber__Scheduler.exec in file "fiber/src/scheduler.ml", line 90, characters 32-61
| Called from Fiber__Scheduler.advance.(fun) in file "fiber/src/scheduler.ml", line 198, characters 2-58
| Called from Fiber.run.loop in file "fiber/src/fiber.ml", line 15, characters 30-61
| Called from Fiber.run.(fun) in file "fiber/src/fiber.ml" (inlined), line 17, characters 17-47
| Called from Lev_fiber.run in file "submodules/lev/lev-fiber/src/lev_fiber.ml", line 1264, characters 10-59
| Re-raised at Stdune__Exn.raise_with_backtrace in file "otherlibs/stdune/exn.ml" (inlined), line 36, characters 27-56
| Called from Stdune__Exn_with_backtrace.reraise in file "otherlibs/stdune/exn_with_backtrace.ml", line 18, characters 33-71
| Called from Stdune__Exn_with_backtrace.try_with in file "otherlibs/stdune/exn_with_backtrace.ml", line 9, characters 8-12
\-----------------------------------------------------------------------


Process ocaml-lsp-server stderr finished

[rgrinberg]

Can you see #1132 fixes the issue for you?