From 3e24f22054e064116dc6a4f6481d598a51b7bd9f Mon Sep 17 00:00:00 2001
From: Sabine Schmaltz <6594573+sabine@users.noreply.github.com>
Date: Thu, 20 Nov 2025 18:15:28 +0100
Subject: [PATCH 1/2] Warn users when RPC connection retry loop is triggered

When dune attempts to forward a build request to an existing dune process
but cannot establish an RPC connection, it silently retries indefinitely.
This adds a warning message to inform users what is happening and how to
resolve the issue by deleting _build/.lock if no other dune process is
actually running.

Signed-off-by: Sabine Schmaltz <6594573+sabine@users.noreply.github.com>
---
 bin/rpc/rpc_common.ml | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/bin/rpc/rpc_common.ml b/bin/rpc/rpc_common.ml
index 1bbbba44569..4069a27893b 100644
--- a/bin/rpc/rpc_common.ml
+++ b/bin/rpc/rpc_common.ml
@@ -77,11 +77,31 @@ let establish_connection_exn () =
 let establish_connection_with_retry () =
   let open Fiber.O in
   let pause_between_retries_s = 0.2 in
+  let warned = ref false in
   let rec loop () =
     establish_connection ()
     >>= function
     | Ok x -> Fiber.return x
     | Error _ ->
+      if not !warned
+      then (
+        warned := true;
+        let lock_file = Path.Build.(relative root ".lock") in
+        let lock_held_by = Dune_util.Global_lock.Lock_held_by.read_lock_file () in
+        let pid_suffix =
+          match lock_held_by with
+          | Dune_util.Global_lock.Lock_held_by.Unknown -> ""
+          | Pid_from_lockfile pid -> sprintf " (pid: %d)" pid
+        in
+        User_warning.emit
+          [ Pp.textf
+              "Another dune instance%s is currently running. Waiting for it to finish..."
+              pid_suffix
+          ; Pp.textf
+              "If you're certain no other dune process is running, you can resolve this by \
+               deleting: %s"
+              (Path.Build.to_string lock_file)
+          ]);
       let* () = Dune_engine.Scheduler.sleep ~seconds:pause_between_retries_s in
       loop ()
   in

From 8b3b1d1f7fc7032d81b11a9fec659336d9368f39 Mon Sep 17 00:00:00 2001
From: Sabine Schmaltz <6594573+sabine@users.noreply.github.com>
Date: Thu, 20 Nov 2025 19:21:05 +0100
Subject: [PATCH 2/2] I think we were actually waiting to connect to it, not
 for it to finish

Signed-off-by: Sabine Schmaltz <6594573+sabine@users.noreply.github.com>
---
 bin/rpc/rpc_common.ml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/bin/rpc/rpc_common.ml b/bin/rpc/rpc_common.ml
index 4069a27893b..515c364e7a8 100644
--- a/bin/rpc/rpc_common.ml
+++ b/bin/rpc/rpc_common.ml
@@ -95,7 +95,7 @@ let establish_connection_with_retry () =
         in
         User_warning.emit
           [ Pp.textf
-              "Another dune instance%s is currently running. Waiting for it to finish..."
+              "Another dune instance%s is currently running. Waiting connect to it..."
               pid_suffix
           ; Pp.textf
               "If you're certain no other dune process is running, you can resolve this by \