Add warning when build directory lock is already held

When dune attempts to acquire the build directory lock and finds it held by
another process, emit a warning message instead of silently waiting. The
warning includes the PID of the lock holder (if available) and the path to
the lock file, making it easier for users and AI assistants to understand
why dune appears to be hanging.

Signed-off-by: sabine <[email protected]>

Author: sabine

src/dune_util/global_lock.ml

@@ -99,13 +99,42 @@ let lock ~timeout =
     else (
       let res =
         match timeout with
-        | None -> Lock.lock ()
+        | None ->
+          (match Lock.lock () with
+           | `Success -> `Success
+           | `Failure ->
+             let lock_held_by = Lock_held_by.read_lock_file () in
+             User_warning.emit
+               [ Pp.textf
+                   "Build directory is locked by another dune process%s. Waiting for the \
+                    lock to be released... (lock file: %s)"
+                   (match lock_held_by with
+                    | Unknown -> ""
+                    | Pid_from_lockfile pid -> sprintf " (pid: %d)" pid)
+                   (Path.Build.to_string lock_file)
+               ];
+             `Failure)
         | Some timeout ->
+          let warned = ref false in
           (match
              with_timeout ~timeout (fun () ->
                match Lock.lock () with
                | `Success -> `Stop
-               | `Failure -> `Continue)
+               | `Failure ->
+                 if not !warned
+                 then (
+                   warned := true;
+                   let lock_held_by = Lock_held_by.read_lock_file () in
+                   User_warning.emit
+                     [ Pp.textf
+                         "Build directory is locked by another dune process%s. Waiting for \
+                          the lock to be released... (lock file: %s)"
+                         (match lock_held_by with
+                          | Unknown -> ""
+                          | Pid_from_lockfile pid -> sprintf " (pid: %d)" pid)
+                         (Path.Build.to_string lock_file)
+                     ]);
+                 `Continue)
            with
            | `Timed_out -> `Failure
            | `Success -> `Success)