Display a warning when dune builds via RPC

When an instance of dune is already running, invoking `dune build` will
still build the target, howwever other command line arguments are
silently ignored by dune as the current RPC interface is too limited to
express them. This may change over time, but currently it's unusual UX
for `dune build` to behave differently depending on whether or not
another instance of `dune build` is already running. To more accurately
set user expectations, print a warning when dune is running with this
reduced functionality.

Signed-off-by: Stephen Sherratt <[email protected]>

Author: gridbugs

bin/build_cmd.ml

@@ -215,6 +215,16 @@ let build =
          perform the RPC call.
       *)
       Scheduler.go_without_rpc_server ~common ~config (fun () ->
+        if not (Common.Builder.equal builder Common.Builder.default)
+        then
+          User_warning.emit
+            [ Pp.textf
+                "Your build request is being forwarded to a running Dune instance%s so \
+                 most command-line arguments will be ignored."
+                (match (lock_held_by : Dune_util.Global_lock.Lock_held_by.t) with
+                 | Unknown -> ""
+                 | Pid_from_lockfile pid -> sprintf " (pid: %d)" pid)
+            ];
         build_via_rpc_server ~print_on_success:true ~targets)
     | Ok () ->
       let request setup =

test/blackbox-tests/test-cases/watching/watching-eager-concurrent-build-command.t

@@ -9,6 +9,7 @@ Build the project once before starting the watch server so the watch server star
   $ dune build --watch &
   Success, waiting for filesystem changes...
   Success, waiting for filesystem changes...
+  Success, waiting for filesystem changes...
   File "foo.ml", line 1, characters 9-21:
   1 | let () = print_endlin "Hello, World!"
                ^^^^^^^^^^^^
@@ -26,6 +27,13 @@ Demonstrate that we can run "dune build" while the watch server is running.
   $ dune build
   Success
 
+Demonstrate that a warning is displayed when extra arguments are passed to
+"dune build", since those arguments will be ignored.
+  $ dune build --auto-promote 2>&1 | sed 's/pid: [0-9]*/pid: PID/g'
+  Warning: Your build request is being forwarded to a running Dune instance
+  (pid: PID) so most command-line arguments will be ignored.
+  Success
+
 Demonstrate that error messages are still printed by "dune build" when it's
 acting as an RPC client while running concurrently with an RPC server.
   $ echo 'let () = print_endlin "Hello, World!"' > foo.ml