[server] emit stderr of `--cmd` as a server message (fixes test for #9690 after #6726)

src/compiler/haxe.ml

@@ -162,6 +162,27 @@ let run_command ctx cmd =
 		if len > 3 && String.sub cmd 0 3 = "cd " then begin
 			Sys.chdir (String.sub cmd 3 (len - 3));
 			0
+		(* Emit stderr as a server message in server mode *)
+		end else if CompilationServer.runs() then begin
+			let binary_string s =
+				if not Globals.is_windows then s else String.concat "\n" (Str.split (Str.regexp "\r\n") s)
+			in
+			let pout, pin, perr = Unix.open_process_full cmd (Unix.environment()) in
+			let bout = Buffer.create 0 in
+			let berr = Buffer.create 0 in
+			let read_content channel buf =
+				Buffer.add_string buf (IO.read_all (IO.input_channel channel));
+			in
+			let tout = Thread.create (fun() -> read_content pout bout) () in
+			read_content perr berr;
+			Thread.join tout;
+			let result = (match Unix.close_process_full (pout,pin,perr) with Unix.WEXITED c | Unix.WSIGNALED c | Unix.WSTOPPED c -> c) in
+			let serr = binary_string (Buffer.contents berr) in
+			let sout = binary_string (Buffer.contents bout) in
+			if serr <> "" then ctx.messages <- CMError((if serr.[String.length serr - 1] = '\n' then String.sub serr 0 (String.length serr - 1) else serr),null_pos) :: ctx.messages;
+			if sout <> "" then ctx.com.print (sout ^ "\n");
+			result
+		(* Direct pass-through of std streams for normal compilation *)
 		end else begin
 			match Unix.system cmd with
 			| WEXITED c | WSIGNALED c | WSTOPPED c -> c