A builtin or setting this build lacks is refused GUI-only, from an exec too

On a terminal, Fonts written to an exec was no builtin there, so it ran as a shell command and ended exit 127; ctl said unknown control message. Both now say Fonts is GUI-only, invalid here (a builtin another platform's build has, not in this build).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 10 insertions, 2 deletions

diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index 68b62186..c2a78fb8 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -388,11 +388,11 @@ fn checkBuiltin(p: *Pardes, req: Req, line: []const u8, scope: builtins.Scope) ?
     const b = std.meta.stringToEnum(Builtin, word) orelse {
         // A setting this build's frontend cannot show (Lift, GripWidth on a
         // terminal) is known, and says so rather than sound misspelt.
-        if (config.Runtime.find(word) != null) {
+        if (builtins.absentWhy(word) orelse if (config.Runtime.find(word) != null) @as([]const u8, "GUI-only") else null) |absent| {
             var why: [64]u8 = undefined;
             // EINVAL, a request this build cannot take, in words 9ns reads
             // as that: `invalid`.
-            return refuse(p, req, std.fmt.bufPrint(&why, "{s} is GUI-only, invalid here", .{word}) catch "GUI-only, invalid here", line);
+            return refuse(p, req, std.fmt.bufPrint(&why, "{s} is {s}, invalid here", .{ word, absent }) catch "GUI-only, invalid here", line);
         }
         return refuse(p, req, "unknown control message", line);
     };
@@ -1704,6 +1704,14 @@ test "a setting this frontend cannot show says GUI-only, and DumpDir reads back
     if (!builtins.capabilities.lift) {
         try testing.expectEqualStrings("Lift is GUI-only, invalid here \"Lift on\"", wr(p, root_ctl, "Lift on\n").reply.ename);
         try testing.expectEqualStrings("GripWidth is GUI-only, invalid here \"GripWidth 120\"", wr(p, root_ctl, "GripWidth 120\n").reply.ename);
+        // A builtin the GUI alone has, written to a ctl or an exec, never
+        // runs as a shell command.
+        if (comptime !pardes.font_picker) {
+            try testing.expectEqualStrings("Fonts is GUI-only, invalid here \"Fonts\"", wr(p, root_ctl, "Fonts\n").reply.ename);
+            const ran = wr(p, root_exec, "Fonts\n");
+            try testing.expectEqual(E.IO, ran.errno());
+            try testing.expectEqualStrings("Fonts is GUI-only, invalid here", ran.reply.ename);
+        }
     }
     try testing.expectEqualStrings("unknown control message \"Liftt on\"", wr(p, root_ctl, "Liftt on\n").reply.ename);
     var buf: [900]u8 = undefined;