A tag write refuses NUL and the other control characters; a Dump that fails says so

A NUL written into a tag went into the dump, which the host then could
not write, and Dump said nothing: a success that wrote nothing. Tag
writes, the workspace's, a column's and a pane's, refuse control
characters but tab and newline, DEL, C1 and bytes not UTF-8, EINVAL; and
Dump reports any failure of its own instead of swallowing it.

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

1 files changed, 11 insertions, 0 deletions

diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index d9a479fe..2958fc38 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -2026,6 +2026,17 @@ test "a served-tree pane spoils no Grep, and Look . from a gone directory is ENO
     try testing.expectStringStartsWith(looked.reply.ename, "look: /tmp/pardes-no-such-dir-zz: no such directory");
 }
 
+test "a tag write refuses NUL and the other control characters" {
+    const p = try withFile(testing.allocator, "x\n");
+    defer p.deinit();
+    for ([_]u64{ @intFromEnum(tree.TopFile.tag), tree.Node.ofCol(pardes.layout.columnSerial(p, 0), .tag), Node.of(serialOf(p), .tag) }) |node| {
+        const refused = wr(p, node, " a\x00b");
+        try testing.expectEqual(E.INVAL, refused.errno());
+        try testing.expectStringStartsWith(refused.reply.ename, "invalid tag text");
+        try testing.expectEqual(E.INVAL, wr(p, node, " \x7f").errno());
+    }
+}
+
 test "size is monotonic: growing is never refused, and a size once taken is taken again" {
     const p = try Pardes.init(testing.allocator, .{ .tty_only = true, .cols = 160, .rows = 60 });
     defer p.deinit();