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, 1 insertions, 0 deletions

diff --git a/src/ninep/cols.zig b/src/ninep/cols.zig
index 3eeadfdc..aae6d364 100644
--- a/src/ninep/cols.zig
+++ b/src/ninep/cols.zig
@@ -78,6 +78,7 @@ pub fn read(p: *Pardes, req: Req, serial: ?u32) Reply {
 pub fn write(p: *Pardes, req: Req, serial: ?u32) Reply {
     const t = (header(p, serial) orelse return Reply.fail(req.tag, E.NOENT)).text;
     if (req.data.len == 0) return .{ .tag = req.tag, .written = 0 };
+    if (pardes.ctlfs.pane.tagFault(req.data)) |why| return tree.failText(req.tag, E.INVAL, why);
     const was = headerText(p, serial).?;
     var data = req.data;
     const rewriting = p.fs.header_rewrite == rewriteKey(serial);