A tag write past the limit is refused whole, naming it

A write to tag longer than the room left was cut and applied in part. It now changes nothing and fails ENOSPC, tag: over 4096 bytes; the docs give the limit.

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

1 files changed, 16 insertions, 2 deletions

diff --git a/src/ninep/pane.zig b/src/ninep/pane.zig
index ec6e8551..4327c403 100644
--- a/src/ninep/pane.zig
+++ b/src/ninep/pane.zig
@@ -570,8 +570,9 @@ fn writeTag(req: Req, pane: *Pane) Reply {
     const pf = &pane.fs;
     const had = tagline.curTail(pane).len;
     const room = limits.max_tag_tail -| had -| @intFromBool(pf.tag_held_newline);
-    if (room == 0) return Reply.fail(req.tag, E.NOSPC);
-    const take = wholeUtf8(req.data[0..@min(req.data.len, room)]);
+    // Whole or not at all: a write that would pass the limit changes nothing.
+    if (req.data.len > room) return tree.failText(req.tag, E.NOSPC, std.fmt.comptimePrint("tag: over {d} bytes", .{limits.max_tag_tail}));
+    const take = wholeUtf8(req.data);
     // A truncating write drops ONE trailing newline, its whole text's: a
     // newline held from a write before goes in once more text follows it.
     if (pf.tag_held_newline) {
@@ -1189,6 +1190,19 @@ test "an open's writes in a row are held and go in as one edit, seen by the next
     try testing.expectEqualStrings("abone\n", rd(p, body, 0, 64).bytes);
 }
 
+test "a tag write past the limit is refused whole, naming the limit" {
+    const p = try withFile(testing.allocator, "x\n");
+    defer p.deinit();
+    const tag = Node.of(serialOf(p), .tag);
+    const before = try testing.allocator.dupe(u8, rd(p, tag, 0, 1 << 16).bytes);
+    defer testing.allocator.free(before);
+    const big = "w" ** (limits.max_tag_tail + 10);
+    const r = wr(p, tag, big);
+    try testing.expectEqual(E.NOSPC, r.errno());
+    try testing.expect(std.mem.startsWith(u8, r.reply.ename, "tag: over "));
+    try testing.expectEqualStrings(before, rd(p, tag, 0, 1 << 16).bytes);
+}
+
 test "two opens writing one body are two undo steps, however their writes interleave" {
     const p = try withFile(testing.allocator, "one\n");
     defer p.deinit();