A tag rewrite counts its text without the newline it drops: echo of exactly 4096 bytes fits

`echo <4096 bytes> > tag` wrote 4097 bytes and was refused "over 4096
bytes", although a rewrite drops that one trailing newline and would have
held exactly 4096. The limit now counts a truncating write's text without
the newline it drops, for a pane's, a column's and the workspace's tag. An
append still counts its newline, which it keeps.

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

1 files changed, 5 insertions, 1 deletions

diff --git a/src/ninep/pane.zig b/src/ninep/pane.zig
index fef0f85f..d5f2f20a 100644
--- a/src/ninep/pane.zig
+++ b/src/ninep/pane.zig
@@ -662,7 +662,11 @@ fn writeTag(p: *Pardes, req: Req, pane: *Pane) Reply {
     // is a text like any other (acme's tag file appends the same way).
     const had = if (pf.tag_trunc_pending) 0 else tagline.curTail(pane).len;
     const room = limits.max_tag_tail -| had -| @intFromBool(pf.tag_held_newline and !pf.tag_trunc_pending);
-    if (req.data.len > room) {
+    // A rewrite drops its one trailing newline: `echo <4096 bytes> > tag`
+    // is 4096 bytes of tag, and fits.
+    const rewrite = pf.tag_rewrite or pf.tag_trunc_pending;
+    const counted = req.data.len - @intFromBool(rewrite and req.data[req.data.len - 1] == '\n');
+    if (counted > room) {
         pf.tag_trunc_pending = false;
         return tree.failText(req.tag, E.NOSPC, e_tag_over);
     }