A write to a closed pane's file says it is gone, and a terminal says why it takes no name

`echo Del > pane/6/exec` just after pane 6 closed failed EACCES: the shell's
lookup failed, it tried to create the file, and the engine refused every
Tcreate as a permission error. pardes now takes Tcreate itself: refused as
before (acme's fsyscreate refuses it too), ENOENT in a pane that has
closed. Renaming a terminal was refused with only EPERM; it now says a
terminal is named by its shell's directory, worded so a mount still maps
it to EPERM.

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

1 files changed, 7 insertions, 2 deletions

diff --git a/src/ninep/pane.zig b/src/ninep/pane.zig
index 9d20c587..3eafdbce 100644
--- a/src/ninep/pane.zig
+++ b/src/ninep/pane.zig
@@ -561,7 +561,10 @@ fn writeName(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
     const name = std.mem.trim(u8, req.data, " \t\r\n");
     if (name.len == 0) return Reply.fail(req.tag, E.INVAL);
     for (name) |c| if (c < ' ') return Reply.fail(req.tag, E.INVAL);
-    if (fileOf(pane) == null) return Reply.fail(req.tag, E.PERM);
+    if (fileOf(pane) == null) return tree.failText(req.tag, E.PERM, if (pane.isTerminal())
+        "rename not allowed: a terminal is named by its shell's directory; cd there, or Tty in another"
+    else
+        "rename not allowed: an image or PDF is named by the file it shows");
     const full = std.fs.path.resolvePosix(p.scratch.allocator(), &.{ pardes.Pardes.paneDir(pane), name }) catch
         return Reply.fail(req.tag, E.NOMEM);
     if (!std.fs.path.isAbsolute(full) or full.len >= 4096) return Reply.fail(req.tag, E.INVAL);
@@ -734,7 +737,9 @@ test "a terminal is listed as term with its directory as name" {
         try std.fmt.bufPrint(&want, "{d} term 0 /work/dir\n", .{pane.serial}),
     ) != null);
     try testing.expectEqualStrings("/work/dir\n", rd(p, Node.of(pane.serial, .name), 0, 4096).bytes);
-    try testing.expectEqual(E.PERM, wr(p, Node.of(pane.serial, .name), "/elsewhere\n").errno());
+    const refused = wr(p, Node.of(pane.serial, .name), "/elsewhere\n");
+    try testing.expectEqual(E.PERM, refused.errno());
+    try testing.expect(std.mem.startsWith(u8, refused.reply.ename, "rename not allowed: a terminal is named"));
 }
 
 test "body reads at any offset and writes append" {