An event write-back past the text says range past end of body

A well-formed record whose range ran past the body (or tag) was refused
as "bad event syntax", which sent a client looking for a typo. It now
says "range past end of body" (or tag); a malformed one keeps the old
words.

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

1 files changed, 5 insertions, 1 deletions

diff --git a/src/ninep/events.zig b/src/ninep/events.zig
index e0e66800..a12cf86d 100644
--- a/src/ninep/events.zig
+++ b/src/ninep/events.zig
@@ -579,7 +579,9 @@ pub fn writeEvent(p: *Pardes, req: Req, id: usize) Reply {
                 else => return tree.failText(req.tag, E.INVAL, tree.e_bad_event),
             }
             const n = if (r.action.onTag()) tag.len else body.len;
-            if (r.q0 > r.q1 or r.q1 > n) return tree.failText(req.tag, E.INVAL, tree.e_bad_event);
+            if (r.q0 > r.q1) return tree.failText(req.tag, E.INVAL, tree.e_bad_event);
+            // Well formed, but not a range of this text: say which.
+            if (r.q1 > n) return tree.failText(req.tag, E.INVAL, if (r.action.onTag()) "range past end of tag" else "range past end of body");
         }
         if (check.i != req.data.len) return tree.failText(req.tag, E.INVAL, tree.e_bad_event);
     }
@@ -816,6 +818,8 @@ test "writing an event record back performs the action it names" {
         try testing.expectEqual(E.INVAL, wr(p, event, bad).errno());
         try testing.expectEqual(@as(usize, 0), pane.msg_len);
     }
+    // Well formed but past the text: said so, not "bad event syntax".
+    try testing.expectEqualStrings("range past end of body", wr(p, event, "MX0 999\n").reply.ename);
 
     _ = call(p, .{ .tag = 23, .op = .open, .node = event });
     p.fs.origin = 'K';