Truncating data or xdata deletes only the addressed range, not the whole body

A shell's echo NEW > data opens data with OTRUNC, and truncate() spliced
the whole body away before the write replaced the addressed range: a
five-byte replacement erased a buffer. data and xdata hold what addr
names, so truncating one now deletes that range and nothing else; > on
data replaces the range and : > data deletes it, and only truncating
body clears the buffer.

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

1 files changed, 8 insertions, 0 deletions

diff --git a/test/fs.py b/test/fs.py
index c7552a2b..ca1ca70e 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -338,6 +338,14 @@ def discovery(binary, embedded=False):
                 assert client.read('/src/pardes.zig').startswith(b'const std')
                 assert client.stat('/src/pardes.zig')['mode'] == 0o444
                 assert b'pub const Pardes' in client.read(f'/pane/{look(client, "/virtual/src/pardes.zig")}/body')
+            # An open of data with OTRUNC (a shell's >) deletes the addressed
+            # range only, so the write after it replaces that range.
+            scratch = new_pane(client, b'hello world\nsecond line\n')
+            client.write(f'/pane/{scratch}/addr', b'#0,#5')
+            client.write(f'/pane/{scratch}/data', b'HOWDY', truncate=True)
+            assert client.read(f'/pane/{scratch}/body') == b'HOWDY world\nsecond line\n'
+            client.remove(f'/pane/{scratch}')
+
     print('9P discovery: listing/stat/find are inert; new, remove, look, exec, name, sel, log, ctl lock, focus, the ctl split and commands behave')