log stats 0, and pardes re-pins cloud9 whose 9ns opens such a file direct and nonseekable: fs.md's follow recipe with cat works through a mount

The log stated the length an open would freeze, a length a mount's
kernel could take as its end, and a splicing cat read it from the
kernel's offset. log now stats 0, as the other streams and generated
views do, and the re-pinned cloud9's 9ns opens any file stating 0
FOPEN_DIRECT_IO | FOPEN_NONSEEKABLE. A look at such a file (the mounted
index) reads a regular file that will not seek as a stream rather than
refusing it as a pipe. selfmount runs the recipe, exec 3<>log; echo
follow >&3; cat <&3, through the mount and hears a record made after it
started.

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

1 files changed, 10 insertions, 0 deletions

diff --git a/test/selfmount.py b/test/selfmount.py
index 86c32fa8..dd59083c 100644
--- a/test/selfmount.py
+++ b/test/selfmount.py
@@ -176,6 +176,16 @@ def run(binary):
             held = run(f"for i in $(seq 40); do cat {mount}/pane/{first}/event > /dev/null & done; "
                        f"sleep 1; timeout 10 cat {mount}/layout > /dev/null; st=$?; kill $(jobs -p); exit $st\n".encode())
             check(held == '0', f'cat layout answers beside 40 held event reads ({held!r})')
+            # fs.md's follow recipe, as written, with plain cat, its output
+            # a pipe (where coreutils cat splices): it hears a record made
+            # after it started.
+            recipe = Path(f'/tmp/{name}.follow')
+            ran = run(f"exec 3<>{mount}/log; echo follow >&3; "
+                      f"(sleep 1; echo 'Msg recipe-heard' > {mount}/exec) & "
+                      f"timeout 5 cat <&3 | cat > {recipe}; exit 0\n".encode())
+            followed = recipe.read_text() if recipe.exists() else ''
+            recipe.unlink(missing_ok=True)
+            check(ran == '0' and 'recipe-heard' in followed, f'the log follow recipe with cat hears a new record ({followed[-120:]!r})')
             client.write('/exec', b'Msg still here\n')
             text = ''.join(cell[0] for cell in client.screen()['cells'])
             check('still here' in text, 'the editor answers after all of that')