Addresses search a line at a time as sam does, say why they fail, and a failed addr leaves no address

addr's regexps ran mvzr over text[from..hi]: ^ and $ anchored only at the
slice's ends, . matched newlines, the search never wrapped, and every
failure read as bad address syntax. The regexps stay mvzr's, called the
way sam searches (editors/acme/regx.c): one line per haystack, so ^ and $
fall at line boundaries and . never crosses a newline; a pattern naming
\\n runs over the whole text with its . made [^\\n]; /re/ wraps unless
limit is set, and ?re? takes the last match before the range. The ceiling
(mvzr's first alternative, not sam's longest) is documented. A failed
address says why (no match for regexp, address out of range, bad regular
expression), and a failed write to addr leaves no address, so data and
xdata refuse until the next good one instead of writing at the old range.

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

1 files changed, 12 insertions, 0 deletions

diff --git a/test/fs.py b/test/fs.py
index ca1ca70e..a6d24e0b 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -344,6 +344,18 @@ def discovery(binary, embedded=False):
             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'
+            # sam's regexps: ^ at any line; a miss says so and leaves no
+            # address, so the data write after it refuses.
+            client.write(f'/pane/{scratch}/addr', b'/^second/')
+            assert client.read(f'/pane/{scratch}/addr') == b'         12          18 '
+            for path, data, why in ((f'/pane/{scratch}/addr', b'/nowhere/', 'no match for regexp'),
+                                    (f'/pane/{scratch}/data', b'LOST', 'no address')):
+                try:
+                    client.write(path, data)
+                    raise AssertionError(f'{path} took {data!r}')
+                except OSError as refused:
+                    assert why in str(refused), refused
+            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')