grep: make echo .|grep '\.' match once again

The same applied for many other backslash-escaped bytes, not just
metacharacters.  The switch to rawmemchr in v3.4-almost-10-g9393b97
made some parts of the code require the usually-guaranteed newline
sentinel at the end of each pattern. Before, some consumers used a
(correct) pattern length and did not care that try_fgrep_pattern could
transform a pattern (with sentinel) like "\\.\n" to "..\n", thus
violating that assumption.
* src/grep.c (try_fgrep_pattern): Preserve the invariant
that each regexp is newline-terminated.
* tests/backslash-dot: New file. Test for this.
* tests/Makefile.am (TESTS): Add it.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/grep.c b/src/grep.c
index ba6b15d6..497780e3 100644
--- a/src/grep.c
+++ b/src/grep.c
@@ -2471,6 +2471,9 @@ try_fgrep_pattern (int matcher, char *keys, size_t *len_p)
     {
       *len_p = p - new_keys;
       memcpy (keys, new_keys, p - new_keys);
+      /* We have just shortened this pattern.
+         Preserve the invariant that it is newline-terminated.  */
+      keys[*len_p] = '\n';
     }
   result = F_MATCHER_INDEX;