[PATCH 0/5] const-correctness in grep.c

While discussing [1], I noticed that the grep code mostly takes non-const buffers, even though it is conceptually a read-only operation to search in them. The culprit is a handful of spots that temporarily tie off NUL-terminated strings by overwriting a byte of the buffer and then restoring it. But I think we no longer need to do so these days, now that we have a regexec_buf() that can take a ptr/size pair.

The first three patches are a bit repetitive, but I broke them up individually because they're the high-risk part. I.e., if my assumptions about needing the NUL are wrong, it could introduce a bug. But based on my reading of the code, plus running the test suite with ASan/UBSan, I feel reasonably confident.

The last two are the bigger cleanups, but should obviously avoid any behavior changes.

  [1/5]: grep: stop modifying buffer in strip_timestamp
  [2/5]: grep: stop modifying buffer in show_line()
  [3/5]: grep: stop modifying buffer in grep_source_1()
  [4/5]: grep: mark "haystack" buffers as const
  [5/5]: grep: store grep_source buffer as const

 grep.c | 87 +++++++++++++++++++++++++++++-----------------------------
 grep.h |  4 +--
 2 files changed, 45 insertions(+), 46 deletions(-)

-Peff

[1] https://lore.kernel.org/git/YUk3zwuse56v76ze@coredump.intra.peff.net/