grep: fix bugs in handling multi-line look-around

This commit hacks in a bug fix for handling look-around across multiple
lines. The main problem is that by the time the matching lines are sent
to the printer, the surrounding context---which some look-behind or
look-ahead might have matched---could have been dropped if it wasn't
part of the set of matching lines. Therefore, when the printer re-runs
the regex engine in some cases (to do replacements, color matches, etc
etc), it won't be guaranteed to see the same matches that the searcher
found.

Overall, this is a giant clusterfuck and suggests that the way I divided
the abstraction boundary between the printer and the searcher is just
wrong. It's likely that the searcher needs to handle more of the work of
matching and pass that info on to the printer. The tricky part is that
this additional work isn't always needed. Ultimately, this means a
serious re-design of the interface between searching and printing. Sigh.

The way this fix works is to smuggle the underlying buffer used by the
searcher through into the printer. Since these bugs only impact
multi-line search (otherwise, searches are only limited to matches
across a single line), and since multi-line search always requires
having the entire file contents in a single contiguous slice (memory
mapped or on the heap), it follows that the buffer we pass through when
we need it is, in fact, the entire haystack. So this commit refactors
the printer's regex searching to use that buffer instead of the intended
bundle of bytes containing just the relevant matching portions of that
same buffer.

There is one last little hiccup: PCRE2 doesn't seem to have a way to
specify an ending position for a search. So when we re-run the search to
find matches, we can't say, "but don't search past here." Since the
buffer is likely to contain the entire file, we really cannot do
anything here other than specify a fixed upper bound on the number of
bytes to search. So if look-ahead goes more than N bytes beyond the
match, this code will break by simply being unable to find the match. In
practice, this is probably pretty rare. I believe that if we did a
better fix for this bug by fixing the interfaces, then we'd probably try
to have PCRE2 find the pertinent matches up front so that it never needs
to re-discover them.

Fixes #1412

1 files changed, 9 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 900162bf..6ba9bd56 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,6 +26,13 @@ discussion on this. Previously, every line in a match was duplicated, even
 when it spanned multiple lines. There are no changes to vimgrep output when
 multi-line mode is disabled.
 
+**In multi-line mode, --count is now equivalent to --count-matches.**
+
+This appears to match how `pcre2grep` implements `--count`. Previously, ripgrep
+would produce outright incorrect counts. Another alternative would be to simply
+count the number of lines---even if it's more than the number of matches---but
+that seems highly unintuitive.
+
 Security fixes:
 
 * [CVE-2021-3013](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3013):
@@ -64,6 +71,8 @@ Bug fixes:
   Document cygwin path translation behavior in the FAQ.
 * [BUG #1311](https://github.com/BurntSushi/ripgrep/issues/1311):
   Fix multi-line bug where a search & replace for `\n` didn't work as expected.
+* [BUG #1412](https://github.com/BurntSushi/ripgrep/issues/1412):
+  Fix multi-line bug with searches using look-around past matching lines.
 * [BUG #1642](https://github.com/BurntSushi/ripgrep/issues/1642):
   Fixes a bug where using `-m` and `-A` printed more matches than the limit.
 * [BUG #1703](https://github.com/BurntSushi/ripgrep/issues/1703):