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

crates/printer/src/summary.rs

@@ -11,7 +11,7 @@ use termcolor::{ColorSpec, NoColor, WriteColor};
 use color::ColorSpecs;
 use counter::CounterWriter;
 use stats::Stats;
-use util::PrinterPath;
+use util::{find_iter_at_in_context, PrinterPath};
 
 /// The configuration for the summary printer.
 ///
@@ -504,6 +504,17 @@ impl<'p, 's, M: Matcher, W: WriteColor> SummarySink<'p, 's, M, W> {
         self.stats.as_ref()
     }
 
+    /// Returns true if and only if the searcher may report matches over
+    /// multiple lines.
+    ///
+    /// Note that this doesn't just return whether the searcher is in multi
+    /// line mode, but also checks if the mater can match over multiple lines.
+    /// If it can't, then we don't need multi line handling, even if the
+    /// searcher has multi line mode enabled.
+    fn multi_line(&self, searcher: &Searcher) -> bool {
+        searcher.multi_line_with_matcher(&self.matcher)
+    }
+
     /// Returns true if this printer should quit.
     ///
     /// This implements the logic for handling quitting after seeing a certain
@@ -579,32 +590,39 @@ impl<'p, 's, M: Matcher, W: WriteColor> Sink for SummarySink<'p, 's, M, W> {
 
     fn matched(
         &mut self,
-        _searcher: &Searcher,
+        searcher: &Searcher,
         mat: &SinkMatch,
     ) -> Result<bool, io::Error> {
-        self.match_count += 1;
-        if let Some(ref mut stats) = self.stats {
-            let mut match_count = 0;
-            self.matcher
-                .find_iter(mat.bytes(), |_| {
-                    match_count += 1;
+        let is_multi_line = self.multi_line(searcher);
+        let sink_match_count = if self.stats.is_none() && !is_multi_line {
+            1
+        } else {
+            // This gives us as many bytes as the searcher can offer. This
+            // isn't guaranteed to hold the necessary context to get match
+            // detection correct (because of look-around), but it does in
+            // practice.
+            let buf = mat.buffer();
+            let range = mat.bytes_range_in_buffer();
+            let mut count = 0;
+            find_iter_at_in_context(
+                searcher,
+                &self.matcher,
+                buf,
+                range,
+                |_| {
+                    count += 1;
                     true
-                })
-                .map_err(io::Error::error_message)?;
-            if match_count == 0 {
-                // It is possible for the match count to be zero when
-                // look-around is used. Since `SinkMatch` won't necessarily
-                // contain the look-around in its match span, the search here
-                // could fail to find anything.
-                //
-                // It seems likely that setting match_count=1 here is probably
-                // wrong in some cases, but I don't think we can do any
-                // better. (Because this printer cannot assume that subsequent
-                // contents have been loaded into memory, so we have no way of
-                // increasing the search span here.)
-                match_count = 1;
-            }
-            stats.add_matches(match_count);
+                },
+            )?;
+            count
+        };
+        if is_multi_line {
+            self.match_count += sink_match_count;
+        } else {
+            self.match_count += 1;
+        }
+        if let Some(ref mut stats) = self.stats {
+            stats.add_matches(sink_match_count);
             stats.add_matched_lines(mat.lines().count() as u64);
         } else if self.summary.config.kind.quit_early() {
             return Ok(false);