searcher: fix roll buffer bug

This commit fixes a subtle bug in how the line buffer was rolling its
contents. Specifically, when ripgrep searches without memory maps,
it uses a "roll" buffer for incremental line oriented search without
needing to read the entire file into memory at once. The roll buffer
works by reading a chunk of bytes from the file into memory, and then
searching everything in that buffer up to the last `\n` byte. The bytes
*after* the last `\n` byte are preserved, since they likely correspond
to *part* of the next line. Once ripgrep is done searching the buffer,
it "rolls" the buffer such that the start of the next line is at the
beginning of the buffer, and then ripgrep reads more data into the
buffer starting at the (possibly) partial end of that line.

The implication of this strategy, necessarily so, is that a buffer must
be big enough to fit a single line in memory. This is because the regex
engine needs a contiguous block of memory to search, so there is no way
to search anything smaller than a single line. So if a file contains a
single line with 7.5 million bytes, then the buffer will grow to be at
least that size. (Many files have super long lines like this, but they
tend to be *binary* files, which ripgrep will detect and stop searching
unless the user forces it with the `-a/--text` flag. So in practice,
they aren't usually a problem. However, in this case, #1335 found a case
where a plain text file had a line with 7.5 million bytes.)

Now, for performance reasons, ripgrep reuses these buffers across its
search. Typically, it will create `N` of these line buffers when it
starts (where `N` is the number of threads it is using), and then reuse
them without creating any new ones as it searches through files.

This means that if you search a file with a very long line, that buffer
will expand to be big enough to store that line. ripgrep never contracts
these buffers, so once it searches the next file, ripgrep will continue
to use this large buffer. While it might be prudent to contract these
buffers in some circumstances, this isn't otherwise inherently a
problem. The memory has already been allocated, and there isn't much
cost to using it, other than the fact that ripgrep hangs on to it and
never gives it back to the OS.

However, the `roll` implementation described above had a really
important bug in it that was impacted by the size of the buffer.
Specifically, it used the following to "roll" the partial line at the
end of the buffer to the beginning:

    self.buf.copy_within_str(self.pos.., 0);

Which means that if the buffer is very large, ripgrep will copy
*everything* from `self.pos` (which might be very small, e.g., for small
files) to the end of the buffer, and move it to the beginning of the
buffer. This will happen repeatedly each time the buffer is used to
search small files, which winds up being quite a large slow down if the
line was exceptionally large (say, megabytes).

It turns out that copying everything is completely unnecessary. We only
need to copy the remainder of the last read to the beginning of the
buffer. Everything *after* the last read in the buffer is just free
space that can be filled for the next read. So, all we need to do is
copy just those bytes:

    self.buf.copy_within_str(self.pos..self.end, 0);

... which is typically much much smaller than the rest of the buffer.

This was likely also causing small performance losses in other cases as
well. For example, when searching a lot of small files, ripgrep would
likely do a lot more copying than necessary. Although, given that the
default buffer size is 8KB, this extra copying was likely pretty small,
and was thus harder to observe.

Fixes #1335

1 files changed, 1 insertions, 1 deletions

diff --git a/grep-searcher/src/line_buffer.rs b/grep-searcher/src/line_buffer.rs
index 4b75fa37..96be997a 100644
--- a/grep-searcher/src/line_buffer.rs
+++ b/grep-searcher/src/line_buffer.rs
@@ -482,7 +482,7 @@ impl LineBuffer {
         }
 
         let roll_len = self.end - self.pos;
-        self.buf.copy_within_str(self.pos.., 0);
+        self.buf.copy_within_str(self.pos..self.end, 0);
         self.pos = 0;
         self.last_lineterm = roll_len;
         self.end = roll_len;