1 files changed, 106 insertions, 0 deletions
diff --git a/lib/streamio/runescanner.go b/lib/streamio/runescanner.go
new file mode 100644
index 0000000..451f32f
--- /dev/null
+++ b/lib/streamio/runescanner.go
@@ -0,0 +1,106 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package streamio
+
+import (
+	"bufio"
+	"context"
+	"io"
+	"os"
+	"time"
+
+	"github.com/datawire/dlib/dlog"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/textui"
+)
+
+type RuneScanner interface {
+	io.RuneScanner
+	io.Closer
+}
+
+type runeScanner struct {
+	ctx            context.Context //nolint:containedctx // For detecting shutdown from methods
+	done           <-chan struct{}
+	progress       textui.Portion[int64]
+	progressWriter *textui.Progress[textui.Portion[int64]]
+	unreadCnt      uint64
+	reader         *bufio.Reader
+	closer         io.Closer
+}
+
+// NewRuneScanner returns an io.RuneScanner (and io.Closer) that
+// bufferes a file, similar to bufio.NewReader.  There are two
+// advantages over bufio.NewReader:
+//
+//   - It takes a Context, and causes reads to fail once the Context is
+//     canceled; allowing large parse operations to be gracefully cut
+//     short.
+//
+//   - It logs the progress of reading the file via textui.Progress.
+func NewRuneScanner(ctx context.Context, fh *os.File) (RuneScanner, error) {
+	fi, err := fh.Stat()
+	if err != nil {
+		return nil, err
+	}
+	ret := &runeScanner{
+		ctx:  ctx,
+		done: ctx.Done(),
+		progress: textui.Portion[int64]{
+			D: fi.Size(),
+		},
+		progressWriter: textui.NewProgress[textui.Portion[int64]](ctx, dlog.LogLevelInfo, textui.Tunable(1*time.Second)),
+		reader:         bufio.NewReader(fh),
+		closer:         fh,
+	}
+	return ret, nil
+}
+
+func isClosed(ch <-chan struct{}) bool {
+	select {
+	case <-ch:
+		return true
+	default:
+		return false
+	}
+}
+
+//nolint:gomnd // False positive: gomnd.ignored-functions=[textui.Tunable] doesn't support type params.
+var runeThrottle = textui.Tunable[int64](64)
+
+// ReadRune implements io.RuneReader.
+func (rs *runeScanner) ReadRune() (r rune, size int, err error) {
+	// According to the profiler, checking if the rs.ctx.Done()
+	// channel is closed is faster than checking if rs.ctx.Err()
+	// is non-nil.
+	if rs.unreadCnt == 0 && isClosed(rs.done) {
+		return 0, 0, rs.ctx.Err()
+	}
+	r, size, err = rs.reader.ReadRune()
+	if rs.unreadCnt > 0 {
+		rs.unreadCnt--
+	} else {
+		rs.progress.N += int64(size)
+		if rs.progress.D < runeThrottle || rs.progress.N%runeThrottle == 0 || rs.progress.N > rs.progress.D-runeThrottle {
+			rs.progressWriter.Set(rs.progress)
+		}
+	}
+	return
+}
+
+// ReadRune implements io.RuneScanner.
+func (rs *runeScanner) UnreadRune() error {
+	if err := rs.reader.UnreadRune(); err != nil {
+		return err
+	}
+	rs.unreadCnt++
+	return nil
+}
+
+// ReadRune implements io.Closer.
+func (rs *runeScanner) Close() error {
+	rs.progressWriter.Done()
+	return rs.closer.Close()
+}