1 files changed, 105 insertions, 0 deletions
diff --git a/pkg/secio/secio.go b/pkg/secio/secio.go
new file mode 100644
index 000000000..fc625efb8
--- /dev/null
+++ b/pkg/secio/secio.go
@@ -0,0 +1,105 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package secio provides support for sectioned I/O.
+package secio
+
+import (
+	"errors"
+	"io"
+)
+
+// ErrReachedLimit is returned when SectionReader.Read or SectionWriter.Write
+// reaches its limit.
+var ErrReachedLimit = errors.New("reached limit")
+
+// SectionReader implements io.Reader on a section of an underlying io.ReaderAt.
+// It is similar to io.SectionReader, but:
+//
+// - Reading beyond the limit returns ErrReachedLimit, not io.EOF.
+//
+// - Limit overflow is handled correctly.
+type SectionReader struct {
+	r     io.ReaderAt
+	off   int64
+	limit int64
+}
+
+// Read implements io.Reader.Read.
+func (r *SectionReader) Read(dst []byte) (int, error) {
+	if r.limit >= 0 {
+		if max := r.limit - r.off; max < int64(len(dst)) {
+			dst = dst[:max]
+		}
+	}
+	n, err := r.r.ReadAt(dst, r.off)
+	r.off += int64(n)
+	if err == nil && r.off == r.limit {
+		err = ErrReachedLimit
+	}
+	return n, err
+}
+
+// NewOffsetReader returns an io.Reader that reads from r starting at offset
+// off.
+func NewOffsetReader(r io.ReaderAt, off int64) *SectionReader {
+	return &SectionReader{r, off, -1}
+}
+
+// NewSectionReader returns an io.Reader that reads from r starting at offset
+// off and stops with ErrReachedLimit after n bytes.
+func NewSectionReader(r io.ReaderAt, off int64, n int64) *SectionReader {
+	// If off + n overflows, it will be < 0 such that no limit applies, but
+	// this is the correct behavior as long as r prohibits reading at offsets
+	// beyond MaxInt64.
+	return &SectionReader{r, off, off + n}
+}
+
+// SectionWriter implements io.Writer on a section of an underlying
+// io.WriterAt. Writing beyond the limit returns ErrReachedLimit.
+type SectionWriter struct {
+	w     io.WriterAt
+	off   int64
+	limit int64
+}
+
+// Write implements io.Writer.Write.
+func (w *SectionWriter) Write(src []byte) (int, error) {
+	if w.limit >= 0 {
+		if max := w.limit - w.off; max < int64(len(src)) {
+			src = src[:max]
+		}
+	}
+	n, err := w.w.WriteAt(src, w.off)
+	w.off += int64(n)
+	if err == nil && w.off == w.limit {
+		err = ErrReachedLimit
+	}
+	return n, err
+}
+
+// NewOffsetWriter returns an io.Writer that writes to w starting at offset
+// off.
+func NewOffsetWriter(w io.WriterAt, off int64) *SectionWriter {
+	return &SectionWriter{w, off, -1}
+}
+
+// NewSectionWriter returns an io.Writer that writes to w starting at offset
+// off and stops with ErrReachedLimit after n bytes.
+func NewSectionWriter(w io.WriterAt, off int64, n int64) *SectionWriter {
+	// If off + n overflows, it will be < 0 such that no limit applies, but
+	// this is the correct behavior as long as w prohibits writing at offsets
+	// beyond MaxInt64.
+	return &SectionWriter{w, off, off + n}
+}